1、介绍
我们知道PathClassLoader是一个应用的默认加载器(而且他只能加载data/app/xxx.apk的文件)，但是我们加载插件一般使用DexClassLoader加载器，所以这里就有问题了，其实如果对于开始的时候，每个人都会认为很简单，很容易想到使用DexClassLoader来加载Activity获取到class对象，在使用Intent启动
2、替换LoadApk里面的mClassLoader
我们知道我们可以将我们使用的DexClassLoader加载器绑定到系统加载Activity的类加载器上就可以了，这个是我们的思路。也是最重要的突破点。下面我们就来通过源码看看如何找到加载Activity的类加载器。加载Activity的时候，有一个很重要的类：LoadedApk.Java，这个类是负责加载一个Apk程序的，我们可以看一下他的源码：
我们知道内部有个mClassLoader成员变量，我们只需要获取它就可以了，因为它不是静态的，所以我们需要先获取LoadApk这个类的对象，我们再去
看看ActivityThread.java这个类
我们可以发现ActivityThread里面有个静态的成员变量sCurrentActivityThread,然后还有一个ArrayMap存放Apk包名和LoadedApk映射关系的数据结构，我们通过反射来获取mClassLoader对象。
如果对ActivityThread.java这个类不熟悉的可以看我这篇博客，/u011068702/article/details/53207039（Android插件化开发之AMS与应用程序(客户端ActivityThread、Instrumentation、Activity)通信模型分析）非常重要，是app程序的入口处。
3、实现具体代码
1）我们先需要一个测试apk,然后把这个测试的test.apk,放到手机sdcard里面去，关键代码如下package com.example.testapkdemo;import android.os.Bundle;import android.support.v7.app.ActionBarActivity;import android.view.View;import android.view.View.OnClickListener;import android.widget.Toast;public class MainActivity extends ActionBarActivity {public static View parentView;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);if (parentView == null) {setContentView(R.layout.activity_main);} else {setContentView(parentView);}findViewById(R.id.button).setOnClickListener(new OnClickListener(){@Overridepublic void onClick(View v) {Toast.makeText(MainActivity.this, "我是来自插件", Toast.LENGTH_SHORT).show();}});}public void setView(View view) {this.parentView = view;}}
效果图如下：
接下来是我宿主代码：
ReflectHelper.java 这个是的我的反射帮助类
package com.example.dexclassloaderactivity;import java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.Method;import java.util.Arrays;/*** 反射辅助函数* @author**/public class ReflectHelper {public static final Class<?>[] PARAM_TYPE_VOID = new Class<?>[]{};public static Object invokeStaticMethod(String className, String methodName, Class<?>[] paramTypes, Object...params) {try {Class<?> clazz = Class.forName(className);Method method = clazz.getMethod(methodName, paramTypes);method.setAccessible(true);return method.invoke(null, params);} catch (Exception e) {e.printStackTrace();}return null;}public static Object invokeMethod(String className, String methodName, Class<?>[] paramTypes, Object obj, Object...params) {try {Class<?> clazz = Class.forName(className);Method method = clazz.getMethod(methodName, paramTypes);method.setAccessible(true);return method.invoke(obj, params);} catch (Exception e) {e.printStackTrace();}return null;}public static Object getStaticField(String className, String fieldName) {try {Class<?> clazz = Class.forName(className);Field field = clazz.getDeclaredField(fieldName);field.setAccessible(true);return field.get(null);} catch (Exception e) {e.printStackTrace();}return null;}public static Object getField(String className, String fieldName, Object obj) {try {Class<?> clazz = Class.forName(className);Field field = clazz.getDeclaredField(fieldName);field.setAccessible(true);return field.get(obj);} catch (Exception e) {e.printStackTrace();}return null;}public static void setStaticField(String className, String fieldName, Object value) {try {Class<?> clazz = Class.forName(className);Field field = clazz.getDeclaredField(fieldName);field.setAccessible(true);field.set(null, value);} catch (Exception e) {e.printStackTrace();}}public static void setField(String className, String fieldName, Object obj, Object value) {try {Class<?> clazz = Class.forName(className);Field field = clazz.getDeclaredField(fieldName);field.setAccessible(true);field.set(obj, value);} catch (Exception e) {e.printStackTrace();}}public static Object createInstance(String className, Class<?>[] paramTypes, Object...params) {Object object = null;try {Class<?> cls = Class.forName(className);Constructor<?