您现在的位置是：首页 > 文章详情

Android ClassLoader加载过程源码分析

日期：2019-07-15点击：375收藏

背景

Android开发过程中，开发的小伙伴对动态加载代码肯定不陌生。使用各个开源框架的中都应该有接触，其主要原理离不开ClassLoader等相关的类。这里我们会从Android中ClassLoader等相关类的源码入手，更好的理解和学习动态加载类的原理。

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，如果没找到在通过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。<br> 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。