一.概述
当项目有加壳子,插件化或热修复等需求的时候,可以使用Proxy/Delegate Application框架的方式,在正常的模式中,一个程序一般只有一个Application入口,而Proxy/Delegate模式中需要有两个Application,原程序的Application改为Delegate Application,再新加一个Proxy Application,由Proxy Application 提供一系列的个性化定制,再将所有的context和context相关的引用全部转化为Delegate Application的实例,让外界包括Delegate Application自身都以为该App的Application入口就是Delegate Application.
二.实例
1.Proxy/Delegate 之前
这里就在Android 4.4原生的DeskClock程序上应用Proxy/Delegate框架为示例
原生的DeskClock程序没有自定义Application,这里先定义一个,并print该程序目前ApplicationContext的名字(在DeskClock中使用的Log是自定义的)
/** * Created by jesse on 15-7-17. */public class MyApplication extends Application{ private final String TAG = MyApplication.class.getSimpleName(); @Override public void onCreate() { super.onCreate(); Log.i(TAG + ", onCreate " + this.getApplicationContext().getClass().getSimpleName()); }} 并且在DeskClock的入口Activity,DeskClock处也print出该程序目前ApplicationContext的名字用于后续Proxy后的对比. Application的Manifest配置是
<application android:name="cn.jesse.MyApplication" android:label="@string/app_label" android:icon="@mipmap/ic_launcher_alarmclock" android:requiredForAllUsers="true" android:supportsRtl="true">过滤后的运行Log: 简单的流程就是先启动自定义MyApplication 之后再launch DeskClock，同时都打印出来ApplicationContext的名字
2.使用Proxy/Delegate框架之后
使用Proxy/Delegate框架,需要重新构建出来一个新的ProxyApplication,用来做代理Application,原先的MyApplication的作用为DelegateApplication
所以Manifest的配置需要更改,app的主入口更改为MyProxyApplication,把DelegateApplication的信息以meta-data子元素的形式存储(当然也可以用其他的方式)
<application android:name="cn.jesse.MyProxyApplication" android:label="@string/app_label" android:icon="@mipmap/ic_launcher_alarmclock" android:requiredForAllUsers="true" android:supportsRtl="true"> <meta-data android:name="DELEGATE_APPLICATION_CLASS_NAME" android:value="cn.jesse.MyApplication" > </meta-data> </application>
定义一个抽象类,提供一个用于替换当前ProxyApplication 的ClassLoader成父类的ClassLoader的抽象方法(或者一些其他的个性化定制)
* Created by jesse on 15-7-17. */public abstract class ProxyApplication extends Application{ protected abstract void initProxyApplication();} 当我们要替换当前ProxyApplication的ClassLoader为父类的ClassLoader,所以这个替换的动作要足够得早(要保证在app Context最早被构建的入口处替换ClassLoader),要不然就会出现替换不干净的情况,就会有程序中大部分使用的DelegateApplication的ClassLoader,而一小部分是使用的ProxyApplication的ClassLoader,这样可能会出现一些意想不到的bug. 通常来说在Application的OnCreate中来做替换就足够了,但是当app有注册ContentProvider的时候ContentProvider：OnCreate的调用是在Application:OnCreate之前的,所以我们必须保证替换ClassLoader的动作要在ContentProvider之前.
通过查看源码可以看到Application是继承自ContextWrapper,而在ContextWrapper中系统在构建完成完善的Context之后第一次回调是通过attachBaseContext方法,既然这样就通过在ProxyApplication中复写该方法来获取刚出炉热喷喷的Context来转换ClassLoader.
/** * Set the base context for this ContextWrapper. All calls will then be * delegated to the base context. Throws * IllegalStateException if a base context has already been set. * * @param base The new base context for this wrapper. */ protected void attachBaseContext(Context base) { if (mBase != null) { throw new IllegalStateException("Base context already set"); } mBase = base; } 转换ClassLoader的入口也确定之后就可以自定义一个MyProxyApplication,继承自ProxyApplication并且复写attachBaseContext方法,print相关信息/** * Created by jesse on 15-7-17. */public class MyProxyApplication extends ProxyApplication { private final String TAG = MyProxyApplication.class.getSimpleName(); private Context mContext; @Override protected void attachBaseContext(Context base) { super.attachBaseContext(base); Log.i(TAG + ", attachBaseContext"); mContext = base; this.initProxyApplication(); } @Override public void onCreate() { super.onCreate(); Log.i(TAG + ", onCreate" + this.getApplicationContext().getClass().getSimpleName()); BootLoader.boot(mContext); } @Override protected void initProxyApplication() { Log.i(TAG + ", initProxyApplication"); BootLoader.resetClassLoader(mContext); }} Log运行的顺序,先进入attachBaseContext->initProxyApplication->onCreate->DeskClock:onCreate (这里DeskClock的onCreate获取到的ApplicationContext的名字是(MyProxyApplication)
入口的顺序没问题了之后,就可以在initProxyApplication方法中替换当前的ClassLoader到父类的ClassLoader,并且在MyProxyApplication的onCreate中将应用层所有的Application的引用全部从ProxyApplication替换成MyApplication(当前在DeskClock程序中没有替换ClassLoader的需求,只需要替换所有的Application的引用就能达到代理的效果,所以在initProxyApplication方法处就写了一个空方法带过).
