源码阅读:类加载器 BaseDexClassLoader

Android 和 Java中的类加载机制都是双亲委派模型

但是Android和Java中实现的类加载器的是不同的

Android中用到的是DexClassLoader()

 

类加载器的构造方法

 public BaseDexClassLoader(String dexPath,
            String librarySearchPath, ClassLoader parent, ClassLoader[] sharedLibraryLoaders,
            ClassLoader[] sharedLibraryLoadersAfter,
            boolean isTrusted)

类加载器的构造方法如上，

parent:是父加载器

sharedLibraryLoader 一部分系统共享的库，暂时不确定这部分是怎样确定的

sharedLibraryLoaderAfter 另一部分系统共享的库

在加载类的吧遍历顺序

1. shareLibraryLoader

2. dexPathList

3. sharedLibraryLoadersAfter

 

双亲委派机制中的具体代码

protected Class<?> loadClass(String name, boolean resolve)throws ClassNotFoundException{// First, check if the class has already been loadedClass<?> c = findLoadedClass(name);if (c == null) {try {if (parent != null) {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) {// If still not found, then invoke findClass in order// to find the class.c = findClass(name);// Android-removed: Android has no jvmstat.// this is the defining class loader; record the stats// PerfCounter.getParentDelegationTime().addTime(t1 - t0);// PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);// PerfCounter.getFindClasses().increment();}}return c;}

加载类的顺序

1. 先让parent类加载器尝试加载

2. 让BootstrapClassLoader尝试加载

3.当前类加载器尝试加载

 

具体的实现时使用 findClass 去加载具体的类

 

BaseDexClassLoader

在Android中，使用PathClassLoader和DexClassLoader进行类的加载，这两个类都是BaseDexClassLoader的基类

 @Overrideprotected Class<?> findClass(String name) throws ClassNotFoundException {// First, check whether the class is present in our shared libraries.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>();Class c = pathList.findClass(name, suppressedExceptions);if (c != null) {return c;}// Now, check whether the class is present in the "after" shared libraries.if (sharedLibraryLoadersAfter != null) {for (ClassLoader loader : sharedLibraryLoadersAfter) {try {return loader.loadClass(name);} catch (ClassNotFoundException ignored) {}}}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;}