OkHttp简析

基于OkHttp 3.13版本进行分析

OkHttp简析

OkHttp定义

OkHttp现如今已成为主流的网络请求框架，连Android源码中都引入其作为基础网络库，可知它的重要性。

所以只会简单的使用是远远不够的，更要深入了解其原理，知道它的设计概念，这才是最重要的。

OkHttp支持SPDY协议，可以合并多个到同一个主机的请求，分享同一个Socket。如果SPDY不可用，会使用连接池的技术减少请求的延迟。

SPDY协议：Google提出的基于TCP的应用层协议，通过压缩、多路复用、优先级来缩短加载时间。

OkHttp使用示例

构造请求

GET请求

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Request request = new Request.Builder()
     .url(url)
     .build();

POST请求

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RequestBody body = RequestBody.create(JSON, json);
Request request = new Request.Builder()
    .url(url)
    .post(body)
    .build();

同步请求

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OkHttpClient client = new OkHttpClient();
Response response = client.newCall(request).execute();
return response.body().string();

异步请求

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OkHttpClient client = new OkHttpClient();

client.newCall(request).enqueue(new Callback() {
    @Override
    public void onFailure(Call call, IOException e) {
    }

    @Override
    public void onResponse(Call call, okhttp3.Response response) throws IOException {
    }
});

OkHttp源码分析

构造OkHttpClient对象

构造OkHttpClient对象

需要先创建一个OkHttpClient用以执行后续请求。内部主要是相关参数配置。

主要功能：通信的客户端，用以统一发起请求与解析返回值。

OkHttpClient.java
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public OkHttpClient() {
  this(new Builder());
}

OkHttpClient(Builder builder) {
  //用于调用网络请求 本质为 线程池
  this.dispatcher = builder.dispatcher;
  //设置代理
  this.proxy = builder.proxy;
  //设置协议
  this.protocols = builder.protocols;
  this.connectionSpecs = builder.connectionSpecs;
  //设置拦截器
  this.interceptors = Util.immutableList(builder.interceptors);
  //设置网络拦截器
  this.networkInterceptors = Util.immutableList(builder.networkInterceptors);
  //回调监听
  this.eventListenerFactory = builder.eventListenerFactory;
  this.proxySelector = builder.proxySelector;
  //Cookie
  this.cookieJar = builder.cookieJar;
  //缓存
  this.cache = builder.cache;
  this.internalCache = builder.internalCache;
  this.socketFactory = builder.socketFactory;

  boolean isTLS = false;
  for (ConnectionSpec spec : connectionSpecs) {
    isTLS = isTLS || spec.isTls();
  }

  //用于Https请求
  if (builder.sslSocketFactory != null || !isTLS) {
    this.sslSocketFactory = builder.sslSocketFactory;
    this.certificateChainCleaner = builder.certificateChainCleaner;
  } else {
    X509TrustManager trustManager = Util.platformTrustManager();
    this.sslSocketFactory = newSslSocketFactory(trustManager);
    this.certificateChainCleaner = CertificateChainCleaner.get(trustManager);
  }

  if (sslSocketFactory != null) {
    Platform.get().configureSslSocketFactory(sslSocketFactory);
  }

  this.hostnameVerifier = builder.hostnameVerifier;
  this.certificatePinner = builder.certificatePinner.withCertificateChainCleaner(
      certificateChainCleaner);
  this.proxyAuthenticator = builder.proxyAuthenticator;
  this.authenticator = builder.authenticator;
  this.connectionPool = builder.connectionPool;
  this.dns = builder.dns;
  this.followSslRedirects = builder.followSslRedirects;
  this.followRedirects = builder.followRedirects;
  //是否需要重试
  this.retryOnConnectionFailure = builder.retryOnConnectionFailure;
  this.callTimeout = builder.callTimeout;
  //链接超时时长
  this.connectTimeout = builder.connectTimeout;
  //读取超时时间
  this.readTimeout = builder.readTimeout;
  //写入超时时间
  this.writeTimeout = builder.writeTimeout;
  this.pingInterval = builder.pingInterval;

  if (interceptors.contains(null)) {
    throw new IllegalStateException("Null interceptor: " + interceptors);
  }
  if (networkInterceptors.contains(null)) {
    throw new IllegalStateException("Null network interceptor: " + networkInterceptors);
  }
}

OkHttpClient是应用建造者模式，通过OkHttpClient.Builder来构造一个OkHttpClient对象，支持数十种参数配置。

构造Request请求对象

OkHttp-构造Request请求对象

创建一个Request对象用以包括请求的所有信息，内部包含了请求地址，请求头，请求内容。

主要功能：封装请求的具体信息。

Request.java
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public final class Request {
  Request(Builder builder) {
    //请求地址
    this.url = builder.url;
    //请求方法 例如：GET、POST、PUT、DELETE等
    this.method = builder.method;
    //请求头信息
    this.headers = builder.headers.build();
    //请求内容构造体
    this.body = builder.body;
    //请求的标签  用于后续对指定标签可进行特殊处理
    this.tags = Util.immutableMap(builder.tags);
  }
    
  public static class Builder {
    @Nullable HttpUrl url;
    String method;
    Headers.Builder headers;
    @Nullable RequestBody body;

    /** A mutable map of tags, or an immutable empty map if we don't have any. */
    Map<Class<?>, Object> tags = Collections.emptyMap();
    
    //默认是GET方法，不带有请求体
    public Builder() {
      this.method = "GET";
      this.headers = new Headers.Builder();
    }

    Builder(Request request) {
      this.url = request.url;
      this.method = request.method;
      this.body = request.body;
      this.tags = request.tags.isEmpty()
          ? Collections.emptyMap()
          : new LinkedHashMap<>(request.tags);
      this.headers = request.headers.newBuilder();
    }
     ... 
  }
}

Request使用的也是建造者模式，通过Request.Builder去构造对应Request。

请求体RequestBody

主要功能：用以提交流、表单等请求信息。

FormBody

支持提交键值对类型。例如userId : 1

使用方法

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FormBody.Builder formBodyBuilder = new FormBody.Builder();
Map<String, Object> map = new ConcurrentHashMap<>();
for (Map.Entry<String, Object> entry : map.entrySet()) {
    formBodyBuilder.add(entry.getKey(), entry.getValue().toString());
}
RequestBody body = formBodyBuilder.build();
MultipartBody

除了支持键值对，还有提交文件功能。

使用方法

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MultipartBody.Builder multipartBuilder = new MultipartBody.Builder().setType(MultipartBody.FORM);
       Map<String, Object> map = new ConcurrentHashMap<>();
       for (Map.Entry<String, Object> entry : map.entrySet()) {
           multipartBuilder.addFormDataPart(entry.getKey(), entry.getValue().toString());
       }
       //可以针对文件新起一个 参数来进行传递
       for (Map.Entry<String, File> entry : message.getFiles().entrySet()) {
           File f = entry.getValue();
           if (f == null)
               continue;
           String name = f.getName();
           String ext = name.substring(name.lastIndexOf('.'));
           String imageFormat = "jpg";
           if (".png".equalsIgnoreCase(ext)) {
               imageFormat = "png";
           }
           multipartBuilder.addFormDataPart(
                   entry.getKey(),
                   entry.getValue().getName(),
                   RequestBody.create(MediaType.parse("image/" + imageFormat), entry.getValue())
           );
       }

       RequestBody body = multipartBuilder.build();

发送Request请求

OkHttp-发送Request请求

通过OkHttpClient.newCall()发送出Request请求

OkHttpClient.java
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@Override public Call newCall(Request request) {
  return RealCall.newRealCall(this, request, false /* for web socket */);
}

返回了一个Call对象，实现类为RealCall

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public interface Call extends Cloneable {
  //获取当前请求
  Request request();
  //执行当前请求 并返回结果
  Response execute() throws IOException;
  //异步请求
  void enqueue(Callback responseCallback);
  //取消请求
  /** Cancels the request, if possible. Requests that are already complete cannot be canceled. */
  void cancel();
  //当前请求是否正在执行
  boolean isExecuted();
  //请求是否已取消
  boolean isCanceled();
  //超时返回
  Timeout timeout();
  //克隆请求 用于重新调用
  Call clone();

  interface Factory {
    Call newCall(Request request);
  }
}
RealCall.java
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final class RealCall implements Call {
    private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    	//前面先行创建的 OkHttpClient
    	this.client = client;
        //创建的请求对象
    	this.originalRequest = originalRequest;
   		//用于建立长连接
    	this.forWebSocket = forWebSocket;
  	}
    
    static RealCall newRealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    	// Safely publish the Call instance to the EventListener.
    	RealCall call = new RealCall(client, originalRequest, forWebSocket);
        //为这次请求设置了事件监听器，包括请求开始、结束、异常等监听
    	call.transmitter = new Transmitter(client, call);
    	return call;
  	}
}

通过newCall()根据传递进来的Request创建一个RealCall实例去发送请求。

同步请求——execute()

直接执行并返回请求结果

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@Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    transmitter.timeoutEnter();
    //请求开始
    transmitter.callStart();
    try {
      //加入 runningSuncCalls 队列中
      client.dispatcher().executed(this);
      //返回响应结果
      return getResponseWithInterceptorChain();
    } finally {
      //从队列中移除 避免重复执行
      client.dispatcher().finished(this);
    }
  }

执行execute()时，监听到请求事件开始，就会加入到Dispatcher.runningSyncCalls中，里面记录的是当前正在进行同步请求的call，然后当call完成时或因异常结束时，再从Dispatcher.runningSyncCalls移除。

异步请求——enqueue(Callback callback)

构造一个异步执行队列，然后把请求加入队列中处理

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  @Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    transmitter.callStart();
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

final class AsyncCall extends NamedRunnable {
    private final Callback responseCallback;
    private volatile AtomicInteger callsPerHost = new AtomicInteger(0);

    AsyncCall(Callback responseCallback) {
      super("OkHttp %s", redactedUrl());
      this.responseCallback = responseCallback;
    }

