Android-Art类加载过程

Dex文件加载

加载Dex文件后会生成DexFile对象，里面储存了多个类文件信息。

通过PathClassLoader或者DexClassLoader去加载Dex文件，最后还是调用到BaseDexClassLoader的加载方法

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class BaseDexClassLoader extends ClassLaoder{
      public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String librarySearchPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, librarySearchPath, null);
2222...
    }
}

  • DexPathList

    DexPathList.java
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    final class DexPathList{
      private Element[] dexElements;
      private final NativeLibraryElement[] nativeLibraryPathElements;
      
      ...
          public DexPathList(ClassLoader definingContext, String dexPath,
                String librarySearchPath, File optimizedDirectory)         {
       ...
       // 记录所有的dexFile
       this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,                       suppressedExceptions, definingContext);
        
       //记录app目录的Native库
       this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
       //记录系统使用的Native库
       this.systemNativeLibraryDirectories =                splitPaths(System.getProperty("java.library.path"), true);
       //记录所有使用的Native库
       this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories);
      }
    }

    为了初始化以下两个字段：

    • dexElements：记录所有的DexFile，按照;进行路径分割
    • nativeLibraryPathElements：记录所有的Native代码库，包括app和系统使用的Native库

  • makeDexElements

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    private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
            List<IOException> suppressedExceptions, ClassLoader loader) {
      Element[] elements = new Element[files.size()];
     for (File file : files) {
      ...
        //以 dex 文件名结尾
        if (name.endsWith(DEX_SUFFIX)) {
          try {
            //加载Dex文件
             DexFile dex = loadDexFile(file, optimizedDirectory, loader, elements);
             if (dex != null) {
                 elements[elementsPos++] = new Element(dex, null);
             }
             } catch (IOException suppressed) {
      
             }
        }else{
          dex = loadDexFile(file, optimizedDirectory, loader, elements);              
              if (dex == null) {
                  elements[elementsPos++] = new Element(file);
              } else {
                  elements[elementsPos++] = new Element(dex, file);
              }
        }
     }
     ... 
    }

    根据传入的Dex文件路径转换Element数组

  • loadDexFile

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    private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader,
                                       Element[] elements)
            throws IOException     {
       //优化后Dex文件 存放地址是否为空
        if (optimizedDirectory == null) {
            //为空创建DexFile对象
            return new DexFile(file, loader, elements);
        } else {
            String optimizedPath = optimizedPathFor(file, optimizedDirectory);
          //不为空将优化后的Dex文件存放到指定目录
            return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements);
        }
    }

    此处为了加载Dex文件

  • DexFile

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    public final class DexFile {
         DexFile(File file, ClassLoader loader, DexPathList.Element[] elements)
                throws IOException {
            this(file.getPath(), loader, elements);
        }
      
      ...
            DexFile(String fileName, ClassLoader loader, DexPathList.Element[] elements) throws IOException {
            mCookie = openDexFile(fileName, null, 0, loader, elements);
            mInternalCookie = mCookie;
            mFileName = fileName;
        }
      ...
            static DexFile loadDex(String sourcePathName, String outputPathName,
            int flags, ClassLoader loader, DexPathList.Element[] elements)     throws IOException     {
            return new DexFile(sourcePathName, outputPathName, flags, loader, elements);
        }
      ...
    }

    loadDex本质也是调用了new DexFile(...)去加载Dex文件的。

  • openDexFile

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    private static Object openDexFile(String sourceName, String outputName, int flags,
            ClassLoader loader, DexPathList.Element[] elements)     throws IOException     {
        // Use absolute paths to enable the use of relative paths when testing on host.
        //加载Dex文件
        return openDexFileNative(new File(sourceName).getAbsolutePath(),
                                 (outputName == null)
                                     ? null
                                     : new File(outputName).getAbsolutePath(),
                                 flags,
                                 loader,
                                 elements);
    }

    openDexFile为了加载Dex文件

  • openDexFileNative

    dalvik_system_DexFile.cc
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    // art/runtime/native/dalvik_system_DexFile.cc
    static jobject DexFile_openDexFileNative(JNIEnv* env,
                                             jclass,
                                             jstring javaSourceName,
                                             jstring javaOutputName ATTRIBUTE_UNUSED,
                                             jint flags ATTRIBUTE_UNUSED,
                                             jobject class_loader,
                                             jobjectArray dex_elements)         {
      ScopedUtfChars sourceName(env, javaSourceName);
      if (sourceName.c_str() == nullptr) {
        return 0;
      }

