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Android ClassLoader加载过程源码分析

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()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&lt;?&gt; 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_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nullptrnullptr使用boot_class_table_来获取class_table, 否则获取当前ClassLoaderClassTableclass_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, pathListnew 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&lt;IOException&gt; suppressedExceptions = new ArrayList&lt;IOException&gt;();
    // 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&lt;File&gt; allNativeLibraryDirectories = new ArrayList&lt;&gt;(nativeLibraryDirectories);
    allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);

    this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories);

    if (suppressedExceptions.size() &gt; 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nativedefineClassNative()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的差不多了,如果想深入的了解具体原理,可以自己看源代码的实现。 这里就介绍到这里。初次写技术分享的文章,如有错误请指正,感谢!

<br>

(360技术原创内容,转载请务必保留文末二维码,谢谢~)

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本文由【360产品技术】发布于开源中国,原文链接:https://my.oschina.net/u/4063048/blog/3074453

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