    AtomicInteger callsPerHost() {
      return callsPerHost;
    }
    ...

    void executeOn(ExecutorService executorService) {
      assert (!Thread.holdsLock(client.dispatcher()));
      boolean success = false;
      try {
        executorService.execute(this);
        success = true;
      } catch (RejectedExecutionException e) {
        InterruptedIOException ioException = new InterruptedIOException("executor rejected");
        ioException.initCause(e);
        transmitter.noMoreExchanges(ioException);
        responseCallback.onFailure(RealCall.this, ioException);
      } finally {
        if (!success) {
          client.dispatcher().finished(this); // This call is no longer running!
        }
      }
    }

    @Override protected void execute() {
      boolean signalledCallback = false;
      transmitter.timeoutEnter();
      try {
        Response response = getResponseWithInterceptorChain();
        signalledCallback = true;
        //回调请求结果
        responseCallback.onResponse(RealCall.this, response);
      } catch (IOException e) {
        if (signalledCallback) {
          // Do not signal the callback twice!
          Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
        } else {
          //回调失败并返回异常
          responseCallback.onFailure(RealCall.this, e);
        }
      } finally {
        //从队列中移除
        client.dispatcher().finished(this);
      }
    }
  }

enqueue()调用到Dispatcher.enqueue()传入的是一个AsyncCall对象，AsyncCall本质是一个Runnable对象，通过Dispatcher中的ExecutorService来执行AsyncCall。

执行Request请求

由execute()或enqueue()发送请求时，最后都是需要有Dispatch去执行请求。

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public final class Dispatcher {
  private int maxRequests = 64;
  private int maxRequestsPerHost = 5;
    
  private @Nullable ExecutorService executorService;
  //正在准备执行的异步请求队列
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();
  //正在执行的异步请求队列
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();
  //正在执行的同步请求队列 
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();
    
  //用以执行异步请求
  public synchronized ExecutorService executorService() {
    if (executorService == null) {
      executorService = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60, TimeUnit.SECONDS,
          new SynchronousQueue<>(), Util.threadFactory("OkHttp Dispatcher", false));
    }
    return executorService;
  }
    
  void enqueue(AsyncCall call) {
    synchronized (this) {
      //加入正在执行的异步队列中
      readyAsyncCalls.add(call);

      // Mutate the AsyncCall so that it shares the AtomicInteger of an existing running call to
      // the same host.
      if (!call.get().forWebSocket) {
        AsyncCall existingCall = findExistingCallWithHost(call.host());
        if (existingCall != null) call.reuseCallsPerHostFrom(existingCall);
      }
    }
    promoteAndExecute();
  }
    
  //加入正在执行的同步队列
  synchronized void executed(RealCall call) {
    runningSyncCalls.add(call);
  }
  //从政在执行的异步队列中移除  
  void finished(AsyncCall call) {
    call.callsPerHost().decrementAndGet();
    finished(runningAsyncCalls, call);
  }
  //执行完毕后  从正在执行的同步队列中移除
  void finished(RealCall call) {
    finished(runningSyncCalls, call);
  }
    
  private <T> void finished(Deque<T> calls, T call) {
    Runnable idleCallback;
    synchronized (this) {
      if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
      idleCallback = this.idleCallback;
    }
    boolean isRunning = promoteAndExecute();
    if (!isRunning && idleCallback != null) {
      idleCallback.run();
    }
  }
    
  private boolean promoteAndExecute() {
    //判定当前线程是否持有锁
    assert (!Thread.holdsLock(this));

    List<AsyncCall> executableCalls = new ArrayList<>();
    boolean isRunning;
    synchronized (this) {
      
      for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
        AsyncCall asyncCall = i.next();
        //正在运行的异步请求不能超过 64个
        if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
        //在同一个Host下的异步请求不能超过5个
        if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.

        i.remove();
        //CAS
        asyncCall.callsPerHost().incrementAndGet();
        //添加至异步执行队列
        executableCalls.add(asyncCall);
        //添加至正在执行异步请求队列
        runningAsyncCalls.add(asyncCall);
      }
      isRunning = runningCallsCount() > 0;
    }

    for (int i = 0, size = executableCalls.size(); i < size; i++) {
      AsyncCall asyncCall = executableCalls.get(i);
      //执行异步请求
      asyncCall.executeOn(executorService());
    }

    return isRunning;
  }
}

Dispatcher是一个任务调度器，内部维护了三个双端队列：

  • readyAsyncCalls：准备执行的异步请求。已经超过请求上限的异步请求就会放在该队列中。
  • runningAsyncCalls：正在执行的异步请求。不超过请求上限时，异步请求会加入到该队列中，超过时，依然放到readyAsyncCalls中。
  • runningSyncCalls：正在执行的同步请求。直接把同步请求添加到该队列中。

通过Dispatcher中的executorService去执行对应请求。

处理Request请求——通过拦截器

OkHttp-处理Request请求

通过Dispatcher执行完请求后，返回回调结果前，需要通过getResponseWithInterceptorChain()通过层层责任链的执行来获得最终的请求结果。

通过责任链模式将请求一层层的通过拦截器进行处理。

RealCall.java
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Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    //加入用户自定义的拦截器
    interceptors.addAll(client.interceptors());
    //重试和重定向拦截器
    interceptors.add(new RetryAndFollowUpInterceptor(client));
    //转化用户请求为网络请求
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    //负责读取缓存以及更新缓存
    interceptors.add(new CacheInterceptor(client.internalCache()));
    //与服务器建立连接
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
       //用户自定义的网络拦截器
      interceptors.addAll(client.networkInterceptors());
    }
    //从服务器读取响应的数据
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, transmitter, null, 0,
        originalRequest, this, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    boolean calledNoMoreExchanges = false;
    try {
      //链式调用拦截器，最终返回 Response
      Response response = chain.proceed(originalRequest);
      if (transmitter.isCanceled()) {
        closeQuietly(response);
        throw new IOException("Canceled");
      }
      return response;
    } catch (IOException e) {
      calledNoMoreExchanges = true;
      throw transmitter.noMoreExchanges(e);
    } finally {
      if (!calledNoMoreExchanges) {
        transmitter.noMoreExchanges(null);
      }
    }
  }

在获得响应结果之前，需要对用户设置的原始请求转换为实际的网络请求，然后通过一系列拦截器，直到最终得到结果，采用链式调用保证这些拦截器的执行顺序。

OkHttp拦截器

所有的拦截器都实现了Interceptor接口，支持用户去自定义拦截器，只要实现Interceptor接口即可。

拦截器可以 用来监控、改写和重试HTTP访问。

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public interface Interceptor {
  //主要实现该接口，控制返回结果
  Response intercept(Chain chain) throws IOException;

  interface Chain {
    
    Request request();

    Response proceed(Request request) throws IOException;

    //返回Request执行后的返回结果
    @Nullable Connection connection();

    Call call();

    int connectTimeoutMillis();

    Chain withConnectTimeout(int timeout, TimeUnit unit);

    int readTimeoutMillis();

    Chain withReadTimeout(int timeout, TimeUnit unit);

    int writeTimeoutMillis();

    Chain withWriteTimeout(int timeout, TimeUnit unit);
  }
}
ApplicationInterceptor

OkHttp-ApplicationInterceptor

该拦截器会被第一个执行，此处得到的Request为最原始状态。但是最终得到的Response是最终的结果。

引用代码

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OkHttpClient client = new OkHttpClient.Builder()
    .addInterceptor(new LoggingInterceptor())
    .build();

ApplicationInterceptor适用于在请求前统一添加一些公共参数，例如App的版本号，系统信息等。

也可用于对返回的Response进行加工。

ApplicationInterceptor有以下特定：

  • 不需要关心后续拦截器进行的操作，因为是会被第一个执行的，只要关心返回结果即可。
  • 只会被响应一次，即使强制缓存获取
  • 可以对后续的拦截器调用进行拦截或者进行多次调用——通过Chain.proceed()进行控制
RetryAndFollowUpInterceptor

OkHttp-RetryAndFollowUpInterceptor

负责失败重试和重定向的拦截器。

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public final class RetryAndFollowUpInterceptor implements Interceptor {
    @Override public Response intercept(Chain chain) throws IOException {
    //从自定义拦截器 那里传递下来的请求
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    //获取事件监听器
    Transmitter transmitter = realChain.transmitter();
    //初始化 重定向次数
    int followUpCount = 0;
    Response priorResponse = null;
    //开启死循环 进行重试操作
    while (true) {
      transmitter.prepareToConnect(request);
      //请求取消
      if (transmitter.isCanceled()) {
        throw new IOException("Canceled");
      }

      Response response;
      boolean success = false;
      try {
        //向下调用 下一个拦截器——BridgeInterceptor
        response = realChain.proceed(request, transmitter, null);
        success = true;
      } catch (RouteException e) {
        // 不需要重试 则抛出异常
        if (!recover(e.getLastConnectException(), transmitter, false, request)) {
          throw e.getFirstConnectException();
        }
        continue;
      } catch (IOException e) {
        // 无法与服务端建立连接
        boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
        if (!recover(e, transmitter, requestSendStarted, request)) throw e;
        continue;
      } finally {
        // The network call threw an exception. Release any resources.
        if (!success) {
          //释放资源
          transmitter.exchangeDoneDueToException();
        }
      }

      // Attach the prior response if it exists. Such responses never have a body.
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }

      Exchange exchange = Internal.instance.exchange(response);
      Route route = exchange != null ? exchange.connection().route() : null;
      //根据返回的 response的Code 判断是否需要进行重定向
      Request followUp = followUpRequest(response, route);

      if (followUp == null) {
        //释放资源
        if (exchange != null && exchange.isDuplex()) {
          transmitter.timeoutEarlyExit();
        }
        return response;
      }

      RequestBody followUpBody = followUp.body();
      if (followUpBody != null && followUpBody.isOneShot()) {
        return response;
      }

      closeQuietly(response.body());
      if (transmitter.hasExchange()) {
        exchange.detachWithViolence();
      }