      Runtime* const runtime = Runtime::Current();
      ClassLinker* linker = runtime->GetClassLinker();
      std::vector<std::unique_ptr<const DexFile>> dex_files;
      std::vector<std::string> error_msgs;
      const OatFile* oat_file = nullptr;

      dex_files = runtime->GetOatFileManager().OpenDexFilesFromOat(sourceName.c_str(),
                                                                   class_loader,
                                                                   dex_elements,
                                                                   /*out*/ &oat_file,
                                                                   /*out*/ &error_msgs);

    ...

        return nullptr;
      }
    }

    openDexFileNative主要处理dex文件，并生成odex到optimizedDirectory里

  • openDexFilesFromOat

    //TODO 版本差异较大

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    //art/runtime/oat_file_manager.cc

    std::vector<std::unique_ptr<const DexFile>> OatFileManager::OpenDexFilesFromOat(
        const char* dex_location,
        const char* oat_location,
        jobject class_loader,
        jobjectArray dex_elements,
        const OatFile** out_oat_file,
        std::vector<std::string>* error_msgs)         {
      
    }

    oat_file_manager.cc
      
    oat_file_assistant.cc
Dex加载过程

Dex中的类文件加载

Dex文件是由多个Class类文件组成，Android加载类需要从Dex中找到对应类进行加载，实际从DexFile找到Class

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public class BaseDexClassLoader extends ClassLoader {
 ...
   private final DexPathList pathList;
   
     @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        // First, check whether the class is present in our shared libraries.
        //加载需要使用的Native库
        if (sharedLibraryLoaders != null) {
            for (ClassLoader loader : sharedLibraryLoaders) {
                try {
                    return loader.loadClass(name);
                } catch (ClassNotFoundException ignored) {
                }
            }
        }
        // Check whether the class in question is present in the dexPath that
        // this classloader operates on.
        List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
        // 找寻Dex中对应class
        Class c = pathList.findClass(name, suppressedExceptions);
        if (c == null) {
            ClassNotFoundException cnfe = new ClassNotFoundException(
                    "Didn't find class \"" + name + "\" on path: " + pathList);
            for (Throwable t : suppressedExceptions) {
                cnfe.addSuppressed(t);
            }
            throw cnfe;
        }
        return c;
    }

}

  • DexPathList - findClass

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    public Class<?> findClass(String name, List<Throwable> suppressed) {
        for (Element element : dexElements) {
            Class<?> clazz = element.findClass(name, definingContext, suppressed);
            if (clazz != null) {
                return clazz;
            }
        }
      
        if (dexElementsSuppressedExceptions != null) {
            suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
        }
        return null;
    }

    findClass为了找到Dex文件中的对应类

    dexElements是由Dex文件加载后得到的DexFile组装成的Element集合形成的。

    findClass实质是去遍历已加载完成的Dex文件中的Class，只要找到对应的Class就会结束循环。

    当两个相同的类出现在不同的Dex时，系统会优先处理排在前面的Dex文件中的类，后面出现的就不会被加载。

    热修复的核心逻辑：将需要修复的类所打包的Dex文件插入到dexElements的首位。

  • Element - findClass

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    public Class<?> findClass(String name, ClassLoader definingContext,
             List<Throwable> suppressed) {
         return dexFile != null ? dexFile.loadClassBinaryName(name, definingContext, suppressed)
                 : null;
     }

  • DexFile - loadClassBinaryName

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    public Class loadClassBinaryName(String name, ClassLoader loader, List<Throwable> suppressed) {
        return defineClass(name, loader, mCookie, this, suppressed);
    }
      
    private static Class defineClass(String name, ClassLoader loader, Object cookie,
                                     DexFile dexFile, List<Throwable> suppressed)         {
        Class result = null;
        try {
            //定义Class
            result = defineClassNative(name, loader, cookie, dexFile);
        } catch (NoClassDefFoundError e) {
            if (suppressed != null) {
                suppressed.add(e);
            }
        } catch (ClassNotFoundException e) {
            if (suppressed != null) {
                suppressed.add(e);
            }
        }
        return result;
    }