> constructor = cls.getConstructor(paramTypes);constructor.setAccessible(true);object = constructor.newInstance(params);}catch (Exception e) {e.printStackTrace();}return object;}/*** Locates a given field anywhere in the class inheritance hierarchy.** @param instance an object to search the field into.* @param name field name* @return a field object* @throws NoSuchFieldException if the field cannot be located*/public static Field findField(Object instance, String name) throws NoSuchFieldException {for (Class<?> clazz = instance.getClass(); clazz != null; clazz = clazz.getSuperclass()) {try {Field field = clazz.getDeclaredField(name);if (!field.isAccessible()) {field.setAccessible(true);}return field;} catch (NoSuchFieldException e) {// ignore and search next}}throw new NoSuchFieldException("Field " + name + " not found in " + instance.getClass());}/*** Locates a given field anywhere in the class inheritance hierarchy.** @param cls to search the field into.* @param name field name* @return a field object* @throws NoSuchFieldException if the field cannot be located*/public static Field findField2(Class<?> cls, String name) throws NoSuchFieldException {Class<?> clazz = null;for (clazz = cls; clazz != null; clazz = clazz.getSuperclass()) {try {Field field = clazz.getDeclaredField(name);if (!field.isAccessible()) {field.setAccessible(true);}return field;} catch (NoSuchFieldException e) {// ignore and search next}}throw new NoSuchFieldException("Field " + name + " not found in " + clazz);}/*** Locates a given method anywhere in the class inheritance hierarchy.** @param instance an object to search the method into.* @param name method name* @param parameterTypes method parameter types* @return a method object* @throws NoSuchMethodException if the method cannot be located*/public static Method findMethod(Object instance, String name, Class<?>... parameterTypes)throws NoSuchMethodException {for (Class<?> clazz = instance.getClass(); clazz != null; clazz = clazz.getSuperclass()) {try {Method method = clazz.getDeclaredMethod(name, parameterTypes);if (!method.isAccessible()) {method.setAccessible(true);}return method;} catch (NoSuchMethodException e) {// ignore and search next}}throw new NoSuchMethodException("Method " + name + " with parameters " +Arrays.asList(parameterTypes) + " not found in " + instance.getClass());}}
MyApplication.java 这个类用实现替换mClassLoader
package com.example.dexclassloaderactivity;import java.io.File;import java.lang.ref.WeakReference;import dalvik.system.DexClassLoader;import android.annotation.SuppressLint;import android.app.Application;import android.os.Environment;import android.util.ArrayMap;import android.util.Log;public class MyApplication extends Application{public static final String TAG = "MyApplication";public static final String AppName = "test.apk";public static int i = 0;public static DexClassLoader mClassLoader;@Overridepublic void onCreate() {Log.d(TAG, "替换之前系统的classLoader");showClassLoader();try {String cachePath = this.getCacheDir().getAbsolutePath();String apkPath = /*Environment.getExternalStorageState() + File.separator*/"/sdcard/"+ AppName;mClassLoader = new DexClassLoader(apkPath, cachePath,cachePath, getClassLoader()); loadApkClassLoader(mClassLoader);} catch (Exception e) {e.printStackTrace();}Log.d(TAG, "替换之后系统的classLoader");showClassLoader();}@SuppressLint("NewApi")public void loadApkClassLoader(DexClassLoader loader) {try {Object currentActivityThread = ReflectHelper.invokeMethod("android.app.ActivityThread", "currentActivityThread", new Class[] {},new Object[] {});String packageName = this.getPackageName();ArrayMap mpackages = (ArrayMap) ReflectHelper.getField("android.app.ActivityThread", "mPackages", currentActivityThread);WeakReference wr= (WeakReference)mpackages.get(packageName);Log.e(TAG, "mClassLoader:" + wr.get()); ReflectHelper.setField("android.app.LoadedApk", "mClassLoader", wr.get(), loader);Log.e(TAG, "load:" + loader); } catch (Exception e) {Log.e(TAG, "load apk classloader error:" + Log.getStackTraceString(e)); }}/*** 打印系统的classLoader*/public void showClassLoader() {ClassLoader classLoader = getClassLoader();if (classLoader != null){Log.i(TAG, "[onCreate] classLoader " + i + " : " + classLoader.toString());while (classLoader.getParent()!=null){classLoader = classLoader.getParent();Log.i(TAG,"[onCreate] classLoader " + i + " : " + classLoader.toString());i++;}}}}