      //超出重定向次数
      if (++followUpCount > MAX_FOLLOW_UPS) {
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }
      //获取重定向结果 赋予 request继续向下请求
      request = followUp;
      priorResponse = response;
    }
  }
    
  private boolean recover(IOException e, Transmitter transmitter,
      boolean requestSendStarted, Request userRequest) {
    // 未开启重试 retryOnConnectionFailure(false)
    if (!client.retryOnConnectionFailure()) return false;
    // 只允许发送一次 isOneShot(){return true;}
    if (requestSendStarted && requestIsOneShot(e, userRequest)) return false;
    // 发生异常
    if (!isRecoverable(e, requestSendStarted)) return false;
    // 设置不允许重试
    if (!transmitter.canRetry()) return false;
    return true;
  }
}
  1. 尝试执行下一个拦截器，即BridgeInterceptor
  2. 抛出异常，需要根据以下情况去判断是否需要重试：
    • 客户端是否开启 retryOnConnectionFailure
    • RequestBody.isOneShot()返回值
    • 判断异常类型，除了ConnectionShutdownException被中断情况外的IOException的子类，都不会进行重试
  3. 根据Response返回的响应码code进行处理
RetryAndFollowUpInterceptor.java
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private Request followUpRequest(Response userResponse, @Nullable Route route) throws IOException {
    if (userResponse == null) throw new IllegalStateException();
    int responseCode = userResponse.code();

    final String method = userResponse.request().method();
    switch (responseCode) {
      // 407 需要进行代理认证
      case HTTP_PROXY_AUTH:
        Proxy selectedProxy = route != null
            ? route.proxy()
            : client.proxy();
        if (selectedProxy.type() != Proxy.Type.HTTP) {
          throw new ProtocolException("Received HTTP_PROXY_AUTH (407) code while not using proxy");
        }
        return client.proxyAuthenticator().authenticate(route, userResponse);
      // 401 未经认证
      case HTTP_UNAUTHORIZED:
        return client.authenticator().authenticate(route, userResponse);
      // 301 永久重定向  302 临时重定向  只有GET、HEAD请求方法才有效
      case HTTP_PERM_REDIRECT:
      case HTTP_TEMP_REDIRECT:
        // "If the 307 or 308 status code is received in response to a request other than GET
        // or HEAD, the user agent MUST NOT automatically redirect the request"
        if (!method.equals("GET") && !method.equals("HEAD")) {
          return null;
        }
        // fall-through
      
      case HTTP_MULT_CHOICE:// 300  多个重定向地址
      case HTTP_MOVED_PERM:// 301 永久移除 指向了新的位置
      case HTTP_MOVED_TEMP://302  临时移除
      case HTTP_SEE_OTHER://303 查看其他位置
        // 开发者是否允许重定向
        if (!client.followRedirects()) return null;
        //重定向后的实际地址
        String location = userResponse.header("Location");
        if (location == null) return null;
        HttpUrl url = userResponse.request().url().resolve(location);

        // Don't follow redirects to unsupported protocols.
        if (url == null) return null;

        // If configured, don't follow redirects between SSL and non-SSL.
        boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());
        if (!sameScheme && !client.followSslRedirects()) return null;

        // Most redirects don't include a request body.
        Request.Builder requestBuilder = userResponse.request().newBuilder();
        if (HttpMethod.permitsRequestBody(method)) {
          final boolean maintainBody = HttpMethod.redirectsWithBody(method);
          if (HttpMethod.redirectsToGet(method)) {
            requestBuilder.method("GET", null);
          } else {
            RequestBody requestBody = maintainBody ? userResponse.request().body() : null;
            requestBuilder.method(method, requestBody);
          }
          if (!maintainBody) {
            requestBuilder.removeHeader("Transfer-Encoding");
            requestBuilder.removeHeader("Content-Length");
            requestBuilder.removeHeader("Content-Type");
          }
        }

        // When redirecting across hosts, drop all authentication headers. This
        // is potentially annoying to the application layer since they have no
        // way to retain them.
        if (!sameConnection(userResponse.request().url(), url)) {
          requestBuilder.removeHeader("Authorization");
        }

        return requestBuilder.url(url).build();
      //408 超时
      case HTTP_CLIENT_TIMEOUT:
        // 408's are rare in practice, but some servers like HAProxy use this response code. The
        // spec says that we may repeat the request without modifications. Modern browsers also
        // repeat the request (even non-idempotent ones.)
        if (!client.retryOnConnectionFailure()) {
          // The application layer has directed us not to retry the request.
          return null;
        }

        RequestBody requestBody = userResponse.request().body();
        if (requestBody != null && requestBody.isOneShot()) {
          return null;
        }

        if (userResponse.priorResponse() != null
            && userResponse.priorResponse().code() == HTTP_CLIENT_TIMEOUT) {
          // We attempted to retry and got another timeout. Give up.
          return null;
        }

        if (retryAfter(userResponse, 0) > 0) {
          return null;
        }

        return userResponse.request();
      //503 服务端不可用
      case HTTP_UNAVAILABLE:
        if (userResponse.priorResponse() != null
            && userResponse.priorResponse().code() == HTTP_UNAVAILABLE) {
          // We attempted to retry and got another timeout. Give up.
          return null;
        }

        if (retryAfter(userResponse, Integer.MAX_VALUE) == 0) {
          // specifically received an instruction to retry without delay
          return userResponse.request();
        }

        return null;

      default:
        return null;
    }
  }

通过followUpRequest()对Response返回的code进行对应操作，在触发到重定向相关的code3XX时，需要对应的转换Request使用获取到的重定向后地址进行请求。

由源码可知，可以重试的最大次数为20次，可以通过retryOnConnectionFailure(true)设置支持重试。但是不支持自定义重试次数，若需要自定义重试次数，需要自定义拦截器去实现。

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class RetryInterceptor(var maxRetry: Int/*最大重试次数*/) : Interceptor {
        //当前重试次数
        private var retryNum = 0

        override fun intercept(chain: Interceptor.Chain): Response {
            val request = chain.request()
            var response = chain.proceed(request)

            while (!response.isSuccessful && retryNum < maxRetry) {
                retryNum++
                response = chain.proceed(request)
            }
            return response
        }
    }
BridgeInterceptor

OkHttp-BridgeInterceptor

用以将用户的请求转换为向服务器的请求，之后再把服务器返回的数据转换成用户直观的数据。主要是对Header进行处理

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public final class BridgeInterceptor implements Interceptor {
    @Override public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();
    //重构用户请求 为 服务器请求格式
    RequestBody body = userRequest.body();
    //如果存在Body
    if (body != null) {
      //对Header进行调整
      MediaType contentType = body.contentType();
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString());
      }

      long contentLength = body.contentLength();
      if (contentLength != -1) {
        requestBuilder.header("Content-Length", Long.toString(contentLength));
        requestBuilder.removeHeader("Transfer-Encoding");
      } else {
        requestBuilder.header("Transfer-Encoding", "chunked");
        requestBuilder.removeHeader("Content-Length");
      }
    }
    //设置Header中的 host
    if (userRequest.header("Host") == null) {
      requestBuilder.header("Host", hostHeader(userRequest.url(), false));
    }

    //设置 connection : Keep-Alive 保持长连接模式
    if (userRequest.header("Connection") == null) {
      requestBuilder.header("Connection", "Keep-Alive");
    }

    // If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
    // the transfer stream.
    boolean transparentGzip = false;
    //默认使用Gzip压缩
    if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
      transparentGzip = true;
      requestBuilder.header("Accept-Encoding", "gzip");
    }
    //设置 Cookie信息
    List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
    if (!cookies.isEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies));
    }
    //设置UA
    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", Version.userAgent());
    }
    //传递至下一个拦截器处理
    Response networkResponse = chain.proceed(requestBuilder.build());

    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

    Response.Builder responseBuilder = networkResponse.newBuilder()
        .request(userRequest);
    //如果服务器支持Gzip压缩，需要进行解压操作
    if (transparentGzip
        && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
        && HttpHeaders.hasBody(networkResponse)) {
      GzipSource responseBody = new GzipSource(networkResponse.body().source());
      Headers strippedHeaders = networkResponse.headers().newBuilder()
          .removeAll("Content-Encoding")
          .removeAll("Content-Length")
          .build();
      responseBuilder.headers(strippedHeaders);
      String contentType = networkResponse.header("Content-Type");
      responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
    }

    return responseBuilder.build();
  }
}

GZip：是一种压缩技术，可以改进Web应用的性能，将请求体明显的减少其大小，如果服务器也支持该格式，就会返回对应格式的内容，客户端需要进行解压操作，可以明显的减少流量消耗。

BridgeInterceptor主要执行了以下3步：

  • 用户请求转换为网络请求

    在原来Request上添加了很多Header，例如Content-Type(定义网络文件的类型和网页的编码)、Content-Length(请求体内容长度)、Transfer-Encoding(请求体的大小)与Content-Length互斥、Accept-Encoding(编码格式)。

    未设置Accept-Encoding默认为gzip。

  • 执行转换后的网络请求

    chain.proceed(requestBuilder.build())

  • 服务器返回的响应结果转换为用户响应结果

    根据上一步获得Response后，需要再次转化为用户直观格式。主要在于服务端返回的信息里是否设置了Accept-Encoding:gzip，设置了则需要进行解压过程，获取最终结果。

CacheInterceptor

OkHttp-CacheInterceptor

主要用于读取缓存以及更新缓存的，为了节省流量和提高响应速度。

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public final class CacheInterceptor implements Interceptor {
    @Override public Response intercept(Chain chain) throws IOException {
    //根据请求的相关信息获取缓存
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();
    //创建缓存策略
    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();①
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

    if (cache != null) {
      cache.trackResponse(strategy);
    }
    //缓存无法使用，关闭获得的Response
    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }

