详细分析ClassLoader加载原理

ClassLoader 的继承关系如下：

这里我们主要分析一下 BaseDexClassLoader.findClass()和 ClassLoader.loadClass()两个函数在系统中是怎么进行查找class的过程。

我们看一下系统加载类ClassLoader.loadClass()函数实现代码，在ClassLoader.java中：

    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
            // 首先 检测是否已经加载过
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                try {
                    if (parent != null) {
                        //去调用父类的loadClass
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    //未找到的情况下，使用findClass在当前dex查找
                    c = findClass(name);
                }
            }
            return c;
    }

    protected Class<?> findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }

1， loadClass()先调用findLoadedClass()来判断当前类是否已加载;
2，未查找到递归去父类中查找是否加载到缓存；
3，均未缓存，去BootClassLoader中查找；
4，以上未发现，自顶级父类依次向下查找，调用findClass()查找当前dex。

findLoadedClass函数分析

下图为findLoadedClass()的调用流程；根据调用流程图配合源代码进行详细的分析原理。

下面介绍对应的源代码实现部分:

    protected final Class<?> findLoadedClass(String name) {
        ClassLoader loader;
        if (this == BootClassLoader.getInstance())
            loader = null;
        else
            loader = this;
        return VMClassLoader.findLoadedClass(loader, name);
    }

函数最终统一调用VMClassLoader.findLoadedClass()进行查找类。

native static Class findLoadedClass(ClassLoader cl, String name);

实现在java_lang_VMClassLoader.cc文件中。

static jclass VMClassLoader_findLoadedClass(JNIEnv* env, jclass, jobject javaLoader,jstring javaName) {
  ....
  ObjPtr<mirror::ClassLoader> loader = soa.Decode<mirror::ClassLoader>(javaLoader);
  ClassLinker* cl = Runtime::Current()->GetClassLinker();

  ObjPtr<mirror::Class> c = VMClassLoader::LookupClass(cl,
                                                       soa.Self(),
                                                       descriptor.c_str(),
                                                       descriptor_hash,
                                                       loader);
  if (c != nullptr && c->IsResolved()) {
    return soa.AddLocalReference<jclass>(c);
  }
  ...
  if (loader != nullptr) {
    // Try the common case.
    StackHandleScope<1> hs(soa.Self());
    c = VMClassLoader::FindClassInPathClassLoader(cl,
                                                  soa,
                                                  soa.Self(),
                                                  descriptor.c_str(),
                                                  descriptor_hash,
                                                  hs.NewHandle(loader));
    if (c != nullptr) {
      return soa.AddLocalReference<jclass>(c);
    }
  }

  return nullptr;
}

  static mirror::Class* LookupClass(ClassLinker* cl,
                                    Thread* self,
                                    const char* descriptor,
                                    size_t hash,
                                    ObjPtr<mirror::ClassLoader> class_loader)
      REQUIRES(!Locks::classlinker_classes_lock_)
      REQUIRES_SHARED(Locks::mutator_lock_) {
    return cl->LookupClass(self, descriptor, hash, class_loader);
  }
  static ObjPtr<mirror::Class> FindClassInPathClassLoader(ClassLinker* cl,
                                                          ScopedObjectAccessAlreadyRunnable& soa,
                                                          Thread* self,
                                                          const char* descriptor,
                                                          size_t hash,
                                                          Handle<mirror::ClassLoader> class_loader)
      REQUIRES_SHARED(Locks::mutator_lock_) {
    ObjPtr<mirror::Class> result;
    if (cl->FindClassInBaseDexClassLoader(soa, self, descriptor, hash, class_loader, &result)) {
      return result;
    }
    return nullptr;
  }

上述代码findLoadedClass()分为两步；

1，通过class_linker_->Lookupclass()进行查找加载类；
2，如果没找到再通过class_linker_->FindClassInPathClassLoader()进行查找。

class_linker_在虚拟机的启动startVM()函数的时候进行的初始化。
Runtime::class_linker_在Runtime::Init()函数的时候做的初始化。

  if (UNLIKELY(IsAotCompiler())) {
    class_linker_ = new AotClassLinker(intern_table_);
  } else {
    class_linker_ = new ClassLinker(intern_table_);
  }

继续来分析ClassLinker::LookupClass()函数的具体实现：

mirror::Class* ClassLinker::LookupClass(Thread* self,
                                        const char* descriptor,
                                        size_t hash,
                                        ObjPtr<mirror::ClassLoader> class_loader) {
  ReaderMutexLock mu(self, *Locks::classlinker_classes_lock_);
  ClassTable* const class_table = ClassTableForClassLoader(class_loader);
  if (class_table != nullptr) {
    ObjPtr<mirror::Class> result = class_table->Lookup(descriptor, hash);
    if (result != nullptr) {
      return result.Ptr();
    }
  }
  return nullptr;
}