  • Dalvik_system_DexFile.cc - DexFile_defineClassNative

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    static jclass DexFile_defineClassNative(JNIEnv* env,
                                            jclass,
                                            jstring javaName,
                                            jobject javaLoader,
                                            jobject cookie,
                                            jobject dexFile)         {
      std::vector<const DexFile*> dex_files;
      const OatFile* oat_file;
      if (!ConvertJavaArrayToDexFiles(env, cookie, /*out*/ dex_files, /*out*/ oat_file)) {
        VLOG(class_linker) << "Failed to find dex_file";
        DCHECK(env->ExceptionCheck());
        return nullptr;
      }

      ScopedUtfChars class_name(env, javaName);
      if (class_name.c_str() == nullptr) {
        VLOG(class_linker) << "Failed to find class_name";
        return nullptr;
      }
      const std::string descriptor(DotToDescriptor(class_name.c_str()));
      const size_t hash(ComputeModifiedUtf8Hash(descriptor.c_str()));
      for (auto& dex_file : dex_files) {
        const DexFile::ClassDef* dex_class_def =
            OatDexFile::FindClassDef(*dex_file, descriptor.c_str(), hash);
        if (dex_class_def != nullptr) {
          ScopedObjectAccess soa(env);
          ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
    222...
            return nullptr;
          }
          //创建目标类对象
          ObjPtr<mirror::Class> result = class_linker->DefineClass(soa.Self(),
                                                                   descriptor.c_str(),
                                                                   hash,
                                                                   class_loader,
                                                                   *dex_file,
                                                                   *dex_class_def);
          // Add the used dex file. This only required for the DexFile.loadClass API since normal
          // class loaders already keep their dex files live.
          class_linker->InsertDexFileInToClassLoader(soa.Decode<mirror::Object>(dexFile),
                                                     class_loader.Get());
          if (result != nullptr) {
            VLOG(class_linker) << "DexFile_defineClassNative returning " << result
                               << " for " << class_name.c_str();
            return soa.AddLocalReference<jclass>(result);
          }
        }
      }
      VLOG(class_linker) << "Failed to find dex_class_def " << class_name.c_str();
      return nullptr;
    }

  • Class_linker.cc - DefineClass

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    mirror::Class* ClassLinker::DefineClass(Thread* self,
                                            const char* descriptor,
                                            size_t hash,
                                            Handle<mirror::ClassLoader> class_loader,
                                            const DexFile& dex_file,
                                            const DexFile::ClassDef& dex_class_def)         {
      ...
      if (klass == nullptr) {
        //加载类实例
        klass.Assign(AllocClass(self, SizeOfClassWithoutEmbeddedTables(dex_file, dex_class_def)));
      }
      ObjPtr<mirror::DexCache> dex_cache = RegisterDexFile(*new_dex_file, class_loader.Get());
      if (dex_cache == nullptr) {
        self->AssertPendingException();
        return nullptr;
      }
      //设置Dex缓存，后续数据从缓存中读取
      klass->SetDexCache(dex_cache);
      //设置Class信息
      SetupClass(*new_dex_file, *new_class_def, klass, class_loader.Get());
      // 把 Class 插入 ClassLoader 的 class_table 中做一个缓存
      ObjPtr<mirror::Class> existing = InsertClass(descriptor, klass.Get(), hash);
      // 加载类属性
      LoadClass(self, *new_dex_file, *new_class_def, klass);

    }

    每当一个类被加载时，ART运行时都会检查该类所属的Dex文件是否已经关联一个dex_cache。

    如果尚未关联，就会创建一个dex_cache与Dex文件建立关联，建立关联后，通过调用RegisterDexFile注册到aRT运行时中去，后续可以直接使用。

    dex_cache用来缓存包含在一个Dex文件里的类型(Type)、方法(Method)、域(Field)、字符串(String)和静态存储区(Static Storage)等信息。

    通过dex_cache间接调用类方法，可以做到延时解析类方法(只有方法第一次调用才会被解析，可以避免解析永远不调用的方法)；一个类方法只会被解析一次，解析的结果存在dex_cache中，下次调用时可以直接从dex_cache进行调用。

加载类成员

参考链接

Android类加载器ClassLoader

相关源码

谈谈 Android 中的 PathClassLoader 和 DexClassLoader

Android

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