    // 根据策略，不使用网络且没有缓存的直接报错，返回504
    if (networkRequest == null && cacheResponse == null) {
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    // 直接返回缓存，不允许使用网络
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      //请求向下传递
      networkResponse = chain.proceed(networkRequest);
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

    // 接受到服务器返回数据，如果返回code为 304 直接使用缓存结果
    if (cacheResponse != null) {
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        cache.trackConditionalCacheHit();
        //更新当前存储的缓存信息
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }
    //读取服务器返回结果
    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();
    //对数据进行缓存
    if (cache != null) {
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }

      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }

    return response;
  }
}

CacheInterceptor的执行流程如下所示：

  1. 先行读取缓存数据
  2. 创建好对应的缓存策略：强制缓存、对比缓存
  3. 根据缓存策略，不使用网络、也没有对应缓存，返回504
  4. 根据缓存策略，不使用网络，存在缓存则直接返回
  5. 前面都没有返回结果，继续向下执行请求：chain.proceed()
  6. 接受到对应网络结果，如果返回code为304，代表直接使用缓存并更新对应缓存信息
  7. 读取网络结果，对数据进行缓存
  8. 返回获取的网络结果

具体的缓存策略请参考缓存策略

ConnectInterceptor

OkHttp-ConnectInterceptor

真正与服务端建立连接，底层是通过Socket进行连接。

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public final class ConnectInterceptor implements Interceptor {
  public final OkHttpClient client;

  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    Transmitter transmitter = realChain.transmitter();

    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    //建立连接
    Exchange exchange = transmitter.newExchange(chain, doExtensiveHealthChecks);
    //继续请求下一个拦截器
    return realChain.proceed(request, transmitter, exchange);
  }
}

ConnectInterceptor主要功能是建立与服务器的连接关系，通过Transmitter.newExchange()建立连接，建立完成后继续向下执行请求。

具体的连接过程可以参考[连接机制](# OkHttp连接机制)

NetworkInterceptor

OkHttp-NetworkInterceptor

用户自定义的网络拦截器，处于第6个拦截器，前面经过了RetryAndFolowUpInterceptor的重定向过程以及BridgeInterceptor的请求头处理，在此处可以获取到更多的连接信息。

引用代码

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OkHttpClient client = new OkHttpClient.Builder()
    .addNetworkInterceptor(new LoggingInterceptor())
    .build();

NetworkInterceptor可以获取到最终请求的Request，以及获取到真正进行过网络请求的得到的Response，从而可以针对Response进行修改然后再回传到上层拦截器。

NetworkInterceptor主要有以下特点：

  • 可以操作经过重定向、重试得到的Response
  • 无法响应缓存数据的请求，因为CacheInterceptor执行在它之前
  • 得到最终进行请求的Request
  • 可以获得连接信息
CallServerInterceptor

OkHttp-CallServerInterceptor

数据的写入过程，也就是客户端和服务端进行交互的过程，客户端发送数据，服务端返回数据。

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public final class CallServerInterceptor implements Interceptor {
@Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Exchange exchange = realChain.exchange();
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();
    //写入请求头
    exchange.writeRequestHeaders(request);

    boolean responseHeadersStarted = false;
    Response.Builder responseBuilder = null;
    //判断当前是否有 请求体body
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
      // Continue" response before transmitting the request body. If we don't get that, return
      // what we did get (such as a 4xx response) without ever transmitting the request body.
      //如果是1XX的话 表示当前需要等服务端响应 
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        exchange.flushRequest();
        responseHeadersStarted = true;
        exchange.responseHeadersStart();
        responseBuilder = exchange.readResponseHeaders(true);
      }

      
      if (responseBuilder == null) {
        //写入请求体
        if (request.body().isDuplex()) {
          // Prepare a duplex body so that the application can send a request body later.
          exchange.flushRequest();
          BufferedSink bufferedRequestBody = Okio.buffer(
              exchange.createRequestBody(request, true));
          request.body().writeTo(bufferedRequestBody);
        } else {
          // Write the request body if the "Expect: 100-continue" expectation was met.
          BufferedSink bufferedRequestBody = Okio.buffer(
              exchange.createRequestBody(request, false));
          request.body().writeTo(bufferedRequestBody);
          bufferedRequestBody.close();
        }
      } else {
        exchange.noRequestBody();
        if (!exchange.connection().isMultiplexed()) {
          // If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection
          // from being reused. Otherwise we're still obligated to transmit the request body to
          // leave the connection in a consistent state.
          exchange.noNewExchangesOnConnection();
        }
      }
    } else {
      exchange.noRequestBody();
    }

    if (request.body() == null || !request.body().isDuplex()) {
        //结束请求
      exchange.finishRequest();
    }

    if (!responseHeadersStarted) {
      exchange.responseHeadersStart();
    }
    //得到响应头
    if (responseBuilder == null) {
      responseBuilder = exchange.readResponseHeaders(false);
    }
    
    Response response = responseBuilder
        .request(request)
        .handshake(exchange.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();
    //读取响应体内容
    int code = response.code();
    if (code == 100) {
      // server sent a 100-continue even though we did not request one.
      // try again to read the actual response
      response = exchange.readResponseHeaders(false)
          .request(request)
          .handshake(exchange.connection().handshake())
          .sentRequestAtMillis(sentRequestMillis)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();

      code = response.code();
    }

    exchange.responseHeadersEnd(response);
    //forWebSocket 表示为socket连接方式
    if (forWebSocket && code == 101) {
      // Connection is upgrading, but we need to ensure interceptors see a non-null response body.
      response = response.newBuilder()
          .body(Util.EMPTY_RESPONSE)
          .build();
    } else {
      response = response.newBuilder()
          .body(exchange.openResponseBody(response))
          .build();
    }
    // close表示关闭连接
    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      exchange.noNewExchangesOnConnection();
    }

    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }

    return response;
  }
}

CallServerInterceptor主要执行了以下过程：

  • 写入请求头
  • 写入请求体(如果存在)
  • 获取状态行及响应头
  • 获取响应体

CallServerInterceptor已经是最后一个拦截器了，接下来就是向上回溯并返回自己获得的Response。

HTTP报文结构：

请求报文：

请求行：声明请求方法、主机域名及协议版本

请求头：声明客户端的部分报文信息

请求体：存放客户端发送给服务器的数据

响应报文

状态行：声明HTTP协议版本、状态码及描述

响应头：声明服务端的部分报文信息

响应体：服务端返回客户端的数据

责任链模式串联

介绍完上述的拦截器后，接下来就是分析如何将这些拦截器进行串联调用。

RealCall.java
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Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    ...
    //构建责任链
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, transmitter, null, 0,
        originalRequest, this, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    boolean calledNoMoreExchanges = false;
    try {
      //开始从头链式调用拦截器
      Response response = chain.proceed(originalRequest);
      if (transmitter.isCanceled()) {
        closeQuietly(response);
        throw new IOException("Canceled");
      }
      return response;
    } catch (IOException e) {
      calledNoMoreExchanges = true;
      throw transmitter.noMoreExchanges(e);
    } finally {
      if (!calledNoMoreExchanges) {
        transmitter.noMoreExchanges(null);
      }
    }
  }

实际执行链式调用的是RealInterceptorChain，由他负责责任链的执行

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public final class RealInterceptorChain implements Interceptor.Chain {
    @Override public Response proceed(Request request) throws IOException {
    return proceed(request, transmitter, exchange);
  }

  public Response proceed(Request request, Transmitter transmitter, @Nullable Exchange exchange)
      throws IOException {
    if (index >= interceptors.size()) throw new AssertionError();

    calls++;

    // 存在已经在使用的流，直接进行复用
    if (this.exchange != null && !this.exchange.connection().supportsUrl(request.url())) {
      throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
          + " must retain the same host and port");
    }

    // If we already have a stream, confirm that this is the only call to chain.proceed().
    if (this.exchange != null && calls > 1) {
      throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
          + " must call proceed() exactly once");
    }

    // 调用该链中的下一个拦截器 实质为 用户自定义的拦截器，不存在则为 RetryAndFollowUpInterceptor
    RealInterceptorChain next = new RealInterceptorChain(interceptors, transmitter, exchange,
        index + 1, request, call, connectTimeout, readTimeout, writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

    // Confirm that the next interceptor made its required call to chain.proceed().
    if (exchange != null && index + 1 < interceptors.size() && next.calls != 1) {
      throw new IllegalStateException("network interceptor " + interceptor
          + " must call proceed() exactly once");
    }

    // Confirm that the intercepted response isn't null.
    if (response == null) {
      throw new NullPointerException("interceptor " + interceptor + " returned null");
    }

    if (response.body() == null) {
      throw new IllegalStateException(
          "interceptor " + interceptor + " returned a response with no body");
    }

    return response;
  }
}

RealIntercrptor为链式调用的起点，调用proceed()之后，继续调用下一层的拦截器，直到得到最终的Response。

后续的拦截器也是按照这个规则向下执行，内部都会调用到chain.proceed()直到没有调用为止。

Request是按照定义的interceptor顺序向下执行，然后Response是逆向向上处理的。

获取请求结果Response

Response：返回HTTP请求响应结果，包含了状态码，响应正文等

在CallServerInterceptor得到最初格式的Response

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//获得状态行及响应头  
public @Nullable Response.Builder readResponseHeaders(boolean expectContinue) throws IOException {
    try {
      Response.Builder result = codec.readResponseHeaders(expectContinue);
      if (result != null) {
        Internal.instance.initExchange(result, this);
      }
      return result;
    } catch (IOException e) {
      eventListener.responseFailed(call, e);
      trackFailure(e);
      throw e;
    }
  }
//获得响应正文
public ResponseBody openResponseBody(Response response) throws IOException {
    try {
      eventListener.responseBodyStart(call);
      String contentType = response.header("Content-Type");
      long contentLength = codec.reportedContentLength(response);
      Source rawSource = codec.openResponseBodySource(response);
      ResponseBodySource source = new ResponseBodySource(rawSource, contentLength);
      return new RealResponseBody(contentType, contentLength, Okio.buffer(source));
    } catch (IOException e) {
      eventListener.responseFailed(call, e);
      trackFailure(e);
      throw e;
    }
  }