先从AndroidManifest配置文件中的metadata拿到DelegateApplication的属性
String className = CLASS_NAME; ApplicationInfo appInfo = getPackageManager().getApplicationInfo(super.getPackageName(), PackageManager.GET_META_DATA); Bundle bundle = appInfo.metaData; if (bundle != null && bundle.containsKey(KEY)) { className = bundle.getString(KEY); if (className.startsWith(".")) className = super.getPackageName() + className; } 根据className反射得到MyApplication,创建MyApplication实例并且取得MyProxyApplication的实例Class delegateClass = Class.forName(className, true, getClassLoader()); Application delegate = (Application) delegateClass.newInstance(); Application proxyApplication = (Application)getApplicationContext();使用反射更换MyProxyApplication context成员中的mOuterContext属性
Class contextImplClass = Class.forName("android.app.ContextImpl"); Field mOuterContext = contextImplClass.getDeclaredField("mOuterContext"); mOuterContext.setAccessible(true); mOuterContext.set(mContext, delegate); 获取MyProxyApplication Context的PackageInfo对象,替换掉其中的mApplication属性
Field mPackageInfoField = contextImplClass.getDeclaredField("mPackageInfo"); mPackageInfoField.setAccessible(true); Object mPackageInfo = mPackageInfoField.get(mContext); Class loadedApkClass = Class.forName("android.app.LoadedApk"); Field mApplication = loadedApkClass.getDeclaredField("mApplication"); mApplication.setAccessible(true); mApplication.set(mPackageInfo, delegate); 再根据之前反射得到的packageInfo对象获取到mActivityThread属性,替换掉其中的mInitialApplication属性
Class activityThreadClass = Class.forName("android.app.ActivityThread"); Field mAcitivityThreadField = loadedApkClass.getDeclaredField("mActivityThread"); mAcitivityThreadField.setAccessible(true); Object mActivityThread = mAcitivityThreadField.get(mPackageInfo); Field mInitialApplicationField = activityThreadClass.getDeclaredField("mInitialApplication"); mInitialApplicationField.setAccessible(true); mInitialApplicationField.set(mActivityThread, delegate); 拿着之前的mActivityThread对象获取到mAllApplications属性,注意该属性是一个list，这里就移除MyProxyApplication添加DelegateApplication,至此应用层MyProxyApplication的Context的引用全部都替换成了MyApplication的引用.
Field mAllApplicationsField = activityThreadClass.getDeclaredField("mAllApplications"); mAllApplicationsField.setAccessible(true); ArrayList<Application> al = (ArrayList<Application>)mAllApplicationsField.get(mActivityThread); al.add(delegate); al.remove(proxyApplication); 给MyApplication通过反射和attach内部方法设置baseContext,并调用MyApplication的onCreate方法完成DelegateApplication的初始化.
Method attach = Application.class.getDeclaredMethod("attach", Context.class); attach.setAccessible(true); attach.invoke(delegate, mContext); delegate.onCreate(); 完成这些步骤之后再重新运行查看Log,观察DeskClock处获取的ApplicationContext的名字已经变成MyApplication.
但是这样还没有完全结束,还记得开头说的ContentProvider吗?他的构造是在Application的onCreate之前的,那么ContentProvider部分有没有需要替换的Context引用呢？从framework/base/core/java/android/app下可以找到ActivityThread.java从其中装载ContentProvider的部分可以看到,如果当前Context的包名和ProviderInfo的包名一样的话,ContentProvider就会引用当前的MyProxyApplication的Context.由于当前的MyProxyApplication只是做代理启动用的,所以在MyProxyApplication处复写getPackageName并且返回空就可以避免ContentProvider复用当前Context了.
private IActivityManager.ContentProviderHolder installProvider(Context context, IActivityManager.ContentProviderHolder holder, ProviderInfo info, boolean noisy, boolean noReleaseNeeded, boolean stable) { ContentProvider localProvider = null; IContentProvider provider; if (holder == null || holder.provider == null) { if (DEBUG_PROVIDER || noisy) { Slog.d(TAG, "Loading provider " + info.authority + ": " + info.name); } Context c = null; ApplicationInfo ai = info.applicationInfo; if (context.getPackageName().equals(ai.packageName)) { c = context; } else if (mInitialApplication != null && mInitialApplication.getPackageName().equals(ai.packageName)) { c = mInitialApplication; } else { try { c = context.createPackageContext(ai.packageName, Context.CONTEXT_INCLUDE_CODE); } catch (PackageManager.NameNotFoundException e) { // Ignore } }三.总结
这篇只是先简单的走了下Proxy/Delegate框架的流程,这个框架其实是有很多使用场景的,例如多dex动态加载,插件化,线上程序热修复bug等可以灵活使用出很多有趣的技术,有时间的话还会再发一篇以Proxy/Delegate实现的线上程序热修复bug的博客.
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