classloader的讲解

我们先从Activity的启动流程开始切入：

//位于android/app/ActivityThread.java中
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {// ........省略代码//通过反射创建activityjava.lang.ClassLoader cl = appContext.getClassLoader();Activity activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);// ........省略代码}

//位于android/app/Instrumentation.java
/*** Perform instantiation of the process's {@link Activity} object.  The* default implementation provides the normal system behavior.* * @param cl The ClassLoader with which to instantiate the object.* @param className The name of the class implementing the Activity*                  object.* @param intent The Intent object that specified the activity class being*               instantiated.* * @return The newly instantiated Activity object.*/public Activity newActivity(ClassLoader cl, String className,Intent intent)throws InstantiationException, IllegalAccessException,ClassNotFoundException {String pkg = intent != null && intent.getComponent() != null? intent.getComponent().getPackageName() : null;return getFactory(pkg).instantiateActivity(cl, className, intent);}

public @NonNull Activity instantiateActivity(@NonNull ClassLoader cl, @NonNull String className,@Nullable Intent intent)throws InstantiationException, IllegalAccessException, ClassNotFoundException {return (Activity) cl.loadClass(className).newInstance();}

我们通过上面代码发现最终是通过classloader.loadClass(className).newInstance()来进行Activity类的实例化的。
classloader通过java.lang.ClassLoader cl = appContext.getClassLoader()构造。

//位于android/app/ContextImpl.java
public ClassLoader getClassLoader() {return mClassLoader != null ? mClassLoader : (mPackageInfo != null ? mPackageInfo.getClassLoader() : ClassLoader.getSystemClassLoader());}

//位于java/lang/ClassLoader.java
public static ClassLoader getSystemClassLoader() {return SystemClassLoader.loader;}

public abstract class ClassLoader {static private class SystemClassLoader {public static ClassLoader loader = ClassLoader.createSystemClassLoader();}private static ClassLoader createSystemClassLoader() {String classPath = System.getProperty("java.class.path", ".");String librarySearchPath = System.getProperty("java.library.path", "");return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());}
}

我们看源码发现
Activity的ClassLoader我们仔细看源码是 PathClassLoader extends BaseDexClassLoader extends ClassLoader 而loadClass这个方法在ClassLoader 中：
在这里插入图片描述
所以cl.loadClass(className)调用的流程如下：

//位于java/lang/ClassLoader.javaprotected Class<?> loadClass(String name, boolean resolve)throws ClassNotFoundException{// First, check if the class has already been loadedClass c = findLoadedClass(name);if (c == null) {// If still not found, then invoke findClass in order// to find the class.c = findClass(name);//这边调用findClass方法，发现这是一个protected类型方法，ClassLoader.java中没有其实现，那就是在子方法中实现了// this is the defining class loader; record the stats}return c;}

BaseDexClassLoader部分源码：

public class BaseDexClassLoader extends ClassLoader {private final DexPathList pathList;/*** Constructs an instance.** @param dexPath the list of jar/apk files containing classes and* resources, delimited by {@code File.pathSeparator}, which* defaults to {@code ":"} on Android* @param optimizedDirectory directory where optimized dex files* should be written; may be {@code null}* @param libraryPath the list of directories containing native* libraries, delimited by {@code File.pathSeparator}; may be* {@code null}* @param parent the parent class loader*/public BaseDexClassLoader(String dexPath, File optimizedDirectory,String libraryPath, ClassLoader parent) {super(parent);this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);}@Overrideprotected Class<?> findClass(String name) throws ClassNotFoundException {List<Throwable> suppressedExceptions = new ArrayList<Throwable>();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部分源码：

//位于/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java
/*package*/ final class DexPathList {private static final String DEX_SUFFIX = ".dex";private static final String JAR_SUFFIX = ".jar";private static final String ZIP_SUFFIX = ".zip";private static final String APK_SUFFIX = ".apk";/** class definition context */private final ClassLoader definingContext;/*** List of dex/resource (class path) elements.* Should be called pathElements, but the Facebook app uses reflection* to modify 'dexElements' (http://b/7726934).*/private final Element[] dexElements;/*** Finds the named class in one of the dex files pointed at by* this instance. This will find the one in the earliest listed* path element. If the class is found but has not yet been* defined, then this method will define it in the defining* context that this instance was constructed with.** @param name of class to find* @param suppressed exceptions encountered whilst finding the class* @return the named class or {@code null} if the class is not* found in any of the dex files*/public Class findClass(String name, List<Throwable> suppressed) {for (Element element : dexElements) {DexFile dex = element.dexFile;if (dex != null) {Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);if (clazz != null) {return clazz;}}}if (dexElementsSuppressedExceptions != null) {suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));}return null;}
}