OkHttp执行流程

OkHttp缓存机制

Http缓存

OkHttp-HTTP缓存机制

Http缓存是web性能优化的重要手段，缓存机制是依赖于header中的参数实现的，这些参数指定了缓存需要从缓存中获取还是从服务端获取。

Http缓存有多种规则，根据是否需要重新向服务器发起请求来进行分类：

强制缓存

当客户端第一次请求数据时，服务端在响应头会携带缓存规则信息，主要为两个字段：Expires、Cache-Control

当再次请求数据时，如果符合缓存规则，则直接使用缓存数据，无需与服务端重新交互。

强制缓存流程

强制缓存在缓存未失效的情况下，可以直接使用缓存数据，接下来介绍判断缓存数据是否失效。
上文提到，强制缓存是根据两个Header字段进行判定的，这两个字段表示了失效规则。

Expires

服务端返回的到期时间，即下一次请求时，请求时间小于服务端返回的到期时间，直接使用缓存数据。
这参数是HTTP1.0的东西了，现在主流的是HTTP1.1。
可能由于客户端时间没有与服务端时间同步而导致缓存命中的误差。

Cache-Control

在HTTP1.1中替代Expires，功能与其一致。
Cache-Control常见的取值有如下几种：

  • private：客户端可以进行缓存
  • public：客户端以及代理服务器都可以进行缓存
  • max-age= XX：缓存数据在 XX秒后失效
  • no-cache：需要使用到对比缓存
  • no-store：所有内容都不进行缓存
  • s-maxage = XX：限定缓存可以在代理服务器中存放多久

对比缓存

需要进行比较判断来确定是否使用缓存，当客户端第一次请求数据时，服务端会返回缓存标识以及数据给客户端，客户端对两者都要进行备份到缓存，当再次请求数据时，客户端会带上缓存标识发送给服务端，服务端对标识进行判断，返回code值。返回若为304，则继续使用缓存。

对比缓存流程

在缓存标识未失效时，可以继续使用缓存数据，每次都需要与服务端进行交互去验证缓存标识。
对比缓存也是依据两个Header字段进行判定的，这两个字段表示了缓存标识。

Last-Modified/If-Modified-Since

Last-Modified：服务端返回给客户端，表示资源的最后修改时间。
If-Modified-Since：客户端发给服务端，表示服务端上次返回的资源最后修改时间。
服务端接收到If-Modified-Since后，与被请求资源的最后修改时间进行比对。

  • 若大于，返回最新资源并返回code为200，客户端需要重新进行缓存
  • 否则，说明资源无修改并返回code为304，客户端继续使用缓存数据
ETag/If-None-Match

ETag：服务端返回给客户端，表示当前资源在服务器的唯一标识。
If-None-Match：客户端发送给服务端，表示服务端上次返回的资源唯一标识。
服务端接收到If-None-Match后，与被请求资源的唯一标识进行比对

  • 标识不同，表示资源被改动过，返回最新资源及设置code为200，客户端需要重新进行缓存
  • 标识相同，表示资源未被改动，返回code为304，客户端继续使用缓存数据

其中ETag/If-None-Match的优先级是高于Last-Modified/If-Modified-Since的

总结

  • 强制缓存的优先级是高于对比缓存的
  • 对于强制缓存，服务端会给予一个过期时间，在有效期内再次请求都只会使用缓存，不会请求服务端。
  • 超过有效期就使用对比缓存策略，将服务端返回的ETag/Last-Modified发还给服务端进行验证，有效则继续使用缓存数据(返回code为304)，无效则重新获取并进行缓存(返回code为200)。

缓存存储

OkHttp缓存实现

介绍完毕Http的缓存机制后，接下来就是看OkHttp中的源码实现

CacheStrategy.java
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CacheStrategy(Request networkRequest, Response cacheResponse) {
  this.networkRequest = networkRequest;
  this.cacheResponse = cacheResponse;
}

缓存策略主要通过CacheStrategy类实现，关键参数为networkRequest(网络请求)、cacheResponse(缓存的响应结果)。
CacheStrategy通过工厂模式进行构建的，最终通过调用getCandidate()来生成不同模式。

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private CacheStrategy getCandidate() {
      // 没缓存直接进行重新请求
      if (cacheResponse == null) {
        return new CacheStrategy(request, null);
      }

      // 如果是HTTPs且握手信息丢失进行重新请求
      if (request.isHttps() && cacheResponse.handshake() == null) {
        return new CacheStrategy(request, null);
      }
      //判断缓存已经失效 重新进行请求
      if (!isCacheable(cacheResponse, request)) {
        return new CacheStrategy(request, null);
      }
      // 
      CacheControl requestCaching = request.cacheControl();
      if (requestCaching.noCache() || hasConditions(request)) {
        return new CacheStrategy(request, null);
      }

      CacheControl responseCaching = cacheResponse.cacheControl();
      //
      long ageMillis = cacheResponseAge();
      long freshMillis = computeFreshnessLifetime();

      if (requestCaching.maxAgeSeconds() != -1) {
        freshMillis = Math.min(freshMillis, SECONDS.toMillis(requestCaching.maxAgeSeconds()));
      }

      long minFreshMillis = 0;
      if (requestCaching.minFreshSeconds() != -1) {
        minFreshMillis = SECONDS.toMillis(requestCaching.minFreshSeconds());
      }

      long maxStaleMillis = 0;
      if (!responseCaching.mustRevalidate() && requestCaching.maxStaleSeconds() != -1) {
        maxStaleMillis = SECONDS.toMillis(requestCaching.maxStaleSeconds());
      }
      //处于强制缓存状态，直接返回缓存数据
      if (!responseCaching.noCache() && ageMillis + minFreshMillis < freshMillis + maxStaleMillis) {
        Response.Builder builder = cacheResponse.newBuilder();
        if (ageMillis + minFreshMillis >= freshMillis) {
          builder.addHeader("Warning", "110 HttpURLConnection \"Response is stale\"");
        }
        long oneDayMillis = 24 * 60 * 60 * 1000L;
        if (ageMillis > oneDayMillis && isFreshnessLifetimeHeuristic()) {
          builder.addHeader("Warning", "113 HttpURLConnection \"Heuristic expiration\"");
        }
        return new CacheStrategy(null, builder.build());
      }

      // Find a condition to add to the request. If the condition is satisfied, the response body
      // will not be transmitted.
      String conditionName;
      String conditionValue;
      if (etag != null) {
        conditionName = "If-None-Match";
        conditionValue = etag;
      } else if (lastModified != null) {
        conditionName = "If-Modified-Since";
        conditionValue = lastModifiedString;
      } else if (servedDate != null) {
        conditionName = "If-Modified-Since";
        conditionValue = servedDateString;
      } else {
        return new CacheStrategy(request, null); // No condition! Make a regular request.
      }
      //交由服务端去进行判断
      Headers.Builder conditionalRequestHeaders = request.headers().newBuilder();
      Internal.instance.addLenient(conditionalRequestHeaders, conditionName, conditionValue);

      Request conditionalRequest = request.newBuilder()
          .headers(conditionalRequestHeaders.build())
          .build();
      return new CacheStrategy(conditionalRequest, cacheResponse);
    }

CacheStrategy根据之前的缓存结果以及要发送的request的header计算缓存策略

networkRequest cacheResponse CacheStrategy
null null 不进行网络请求且缓存不存在或过期
返回504错误
null not null 不进行网络请求但是存在缓存且有效
直接返回缓存数据
not null null 进行网络请求且缓存不存在或过期
直接进行网络请求获取数据
not null not null 进行网络请求，请求头包含ETag/Last-Modified且缓存存在
根据网络请求结果判断
返回304，使用缓存
返回200，使用请求数据且更新缓存

OkHttp连接机制

在ConnectInterceptor中进行了与服务端的连接，通过Exchange类进行连接。

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Exchange newExchange(Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
  synchronized (connectionPool) {
    if (noMoreExchanges) throw new IllegalStateException("released");
    if (exchange != null) throw new IllegalStateException("exchange != null");
  }
  //建立连接
  ExchangeCodec codec = exchangeFinder.find(client, chain, doExtensiveHealthChecks);
  Exchange result = new Exchange(this, call, eventListener, exchangeFinder, codec);

  synchronized (connectionPool) {
    this.exchange = result;
    this.exchangeRequestDone = false;
    this.exchangeResponseDone = false;
    return result;
  }
}

通过Socket连接服务端

通过ExchangeCodec.find()来设置连接或者复用

ExchangeCodec.java
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public ExchangeCodec find(
      OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
    int connectTimeout = chain.connectTimeoutMillis();
    int readTimeout = chain.readTimeoutMillis();
    int writeTimeout = chain.writeTimeoutMillis();
    int pingIntervalMillis = client.pingIntervalMillis();
    boolean connectionRetryEnabled = client.retryOnConnectionFailure();

    try {
      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
          writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);
      return resultConnection.newCodec(client, chain);
    } catch (RouteException e) {
      trackFailure();
      throw e;
    } catch (IOException e) {
      trackFailure();
      throw new RouteException(e);
    }
  }
  //返回一个健康的连接 
  private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
      int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled,
      boolean doExtensiveHealthChecks) throws IOException {
    while (true) {
      RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
          pingIntervalMillis, connectionRetryEnabled);

      // If this is a brand new connection, we can skip the extensive health checks.
      synchronized (connectionPool) {
        if (candidate.successCount == 0) {
          return candidate;
        }
      }

      // Do a (potentially slow) check to confirm that the pooled connection is still good. If it
      // isn't, take it out of the pool and start again.
      if (!candidate.isHealthy(doExtensiveHealthChecks)) {
        candidate.noNewExchanges();
        continue;
      }

      return candidate;
    }
  }
  
  private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
      int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
    boolean foundPooledConnection = false;
    RealConnection result = null;
    Route selectedRoute = null;
    RealConnection releasedConnection;
    Socket toClose;
    synchronized (connectionPool) {
      if (transmitter.isCanceled()) throw new IOException("Canceled");
      hasStreamFailure = false; // This is a fresh attempt.
      