然后就是MainActivity.java文件，里面包含了下面另外一种方式，打开activity，所以我把函数inject(DexClassLoader loader)先注释掉
package com.example.dexclassloaderactivity;import java.io.File;import java.lang.reflect.Array;import java.lang.reflect.Field;import android.content.Intent;import android.os.Bundle;import android.os.Environment;import android.support.v7.app.ActionBarActivity;import android.util.Log;import android.view.View;import android.view.View.OnClickListener;import android.widget.TextView;import dalvik.system.DexClassLoader;import dalvik.system.PathClassLoader;public class MainActivity extends ActionBarActivity{public static final String TAG = "MainActivity";public static final String AppName = "test.apk";public static DexClassLoader mDexClassLoader = null;public static final String APPName = "test.apk";public TextView mText;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);findViewById(R.id.text2).setOnClickListener(new OnClickListener(){@Overridepublic void onClick(View v) {try {// inject(MyApplication.mClassLoader);String apkPath = Environment.getExternalStorageDirectory().toString() + File.separator + APPName;Class clazz = MyApplication.mClassLoader.loadClass("com.example.testapkdemo.MainActivity");Intent intent = new Intent(MainActivity.this, clazz);startActivity(intent);finish();} catch (Exception e) {Log.e(TAG, "name:" + Log.getStackTraceString(e));}}});}private void inject(DexClassLoader loader){ PathClassLoader pathLoader = (PathClassLoader) getClassLoader(); try { Object dexElements = combineArray( getDexElements(getPathList(pathLoader)), getDexElements(getPathList(loader))); Object pathList = getPathList(pathLoader); setField(pathList, pathList.getClass(), "dexElements", dexElements); } catch (IllegalArgumentException e) { e.printStackTrace(); } catch (NoSuchFieldException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } catch (ClassNotFoundException e) { e.printStackTrace(); } } private static Object getPathList(Object baseDexClassLoader) throws IllegalArgumentException, NoSuchFieldException, IllegalAccessException, ClassNotFoundException { ClassLoader bc = (ClassLoader)baseDexClassLoader; return getField(baseDexClassLoader, Class.forName("dalvik.system.BaseDexClassLoader"), "pathList"); } private static Object getField(Object obj, Class<?> cl, String field) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException { Field localField = cl.getDeclaredField(field); localField.setAccessible(true); return localField.get(obj); } private static Object getDexElements(Object paramObject) throws IllegalArgumentException, NoSuchFieldException, IllegalAccessException { return getField(paramObject, paramObject.getClass(), "dexElements"); } private static void setField(Object obj, Class<?> cl, String field, Object value) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException { Field localField = cl.getDeclaredField(field); localField.setAccessible(true); localField.set(obj, value); } private static Object combineArray(Object arrayLhs, Object arrayRhs) { Class<?> localClass = arrayLhs.getClass().getComponentType(); int i = Array.getLength(arrayLhs); int j = i + Array.getLength(arrayRhs); Object result = Array.newInstance(localClass, j); for (int k = 0; k < j; ++k) { if (k < i) { Array.set(result, k, Array.get(arrayLhs, k)); } else { Array.set(result, k, Array.get(arrayRhs, k - i)); } } return result; } }
这里一定要注意，我们犯了3个错，
1、test.apk的路径写错了，下次写文件路径的时候，我们应该需要加上File file = new File(path); 用file.exist()函数来判断是否存在
2、从sdcard卡里面读取test.apk，没加上权限， <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