LookupClass()函数通过class_loader是否为nullptr，nullptr使用boot_class_table_来获取class_table, 否则获取当前ClassLoader的ClassTable。 class_table存放当前已经加载过的class，其实可以理解为class cache。如何进行dex 解析和aot等加载系统类和解析映射到内存中的不在此处展开分析。可以了解art虚拟机启动进行详细分析。

findClass()函数分析

下图是findClass的调用流程；根据调用流程图配合下面的代码进行详细的分析了解：

下面我们介绍对应的源代码实现部分。

findClass()函数在BaseDexClassLoader.java实现, 该函数主要做的事情就是在当前dex中查找类。如果类在当前dex中即返回。

代码如下：

 @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
        Class c = pathList.findClass(name, suppressedExceptions);
        if (c == null) {
            ...
            throw cnfe;
        }
        return c;
    }

pathList类型为DexPathList用来保存dexfile文件的句柄等dex的操作。pathList.findClass()实现在当前dex中查找类, pathList在new DexClassLoader()构造时初始化。

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

DexPathList.java

public DexPathList(ClassLoader definingContext, String dexPath,
            String librarySearchPath, File optimizedDirectory) {

        ...
        this.definingContext = definingContext;
        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
        // save dexPath for BaseDexClassLoader
        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions, definingContext);

        this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
        this.systemNativeLibraryDirectories =
                splitPaths(System.getProperty("java.library.path"), true);
        List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
        allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);

        this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories);

        if (suppressedExceptions.size() > 0) {
            this.dexElementsSuppressedExceptions =
                suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
        } else {
            dexElementsSuppressedExceptions = null;
        }
    }

dexElements数组保存dexfile文件句柄。具体实现在makeDexElements()函数中调用loadDexFile()函数加载dex。该函数实现：

DexFile.java
private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader, Element[] elements) throws IOException {
        if (optimizedDirectory == null) {
            return new DexFile(file, loader, elements);
        } else {
            String optimizedPath = optimizedPathFor(file, optimizedDirectory);
            return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements);
        }
    }

DexFile.loadDex()进行解析加载dex文件。关键代码如下：

private DexFile(String sourceName, String outputName, int flags, ClassLoader loader, DexPathList.Element[] elements) throws IOException {
    ...
    mCookie = openDexFile(sourceName, outputName, flags, loader, elements);
    mInternalCookie = mCookie;
    mFileName = sourceName;
    ...
}

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.
    return openDexFileNative(new File(sourceName).getAbsolutePath(),
                              (outputName == null)
                              ? null
                              : new File(outputName).getAbsolutePath(),
                              flags,loader,elements);
}

private static native Object openDexFileNative(String sourceName, String outputName, int flags, ClassLoader loader, DexPathList.Element[] elements);

最终打开dexfile是通过native方法实现，并且返回mCookie, mCookie类型是int用来标识dex的唯一性。 openDexFileNative()实现代码：

//`dalvik_system_DexFile.cc`
static jobject DexFile_openDexFileNative(JNIEnv* env,
                                         jclass,
                                         jstring javaSourceName,
                                         jstring javaOutputName,
                                         jint flags ATTRIBUTE_UNUSED,
                                         jobject class_loader,
                                         jobjectArray dex_elements)
{
  ...
  Runtime* const runtime = Runtime::Current();
  ClassLinker* linker = runtime->GetClassLinker();
  
  ...

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

上述代码通过aotManager打开并返回mCookie，进一步的打开实现不在此处展开。即上述已经填充elements[],下面开始展开pathList.findClass()函数的查找方式。

    //BaseDexClassLoader.java
    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()会遍历elements[], 每个element保存了dex的DexFile句柄，然后调用loadClassBinaryName()函数进行当前dex查找类。

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

  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 {
          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;
  }

真正去dex或者内存中查找类的函数在native中defineClassNative()实现，我们来分析一下真正的实现过程：

private static native Class defineClassNative(String name, ClassLoader loader, Object cookie, DexFile dexFile)

//dalvik_system_DexFile.cc
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)) {
    ...
    return nullptr;
  }

  ScopedUtfChars class_name(env, javaName);
  ...

  const std::string descriptor(DotToDescriptor(class_name.c_str()));
  const size_t hash(ComputeModifiedUtf8Hash(descriptor.c_str()));
  for (auto& dex_file : dex_files) {
      ...
      ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
      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());
      ....
        return soa.AddLocalReference<jclass>(result);
      }
    }
    ...
  return nullptr;
}

通过Runtime拿到当前的ClassLinker对象，然后通过class_linker->DefineClass()在当前dex中进行查找类。然后把找到的类通过class_linker->InsertDexFileInToClassLoader()插入到class_table中进行缓存，返回查找到的类。这里不进一步展开分析。

Android ClassLoader加载过程的源代码分析到此已经分析得差不多了，如果想深入地了解具体原理，可以自己看源代码的实现。本文就介绍到这里。初次写技术分享的文章，如有错误请指正，感谢！

本文转载自公众号360技术（ID：qihoo_tech）。

原文链接：

https://mp.weixin.qq.com/s/31MGkrTTjiHfXffUDydONg

创作场景

Android ClassLoader 加载过程源码分析