      Route previousRoute = retryCurrentRoute()
          ? transmitter.connection.route()
          : null;

      // Attempt to use an already-allocated connection. We need to be careful here because our
      // already-allocated connection may have been restricted from creating new exchanges.
      releasedConnection = transmitter.connection;
      toClose = transmitter.connection != null && transmitter.connection.noNewExchanges
          ? transmitter.releaseConnectionNoEvents()
          : null;

      if (transmitter.connection != null) {
        // We had an already-allocated connection and it's good.
        result = transmitter.connection;
        releasedConnection = null;
      }

      if (result == null) {
        // Attempt to get a connection from the pool.
        if (connectionPool.transmitterAcquirePooledConnection(address, transmitter, null, false)) {
          foundPooledConnection = true;
          result = transmitter.connection;
        } else {
          selectedRoute = previousRoute;
        }
      }
    }
    closeQuietly(toClose);

    if (releasedConnection != null) {
      eventListener.connectionReleased(call, releasedConnection);
    }
    if (foundPooledConnection) {
      eventListener.connectionAcquired(call, result);
    }
    if (result != null) {
      // If we found an already-allocated or pooled connection, we're done.
      return result;
    }

    // If we need a route selection, make one. This is a blocking operation.
    boolean newRouteSelection = false;
    if (selectedRoute == null && (routeSelection == null || !routeSelection.hasNext())) {
      newRouteSelection = true;
      routeSelection = routeSelector.next();
    }

    List<Route> routes = null;
    synchronized (connectionPool) {
      if (transmitter.isCanceled()) throw new IOException("Canceled");

      if (newRouteSelection) {
        // Now that we have a set of IP addresses, make another attempt at getting a connection from
        // the pool. This could match due to connection coalescing.
        routes = routeSelection.getAll();
        if (connectionPool.transmitterAcquirePooledConnection(
            address, transmitter, routes, false)) {
          foundPooledConnection = true;
          result = transmitter.connection;
        }
      }
      //没有从连接池中获取到连接需要重新建立
      if (!foundPooledConnection) {
        if (selectedRoute == null) {
          selectedRoute = routeSelection.next();
        }

        // Create a connection and assign it to this allocation immediately. This makes it possible
        // for an asynchronous cancel() to interrupt the handshake we're about to do.
        result = new RealConnection(connectionPool, selectedRoute);
        connectingConnection = result;
      }
    }

    // If we found a pooled connection on the 2nd time around, we're done.
    if (foundPooledConnection) {
      eventListener.connectionAcquired(call, result);
      return result;
    }

    // Do TCP + TLS handshakes. This is a blocking operation.
    // 开始TCP三次握手以及TLS操作，为阻塞操作
    result.connect(connectTimeout, readTimeout, writeTimeout, pingIntervalMillis,
        connectionRetryEnabled, call, eventListener);
    connectionPool.routeDatabase.connected(result.route());

    Socket socket = null;
    synchronized (connectionPool) {
      connectingConnection = null;
      // Last attempt at connection coalescing, which only occurs if we attempted multiple
      // concurrent connections to the same host.
      if (connectionPool.transmitterAcquirePooledConnection(address, transmitter, routes, true)) {
        // We lost the race! Close the connection we created and return the pooled connection.
        result.noNewExchanges = true;
        socket = result.socket();
        result = transmitter.connection;
      } else {
        //加入连接池 等待复用
        connectionPool.put(result);
        transmitter.acquireConnectionNoEvents(result);
      }
    }
    closeQuietly(socket);

    eventListener.connectionAcquired(call, result);
    return result;
  }

最终通过Socket.connect()进行连接。

连接池(ConnectionPool)

![OkHttp-连接池 ConnectionPool](/images/OkHttp-连接池 ConnectionPool.png)

频繁的进行Socket连接(三次握手)和Socket断开(四次挥手)非常消耗网络资源以及时间。在HTTP1.1之后提供了keep-alive这个header，可以实现长连接，有效的降低了延迟并提升了处理速度。
连接池就是为了复用已存在连接，可以有效降低创建连接的开销。

连接池构造方法以及成员变量

ConnectionPool.java
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public final class ConnectionPool {
  //后台清理线程
   private static final Executor executor = new ThreadPoolExecutor(0 /* corePoolSize */,
      Integer.MAX_VALUE /* maximumPoolSize */, 60L /* keepAliveTime */, TimeUnit.SECONDS,
      new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp ConnectionPool", true)); 
    //最大的空闲连接数
    private final int maxIdleConnections;
    //连接最大持续时间
    private final long keepAliveDurationNs;
    //存储连接的双向队列
    private final Deque<RealConnection> connections = new ArrayDeque<>();
  
    public ConnectionPool() {
      this(5, 5, TimeUnit.MINUTES);
    }

  public ConnectionPool(int maxIdleConnections, long keepAliveDuration, TimeUnit timeUnit) {
    this.maxIdleConnections = maxIdleConnections;
    this.keepAliveDurationNs = timeUnit.toNanos(keepAliveDuration);

    // Put a floor on the keep alive duration, otherwise cleanup will spin loop.
    if (keepAliveDuration <= 0) {
      throw new IllegalArgumentException("keepAliveDuration <= 0: " + keepAliveDuration);
    }
  }
}

ConnectionPool支持配置以下变量

  • maxIdleConnections：最大空闲连接数，默认5
  • keepAliveDurationNs：最大连接保持时间，默认5min

Connection连接池中的Connection任一超出以上配置，就需要执行清理。

可以通过以下方法配置连接池

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OkHttpClient.Builder builder = new OkHttpClient.Builder()
        .connectionPool(new ConnectionPool()); //配置连接池

连接池加入连接

双端队列

通过connections存储Connection

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void put(RealConnection connection) {
  assert (Thread.holdsLock(this));
  if (!cleanupRunning) {//当前清理线程没有运行
    cleanupRunning = true;
    executor.execute(cleanupRunnable);//开启清理过程
  }
  connections.add(connection);//加入队列
}

在外部执行put()时，连接加入连接池，并且开启清理线程去清理那些超出配置的连接。

外部执行put()路径如下

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// ConnectInterceptor 连接拦截器，其中执行连接过程
public final class ConnectInterceptor implements Interceptor {
    @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();

    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);//建立新连接
    RealConnection connection = streamAllocation.connection();

    return realChain.proceed(request, streamAllocation, httpCodec, connection);
}
  
// StreamAllocation 
   public HttpCodec newStream(
      OkHttpClient client, Interceptor.Chain chain, boolean doExtensiveHealthChecks) {
       ...

    try {
      RealConnection resultConnection = findHealthyConnection(connectTimeout, readTimeout,
          writeTimeout, pingIntervalMillis, connectionRetryEnabled, doExtensiveHealthChecks);//寻找可用的连接
      HttpCodec resultCodec = resultConnection.newCodec(client, chain, this);

      synchronized (connectionPool) {
        codec = resultCodec;
        return resultCodec;
      }
    } catch (IOException e) {
      throw new RouteException(e);
    }
  } 
  
  private RealConnection findHealthyConnection(int connectTimeout, int readTimeout,
      int writeTimeout, int pingIntervalMillis, boolean connectionRetryEnabled,
      boolean doExtensiveHealthChecks) throws IOException {
    while (true) {
      RealConnection candidate = findConnection(connectTimeout, readTimeout, writeTimeout,
          pingIntervalMillis, connectionRetryEnabled);
      ...

      return candidate;
    }
  }
  
   private RealConnection findConnection(int connectTimeout, int readTimeout, int writeTimeout,
      int pingIntervalMillis, boolean connectionRetryEnabled) throws IOException {
    ...
     synchronized (connectionPool) {
      if (released) throw new IllegalStateException("released");
      if (codec != null) throw new IllegalStateException("codec != null");
      if (canceled) throw new IOException("Canceled");

      // Attempt to use an already-allocated connection. We need to be careful here because our
      // already-allocated connection may have been restricted from creating new streams.
      releasedConnection = this.connection;
      toClose = releaseIfNoNewStreams();
      ...
      if (result == null) {
        // Attempt to get a connection from the pool.
        Internal.instance.get(connectionPool, address, this, null);//根据address从连接池中获取对应连接
        if (connection != null) {
          foundPooledConnection = true;
          result = connection;
        } else {
          selectedRoute = route;
        }
      }
    }
     ...
    if (result != null) {
      // If we found an already-allocated or pooled connection, we're done.
      return result;
    }  
     
     ...
       
    synchronized (connectionPool) {
      if (canceled) throw new IOException("Canceled");

      if (newRouteSelection) {
        // Now that we have a set of IP addresses, make another attempt at getting a connection from
        // the pool. This could match due to connection coalescing.
        List<Route> routes = routeSelection.getAll();
        for (int i = 0, size = routes.size(); i < size; i++) {
          Route route = routes.get(i);
          Internal.instance.get(connectionPool, address, this, route);
          if (connection != null) {
            foundPooledConnection = true;
            result = connection;
            this.route = route;
            break;
          }
        }
      }

      if (!foundPooledConnection) {
        if (selectedRoute == null) {
          selectedRoute = routeSelection.next();
        }

        // Create a connection and assign it to this allocation immediately. This makes it possible
        // for an asynchronous cancel() to interrupt the handshake we're about to do.
        route = selectedRoute;
        refusedStreamCount = 0;
        result = new RealConnection(connectionPool, selectedRoute);//连接池未找到对应连接，建立新连接
        acquire(result, false);
      }
    }
     
     ...
    
    synchronized (connectionPool) {
      reportedAcquired = true;