3、写了Application,忘了在AndroidManifest.xml文件里面声明。
我们还需要注意要加上开启activity 在AndroidManifest.xml里面注册，切记，希望下次不要再次犯错。
AndroidManifest.xml文件如下：
<?xml version="1.0" encoding="utf-8"?><manifest xmlns:android="/apk/res/android"package="com.example.dexclassloaderactivity"android:versionCode="1"android:versionName="1.0" ><uses-sdkandroid:minSdkVersion="11"android:targetSdkVersion="21" /><uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/><applicationandroid:name="com.example.dexclassloaderactivity.MyApplication"android:allowBackup="true"android:icon="@drawable/ic_launcher"android:label="@string/app_name"android:theme="@style/AppTheme" ><activityandroid:name=".MainActivity"android:label="@string/app_name" ><intent-filter><action android:name="android.intent.action.MAIN" /><category android:name="android.intent.category.LAUNCHER" /></intent-filter></activity><activity android:name="com.example.testapkdemo.MainActivity"> </activity> </application></manifest>
运行结果如下：
然后这里又出现了插件资源和宿主资源冲突问题，我们后面再来研究。
4、合并PathClassLoader和DexClassLoader中的dexElements数组
我们首先来看一下PathClassLoader和DexClassLoader类加载器的父类BaseDexClassloader的源码：
(这里需要注意的是PathClassLoader和DexClassLoader类的父加载器是BaseClassLoader,他们的父类是BaseDexClassLoader)
这里有一个DexPathList对象，在来看一下DexPathList.java源码：
Elements数组，我们看到这个变量他是专门存放加载的dex文件的路径的，系统默认的类加载器是PathClassLoader，本身一个程序加载之后会释放一个dex出来，这时候会将dex路径放到里面，当然DexClassLoader也是一样的，那么我们会想到，我们是否可以将DexClassLoader中的dexElements和PathClassLoader中的dexElements进行合并，然后在设置给PathClassLoader中呢？这也是一个思路。我们来看代码：
/*** 以下是一种方式实现的* @param loader*/private void inject(DexClassLoader loader){PathClassLoader pathLoader = (PathClassLoader) getClassLoader();try {Object dexElements = combineArray(getDexElements(getPathList(pathLoader)),getDexElements(getPathList(loader)));Object pathList = getPathList(pathLoader);setField(pathList, pathList.getClass(), "dexElements", dexElements);} catch (IllegalArgumentException e) {e.printStackTrace();} catch (NoSuchFieldException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} catch (ClassNotFoundException e) {e.printStackTrace();}}private static Object getPathList(Object baseDexClassLoader)throws IllegalArgumentException, NoSuchFieldException, IllegalAccessException, ClassNotFoundException {ClassLoader bc = (ClassLoader)baseDexClassLoader;return getField(baseDexClassLoader, Class.forName("dalvik.system.BaseDexClassLoader"), "pathList");}private static Object getField(Object obj, Class<?> cl, String field)throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {Field localField = cl.getDeclaredField(field);localField.setAccessible(true);return localField.get(obj);}private static Object getDexElements(Object paramObject)throws IllegalArgumentException, NoSuchFieldException, IllegalAccessException {return getField(paramObject, paramObject.getClass(), "dexElements");}private static void setField(Object obj, Class<?> cl, String field,Object value) throws NoSuchFieldException,IllegalArgumentException, IllegalAccessException {Field localField = cl.getDeclaredField(field);localField.setAccessible(true);localField.set(obj, value);}private static Object combineArray(Object arrayLhs, Object arrayRhs) {Class<?> localClass = arrayLhs.getClass().getComponentType();int i = Array.getLength(arrayLhs);int j = i + Array.getLength(arrayRhs);Object result = Array.newInstance(localClass, j);for (int k = 0; k < j; ++k) {if (k < i) {Array.set(result, k, Array.get(arrayLhs, k));} else {Array.set(result, k, Array.get(arrayRhs, k - i));}}return result;}
然后运行的时候把MyApplication.java文件里面的函数loadApkClassLoader(mClassLoader);注释掉，然后把MainActivity.java文件里面的inject(MyApplication.mClassLoader)不要注释，运行效果一样。
总结：
我们在使用反射机制来动态加载Activity的时候，有两个思路：
1>、替换LoadApk类中的mClassLoader变量的值，将我们动态加载类DexClassLoader设置为mClassLoader的值
2>、合并系统默认加载器PathClassLoader和动态加载器DexClassLoader中的dexElements数组
这两个的思路原理都是一样的：就是让我们动态加载进来的Activity能够具备正常的启动流程和生命周期。
我们还没解决资源冲突问题，后面再解决，有点复杂。
