Android中apk文件里的dex文件是对java编译出来的.class文件进行重新打包,当然在打包之前会利用自己的协议做一些数据处理,例如优化函数表和变量表。在java程序中是使用classloader来加载这些编译生成的.class文件,然而在android程序中是通过DexClassLoader来装载这些文件的.这里我们就可以通过DexClassLoader在程序A里面动态装载程序B中的类,并且来调用B程序中的方法.

     1.首先先建立一个普通的Android工程,在这个工程中定义一个叫做plugin的类,类中实现一个简单的方法,如下所示:

<span style="font-size:14px;">public class PluginClass {
	private static String TAG = PluginClass.class.getSimpleName();
	public PluginClass(){
		Log.i(TAG, "initialized");
	}
	public void invoke(String s){
		Log.i(TAG, s);
	}
}</span>

      3.再重新建立一个Android工程,其中定义一个叫做host的类,在这个类中实现DexClassLoader动态加载第一个工程中的plugin类,如下所示:

<span style="font-size:14px;">public class HostClass {
	private static String TAG = HostClass.class.getSimpleName();
	private Context mContext = null;
	public HostClass(Context contect){
		mContext = contect;
	}
	public void useDexClassLoader(){
		Intent intent = new Intent();
		intent.setPackage("com.example.plugin");
		PackageManager pm = mContext.getPackageManager();
		final List<ResolveInfo> plugins = pm.queryIntentActivities(intent,0);
		if(plugins.size() <= 0){
			Log.i(TAG, "resolve info size is:" + plugins.size());
			return;
		}
		ResolveInfo resolveInfo = plugins.get(0);
		ActivityInfo activityInfo = resolveInfo.activityInfo;
		
		String div = System.getProperty("path.separator");
		String packageName = activityInfo.packageName;
		String dexPath = activityInfo.applicationInfo.sourceDir;
		//目标类所在的apk或者jar的路径，class loader会通过这个路径来加载目标类文件
		String dexOutputDir = mContext.getApplicationInfo().dataDir;
		//由于dex文件是包含在apk或者jar文件中的,所以在加载class之前就需要先将dex文件解压出来，dexOutputDir为解压路径
		String libPath = activityInfo.applicationInfo.nativeLibraryDir;
		//目标类可能使用的c或者c++的库文件的存放路径
		
		Log.i(TAG, "div:" + div + "   " + 
					"packageName:" + packageName + "   " +
					"dexPath:" + dexPath + "   " +
					"dexOutputDir:" + dexOutputDir + "   " +
					"libPath:" + libPath);
		
		DexClassLoader dcLoader = new DexClassLoader(dexPath,dexOutputDir,libPath,this.getClass().getClassLoader());
		try {
			Class<?> clazz = dcLoader.loadClass(packageName + ".PluginClass");
			Object obj = clazz.newInstance();
			Class[] param = new Class[1];
			param[0] = String.class;
			Method action = clazz.getMethod("invoke", param);
			action.invoke(obj, "test this function");
		} catch (ClassNotFoundException e) {
			Log.i(TAG, "ClassNotFoundException");
		} catch (InstantiationException e) {
			Log.i(TAG, "InstantiationException");
		} catch (IllegalAccessException e) {
			Log.i(TAG, "IllegalAccessException");
		} catch (NoSuchMethodException e) {
			Log.i(TAG, "NoSuchMethodException");
		} catch (IllegalArgumentException e) {
			Log.i(TAG, "IllegalArgumentException");
		} catch (InvocationTargetException e) {
			Log.i(TAG, "InvocationTargetException");
		}
	}
}</span>

<span style="font-size:14px;">I/HostClass( 8341): div::   packageName:com.example.plugin   dexPath:/data/app/com.example.plugin-1.apk   dexOutputDir:/data/data/com.example.host   libPath:/data/app-lib/com.example.plugin-1
D/dalvikvm( 8341): DexOpt: --- BEGIN 'com.example.plugin-1.apk' (bootstrap=0) ---
D/dalvikvm( 8341): DexOpt: --- END 'com.example.plugin-1.apk' (success) ---
D/dalvikvm( 8341): DEX prep '/data/app/com.example.plugin-1.apk': unzip in 39ms, rewrite 723ms
I/PluginClass( 8341): initialized
I/PluginClass( 8341): test this function
D/libEGL  ( 8341): loaded /system/lib/egl/libEGL_mali.so
D/libEGL  ( 8341): loaded /system/lib/egl/libGLESv1_CM_mali.so
D/libEGL  ( 8341): loaded /system/lib/egl/libGLESv2_mali.so
D/OpenGLRenderer( 8341): Enabling debug mode 0
I/HostClass( 8341): div::   packageName:com.example.plugin   dexPath:/data/app/com.example.plugin-1.apk   dexOutputDir:/data/data/com.example.host   libPath:/data/app-lib/com.example.plugin-1
I/PluginClass( 8341): initialized
I/PluginClass( 8341): test this function</span>

Android 使用DexClassLoader来运行其他apk中的方法