      // Pool the connection.
      Internal.instance.put(connectionPool, result);//将新建的连接加入到连接池内

      // If another multiplexed connection to the same address was created concurrently, then
      // release this connection and acquire that one.
      if (result.isMultiplexed()) {
        socket = Internal.instance.deduplicate(connectionPool, address, this);
        result = connection;
      }
    }
     
   }
  
  //OkHttpClient
  public class OkHttpClient implements Cloneable, Call.Factory, WebSocket.Factory {
    static {
      Internal.instance = new Internal() {
       ...
        @Override public RealConnection get(ConnectionPool pool, Address address,
          StreamAllocation streamAllocation, Route route) {
           return pool.get(address, streamAllocation, route);//从连接池获取连接
        } 
         
        @Override public void put(ConnectionPool pool, RealConnection connection) {
          pool.put(connection);//向连接池添加连接
        }
       ...
      }
  }

通过ConnectInterceptor.intercept()去建立连接，向下调用到StreamAllocation.newStream()

在newStream()中继续执行到findConnection()，其中主要执行了两步

  1. 从ConnectionPool寻找是否存有当前address对应的连接，调用ConnectionPool.get(XXX)，存在就返回对应连接。
  2. 不存在对应连接，执行new RealConnection()新建连接，并调用ConnectionPool.put()存储新连接

对应的get()、put()都是通过Internal.instance调用的，其中Internal是一个抽象类，具体实现类对应的就是OkHttpClient。

连接池清理连接

在使用连接池时，初始化了一个Executor线程池，这个主要就是为了在清理无效连接时去开启清理线程用的

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  /**
   * Background threads are used to cleanup expired connections. There will be at most a single
   * thread running per connection pool. The thread pool executor permits the pool itself to be
   * garbage collected.
   * 清理过期的连接，且保证最多只能运行一个清理线程。
   */
  private static final Executor executor = new ThreadPoolExecutor(0 /* corePoolSize */,
      Integer.MAX_VALUE /* maximumPoolSize */, 60L /* keepAliveTime */, TimeUnit.SECONDS,
      new SynchronousQueue<Runnable>(), Util.threadFactory("OkHttp ConnectionPool", true));

//清理过期连接任务
  private final Runnable cleanupRunnable = new Runnable() {
    @Override public void run() {
      while (true) {
        long waitNanos = cleanup(System.nanoTime());//返回下次需要清理连接的时间
        if (waitNanos == -1) return;
        if (waitNanos > 0) {
          long waitMillis = waitNanos / 1000000L;
          waitNanos -= (waitMillis * 1000000L);
          synchronized (ConnectionPool.this) {
            try {
              ConnectionPool.this.wait(waitMillis, (int) waitNanos);//阻塞等待
            } catch (InterruptedException ignored) {
            }
          }
        }
      }
    }
  };
cleanup()

内部主要执行的是标记空闲连接、清理空闲连接，返回下次清理时间这几步

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long cleanup(long now) {
  int inUseConnectionCount = 0;
  int idleConnectionCount = 0;
  RealConnection longestIdleConnection = null;
  long longestIdleDurationNs = Long.MIN_VALUE;

  // Find either a connection to evict, or the time that the next eviction is due.
  synchronized (this) {
    for (Iterator<RealConnection> i = connections.iterator(); i.hasNext(); ) {//遍历连接储存队列
      RealConnection connection = i.next();

      // If the connection is in use, keep searching.
      if (pruneAndGetAllocationCount(connection, now) > 0) {//标记正在使用的活跃连接
        inUseConnectionCount++;
        continue;
      }

      idleConnectionCount++;//非活跃标记为空闲连接

      // If the connection is ready to be evicted, we're done.
      long idleDurationNs = now - connection.idleAtNanos;
      if (idleDurationNs > longestIdleDurationNs) {
        longestIdleDurationNs = idleDurationNs;
        longestIdleConnection = connection;//得到最长空闲时间的连接
      }
    }

    if (longestIdleDurationNs >= this.keepAliveDurationNs
        || idleConnectionCount > this.maxIdleConnections) {
      // We've found a connection to evict. Remove it from the list, then close it below (outside
      // of the synchronized block).
      // 空闲连接超过`maxIdleConnections`个或者空闲时间超过`keepAliveDurationNs`，需要清理该连接
      connections.remove(longestIdleConnection);
    } else if (idleConnectionCount > 0) {
      // A connection will be ready to evict soon.
      // 返回最大空闲连接的到期时间，等待到达时间后进行清理
      return keepAliveDurationNs - longestIdleDurationNs;
    } else if (inUseConnectionCount > 0) {
      // All connections are in use. It'll be at least the keep alive duration 'til we run again.
      // 所有都是活跃连接，返回最大空闲连接时间，等待到达时间后清理
      return keepAliveDurationNs;
    } else {
      // No connections, idle or in use.
      // 当前不存在连接，直接返回 -1，不进行清理任务
      cleanupRunning = false;
      return -1;
    }
  }
// 立即关闭过期连接
  closeQuietly(longestIdleConnection.socket());

  // Cleanup again immediately.
  return 0;
}

cleanup()执行流程如下：

  • 遍历ConnectionPool的connections，通过pruneAndGetAllocationCount()判断connection是否空闲
  • 遍历完毕后，找到最长时间的空闲连接(longestIdleConnection)
  • 得到longestIdleConnection后，先是比较当前的idleConnectionCount是否大于maxIdleConnections或者longestIdleDurarionNs是否大于keepAliveDurationNs，两者满足其一，则清理掉longestIdleConnection
  • 不满足其上条件，继续判断idleConnectionCount > 0 ，表示当前存在空闲连接，就返回距离最大空闲连接时间差keepAliveDurationNs - longestIdleDurationNs，等待到时清理
  • 不满足其上条件，继续判断inUseConnectionCount > 0，表示当前都是活跃连接，返回keepAliveDurationNs，等待达到时间清理
  • 以上条件都不满足，表示当前没有连接，直接返回-1
  • 存在longestIdleConnection，即调用longestIdleConnection.socket().close()关闭连接即可
pruneAndGetAllocationCount(Connection)

判断当前连接是否正在活跃，采用了引用计数法

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private int pruneAndGetAllocationCount(RealConnection connection, long now) {
  //连接弱引用列表
  List<Reference<StreamAllocation>> references = connection.allocations;
  for (int i = 0; i < references.size(); ) {
    //获取引用连接
    Reference<StreamAllocation> reference = references.get(i);
    //不为null ，表示当前连接尚未回收
    if (reference.get() != null) {
      i++;
      continue;
    }

    // We've discovered a leaked allocation. This is an application bug.
    StreamAllocation.StreamAllocationReference streamAllocRef =
        (StreamAllocation.StreamAllocationReference) reference;
    String message = "A connection to " + connection.route().address().url()
        + " was leaked. Did you forget to close a response body?";
    Platform.get().logCloseableLeak(message, streamAllocRef.callStackTrace);

    //移除引用
    references.remove(i);
    connection.noNewStreams = true;

    // If this was the last allocation, the connection is eligible for immediate eviction.
    //所有引用都被移除，表示当前连接处于空闲
    if (references.isEmpty()) {
      connection.idleAtNanos = now - keepAliveDurationNs;
      return 0;
    }
  }

  return references.size();
}

StreamAllocation引用是在StreamAllocation.acquire()时加入的

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public void acquire(RealConnection connection, boolean reportedAcquired) {
  assert (Thread.holdsLock(connectionPool));
  if (this.connection != null) throw new IllegalStateException();

  this.connection = connection;
  this.reportedAcquired = reportedAcquired;
  connection.allocations.add(new StreamAllocationReference(this, callStackTrace));
}

通过这种引用计数法来判断当前是否为空闲连接

OkHttp-Cookie机制

OkHttp-Cookie机制

使用方法

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OkHttpClient.Builder()
  .//其他参数
  .cookieJar(object : CookieJar {
     override fun saveFromResponse(url:HttpUrl,cookies:MutableList<Cookie>){
       //从Response 获取Cookie信息 并存储到本地
     }
    
     override fun loadForRequest(url:HttpUrl) : MutableList<Cookie> {
       //根据url获取存储本地的Cookie信息 放到 Request请求中
     }
  })

Cookie一般都会在请求中进行使用，多用于请求头的Cookie字段，大致可以猜测相关的处理位于BridgeInterceptor中。

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//BridgeInterceptor.java
public final class BridgeInterceptor implements Interceptor{
  //这个也就是在 构建Client时放入的CookieJar对象
  private final CookieJar cookieJar;
  
    @Override public Response intercept(Chain chain) throws IOException {
  ...
    //从配置的cookiejar中获取信息
     List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
     if (!cookies.isEmpty()) {
       //在请求头中添加 Cookie请求头，携带Cookie参数
       requestBuilder.header("Cookie", cookieHeader(cookies));
     }
  ...   
    //请求最后得到的 response结果
    Response networkResponse = chain.proceed(requestBuilder.build());
    //从rensponse中解析Cookie并缓存到 CookieJar中
    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
      
    }
}

//HttpHeaders.java
  public static void receiveHeaders(CookieJar cookieJar, HttpUrl url, Headers headers) {
    if (cookieJar == CookieJar.NO_COOKIES) return;

    List<Cookie> cookies = Cookie.parseAll(url, headers);
    if (cookies.isEmpty()) return;
    //获取到Cookie对象 缓存中 CookieJar中
    cookieJar.saveFromResponse(url, cookies);
  }

//Cookie.java
  public static List<Cookie> parseAll(HttpUrl url, Headers headers) {
    //从响应头中读取到 Set-Cookie字段，并转换成 Cookie对象
    List<String> cookieStrings = headers.values("Set-Cookie");
    List<Cookie> cookies = null;

    for (int i = 0, size = cookieStrings.size(); i < size; i++) {
      Cookie cookie = Cookie.parse(url, cookieStrings.get(i));
      if (cookie == null) continue;
      if (cookies == null) cookies = new ArrayList<>();
      cookies.add(cookie);
    }

    return cookies != null
        ? Collections.unmodifiableList(cookies)
        : Collections.<Cookie>emptyList();
  }

