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，（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;
@Override
protected 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(){
@Override
public 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;
}
}
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;
@Override
public 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;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
findViewById(R.id.text2).setOnClickListener(new OnClickListener(){
@Override
public 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="http://schemas.android.com/apk/res/android"
package="com.example.dexclassloaderactivity"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="11"
android:targetSdkVersion="21" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<application
android:name="com.example.dexclassloaderactivity.MyApplication"
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android: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。