Cookie原理简述：

服务端返回的Response通过Set-Cookie响应头返回对应的Cookie信息，以便在下次进行请求时在Request的请求头中添加Cookie字段以携带Cookie信息。

OkHttp通过setCookieJar对所有的Cookie文件进行管理。

  • saveFromResponse()：可以获取对应域名的Cookie信息
  • loadForRequest()：可以在请求域名时添加Cookie信息

可以通过一个全局的CookieJar类来实现应用内Cookie文件的管理。

OkHttp-DNS功能

DNS介绍

Domain Name System：根据域名查出IP地址，是HTTP协议的前提，只有将域名正确的进行解析，得到IP地址后，才可以继续进行网络连接。

DNS服务器结构如下：

  • 根DNS服务器：返回顶级DNS服务器的IP地址
  • 顶级域DNS服务器：返回权威DNS服务器的IP地址
  • 权威DNS服务器：返回对应主机的IP地址

img

LocalDNS

OkHttp-LocalDNS

运营商提供的DNS服务器，请求时优先查询LocalDNS 缓存，存在直接使用。不存在就需要从根域名服务器 -> 顶级域名服务器 -> 权威域名服务器往上查询可用的IP地址。

img

缺陷

  1. 不稳定

    DNS劫持或者服务器故障，导致解析服务不可用

  2. 不准确

    LocalDNS调度不一定是就近原则。某些运营商会把解析请求转发到其他运营商的LocalDNS服务器。

    就会导致解析出的IP不是就近服务器，致使访问变慢甚至无法访问。

  3. 不及时

    运营商可能修改DNS的TTL(Time-To-Live，DNS缓存时间)，导致DNS解析结构发生修改，但是在当前请求条件下尚未生效。

img

HttpDNS

OkHttp-HTTPDNS

HTTPDNS利用HTTP协议与DNS服务交互，绕开了运营商LocalDNS服务，有效防止了域名劫持以及提高了域名解析成功率。

原理

img

  1. 客户端直接访问HttpDNS接口，获取域名在HTTPDNS服务器上的最优IP(从容灾方面考虑，还需要保留LocalDNS请求)
  2. 客户端获取到IP后，直接向该IP发起HTTP请求
优势
  1. 降低了UnknownHostException异常发生
  2. 调度精准，根据用户IP，精准获取域名对应IP
  3. 扩展性强，可以自定义域名对应IP规则

OkHttp-HttpDNS实现

OkHttp提供了Dns接口，可以进行自定义拓展替代本身的LocalDNS解析方式

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//自定义DNS
class OptimizeLocalDNS : Dns{
    override fun lookup(hostname: String): List<InetAddress> {
        return DNSLookUpUtil.loadLocalDNS(hostname)
    }
}

//设置LocalDNS超时取消
        fun loadLocalDNS(hostname: String, timeout: Long = 10L): List<InetAddress> {
            try {
                val task = FutureTask<List<InetAddress>>(Callable<List<InetAddress>> {
                    //返回去重结果
                    InetAddress.getAllByName(hostname).toList().distinct()
                })
                Thread(task).start()
                return task.get(timeout, TimeUnit.SECONDS)
            } catch (e: Exception) {
            }
            return listOf()
        }

//设置自定义DNS
mOkHttpClient = httpBuilder
        .dns(OptimizeLocalDNS())
        .build();

OkHttp-DNS原理

配置的dns()初始使用位于RetryAndFollowUpInterceptor.intercept()中

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public final class RetryAndFollowUpInterceptor implements Interceptor {
  ...
   @Override public Response intercept(Chain chain) throws IOException {
    ...
        StreamAllocation streamAllocation = new StreamAllocation(client.connectionPool(),
        createAddress(request.url()), call, eventListener, callStackTrace);
   }
  
   private Address createAddress(HttpUrl url) {
    ...
    return new Address(url.host(), url.port(), client.dns(), client.socketFactory(),
        sslSocketFactory, hostnameVerifier, certificatePinner, client.proxyAuthenticator(),
        client.proxy(), client.protocols(), client.connectionSpecs(), client.proxySelector());
  }
 
  ...
}

构造出一个Address对象，里面包含了主机名(host)、端口(port)、DNS配置(DNS)、SSL配置(sslSocketFactory,certificatePinner)、代理设置

得到Address，通过StreamAllocation构造了RouteSelector对象

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public StreamAllocation(ConnectionPool connectionPool, Address address, Call call,
    EventListener eventListener, Object callStackTrace) {
  this.connectionPool = connectionPool;
  this.address = address;
  this.call = call;
  this.eventListener = eventListener;
  this.routeSelector = new RouteSelector(address, routeDatabase(), call, eventListener);
  this.callStackTrace = callStackTrace;
}

RouteSelector主要为了Select Route(选择路由)，返回一个可用的Route对象。

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private void resetNextInetSocketAddress(Proxy proxy) throws IOException {
    // Clear the addresses. Necessary if getAllByName() below throws!
    inetSocketAddresses = new ArrayList<>();

    String socketHost;
    int socketPort;
    if (proxy.type() == Proxy.Type.DIRECT || proxy.type() == Proxy.Type.SOCKS) {//存在代理
      socketHost = address.url().host();
      socketPort = address.url().port();
    } else {
      SocketAddress proxyAddress = proxy.address();
      if (!(proxyAddress instanceof InetSocketAddress)) {
        throw new IllegalArgumentException(
            "Proxy.address() is not an " + "InetSocketAddress: " + proxyAddress.getClass());
      }
      InetSocketAddress proxySocketAddress = (InetSocketAddress) proxyAddress;
      socketHost = getHostString(proxySocketAddress);
      socketPort = proxySocketAddress.getPort();
    }

    if (socketPort < 1 || socketPort > 65535) {
      throw new SocketException("No route to " + socketHost + ":" + socketPort
          + "; port is out of range");
    }

    if (proxy.type() == Proxy.Type.SOCKS) {//解析的直接为代理地址
      inetSocketAddresses.add(InetSocketAddress.createUnresolved(socketHost, socketPort));
    } else {
      //dns开始解析监听
      eventListener.dnsStart(call, socketHost);

      // Try each address for best behavior in mixed IPv4/IPv6 environments.
      List<InetAddress> addresses = address.dns().lookup(socketHost);//通过配置的DNS去解析对应域名的IP列表
      if (addresses.isEmpty()) {
        throw new UnknownHostException(address.dns() + " returned no addresses for " + socketHost);
      }
      //dns解析结束监听
      eventListener.dnsEnd(call, socketHost, addresses);

      for (int i = 0, size = addresses.size(); i < size; i++) {
        InetAddress inetAddress = addresses.get(i);
        inetSocketAddresses.add(new InetSocketAddress(inetAddress, socketPort));
      }
    }
  }

resetNextInetSocketAddress()返回List<InetSocketAddress>，区分了一下两种情况

  • 设置了proxies代理服务器，直接返回InetSocketAddress(socketHost,socketPort)代理服务器对应的地址和端口
  • 未设置代理服务器，通过设置的dns去解析对应域名(dns.lookup(host))得到List<InetAddress>对应的IP列表，在返回对应地址

OkHttp-HTTP2.0协议支持

OkHttp-HTTP2.0

基于二进制分帧、首部压缩和服务端推送进行分析

//TODO

OkHttp拓展

请求时间获取

img

EventListener是OkHttp提供的监听回调，可以通过实现这个抽象类监听到网络请求各阶段的时间点

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public abstract class EventListener {
   //请求相关回调
   public void callStart(Call call) {} 
   public void callEnd(Call call) {}
   public void callFailed(Call call, IOException ioe) {}  
  
   //dns解析回调
   public void dnsStart(Call call, String domainName) {}
   public void dnsEnd(Call call, String domainName, List<InetAddress> inetAddressList) {}
  
   //请求连接相关回调
   public void connectStart(Call call, InetSocketAddress inetSocketAddress, Proxy proxy){}
   public void connectEnd(Call call, InetSocketAddress inetSocketAddress, Proxy proxy,@Nullable Protocol protocol) {}
   public void connectFailed(Call call, InetSocketAddress inetSocketAddress, Proxy proxy,
      @Nullable Protocol protocol, IOException ioe) {}
   public void connectionAcquired(Call call, Connection connection){}
   public void connectionReleased(Call call, Connection connection){}
  
   ...
}

DNS解析耗时

只要监听dnsStart()和dnsEnd()之间的时间差即可

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private long dnsStartTime;

private long dnsDuration = -1L;    

@Override
public void dnsStart(Call call, String domainName) {
    super.dnsStart(call, domainName);
    recordEventLog("dnsStart");
    dnsStartTime = System.nanoTime();
}

@Override
public void dnsEnd(Call call, String domainName, List<InetAddress> inetAddressList) {
    super.dnsEnd(call, domainName, inetAddressList);
    recordEventLog("dnsEnd");
    dnsDuration = (System.nanoTime() - dnsStartTime) / 1000000;
}

public long getDnsDuration() {
    return dnsDuration;
}

dnsDuration即为DNS解析耗时

请求连接耗时

初始连接耗时

使用Socket建立TCP连接，初始连接表示的就是Socket建立连接的过程

只要监听connectStart()和connectEnd()之间的时间差。

复用连接耗时

OkHttp设置ConnectionPool，可以复用已存在的连接

需要监听connectAcquired()和connectReleased()之间的时间差

IP直连问题

//TODO

内容引用

开源框架源码鉴赏：OkHttp
彻底弄懂HTTP缓存机制及原理

百度App网络深度优化系列《一》DNS优化

HTTP2.0相关

HPACK算法

HTTP/2首部压缩的OkHttp3实现

#源码解析
OkHttp简析
https://leo-wxy.github.io/2019/03/08/OkHttp简析/
作者
Leo-Wxy
发布于
2019年3月8日
许可协议