Framework分析-Application的onCreate

ActivityThread并不是一个Thread，而是一个单纯的Java类，查看一下 ActivityThread 的源码final class ActivityThread，并没有继承Thread或者实现Runnable接口，ActivityThread 其中包含 main 方法,程序的入口地方，怎么看出来的呢? 我们开发过程中总会出现程序异常信息，细心看一下log，查看最下面的几行，最终问题出在 android.app.ActivityThread.main.

AndroidRuntime:     at android.app.Activity.performCreate(Activity.java:5133)
AndroidRuntime:     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
AndroidRuntime:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
AndroidRuntime:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261) 
AndroidRuntime:     at android.app.ActivityThread.access$600(ActivityThread.java:141) 
AndroidRuntime:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256) 
AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:99) 
AndroidRuntime:     at android.os.Looper.loop(Looper.java:137) 
AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5103) 
AndroidRuntime:     at java.lang.reflect.Method.invokeNative(Native Method) 
AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:525) 
AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737) 
AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
AndroidRuntime:     at dalvik.system.NativeStart.main(Native Method) 

介绍相关几个类的结构:
class ApplicationThread extends ApplicationThreadNative
abstract class ApplicationThreadNative extends Binder implements IApplicationThread
ApplicationThread (Binder)对象。其中 Binder负责接收远程AMS的 IPC调用，接收到调用
后，则通过Handler把消息发送到消息队列，UI主线程会异步地从消息队列中取出消息并执行相应操作，比如 start、stop、pause 等。

class ActivityManagerService extends ActivityManagerNative
abstract class ActivityManagerNative extends Binder implements IActivityManager

从 ActivityThread 的main()方法开始分析，

ActivityThread的main方法

    public static void main(String[] args) {

        //...

        Looper.prepareMainLooper(); //主线程的Looper对象

        ActivityThread thread = new ActivityThread(); // 创建ActivityThread
        thread.attach(false); //

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler(); //主线程的Handler
        }

        AsyncTask.init();

        Looper.loop(); //消息循环

        //...
    } 

ActivityThread的attach方法

 private void attach(boolean system) {

        if (!system) {
        // ActivityManagerService(AMS)的代理对象，用于跟AMS通信, IActivityManager具体实现类是 ActivityManagerProxy
        // ActivityManagerNative.getDefault返回ActivityManagerService的远程接口，即ActivityManagerProxy接口
            IActivityManager mgr = ActivityManagerNative.getDefault(); 
            try {
                mgr.attachApplication(mAppThread);//  mAppThread是ActivityThread的成员变量， mAppThread = new ApplicationThread();
            } catch (RemoteException ex) {
                // Ignore
            }
        } else {
            // ...
        }       
}

attach最终调用了ActivityManagerService的远程接口ActivityManagerProxy的attachApplication函数，传入的参数是mAppThread(ApplicationThread类型的Binder对象)，它的作用是用来进行进程间通信的.

ActivityManagerProxy的attachApplication

public void attachApplication(IApplicationThread app) throws RemoteException
{
    Parcel data = Parcel.obtain();
    Parcel reply = Parcel.obtain();
    data.writeInterfaceToken(IActivityManager.descriptor);
    data.writeStrongBinder(app.asBinder()); //将app对象加到data
    mRemote.transact(ATTACH_APPLICATION_TRANSACTION, data, reply, 0);
    //跟服务端通信，transact的最后一个参数的含义是执行IPC调用的模式，0 表示服务端执行完指定服务后会返回一定的数据；1 表示不返回任何数据
    reply.readException();
    data.recycle();
    reply.recycle();
}

可以看出attachApplication方法其实就是进行远程通信，向服务端发起一个 ATTACH_APPLICATION_TRANSACTION的消息.

补充一下Android中的 Binder机制，来自《Android内核剖析》

Binder,英文的意思是别针、回形针。我们经常用别针把两张纸“别” 在一起，而在Andmid中，
Binder用于完成进程间通信（IPC)，即把多个进程“别”在一起。比如，普通应用程序可以调用音乐播放服务提供的播放、暂停、停止等功能。Binder工作在Linux层面，属于一个驱动，只是这个驱动不需要硬件，或者说其操作的硬件是基于一小段内存。从线程的角度来讲，Binder驱动代码运行在内核态，客户端程序调用Binder是通过系统调用完成的.
Binder是一种架构，这种架构提供了服务端接口、Binder驱动、客户端接口三个模块。
首先来看服务端。一个Binder服务端实际上就是一个Binder类的对象，该对象一旦创建，内部就
启动一个隐藏线程。该线程接下来会接收Binder驱动发送的消息，收到消息后，会执行到Binder对象中的onTransact()函数，并按照该函数的参数执行不同的服务代码。因此，要实现一个Binder服务，就必须重载onTransact()方法。
可以想象，重载 onTransact()函数的主要内容是把onTransact()函数的参数转换为服务函数的参数，而onTransact()函数的参数来源是客户端调用transact()函数时输入的，因此，如果transact()有固定格式的输入，那么 onTransact()就会有固定格式的输出。
下面再看Binder驱动。任意一个服务端Binder对象被创建时，同时会在Binder驱动中创建一个
mRemote对象，该对象的类型也是Binder类。客户端要访问远程服务时，都是通过mRemote对象。

根据上面的表示，客户端调用 transact()函数之后，服务端会在 onTransact()中收到客户端传递的消息, 那么在ActivityManagerService的onTransact()方法中查找：

ActivityManagerService的onTransact

    @Override
    public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
            throws RemoteException {
        // ... 
        try {
            return super.onTransact(code, data, reply, flags); //其实是调用父类ActivityManagerNative的onTransact处理
        } catch (RuntimeException e) {
            // The activity manager only throws security exceptions, so let's
            // log all others.
            if (!(e instanceof SecurityException)) {
                Slog.wtf(TAG, "Activity Manager Crash", e);
            }
            throw e;
        }
    }

ActivityManagerNative的onTransact

public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
            throws RemoteException {
        switch (code) {
        // ...
        case ATTACH_APPLICATION_TRANSACTION: {
            data.enforceInterface(IActivityManager.descriptor);
            IApplicationThread app = ApplicationThreadNative.asInterface(
                    data.readStrongBinder()); // 取出客户端传过来的 app
            if (app != null) {
                attachApplication(app); //调用 ActivityManagerService的attachApplication 方法
            }
            reply.writeNoException();
            return true;
        }

ActivityManagerService的attachApplication

 public final void attachApplication(IApplicationThread thread) {
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final long origId = Binder.clearCallingIdentity();
            attachApplicationLocked(thread, callingPid); //调用 attachApplicationLocked
            Binder.restoreCallingIdentity(origId);
        }
    }

ActivityManagerService的attachApplicationLocked

private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid) {
        // ...

        thread.bindApplication(processName, appInfo, providers,
                app.instrumentationClass, profileFile, profileFd, profileAutoStop,
                app.instrumentationArguments, app.instrumentationWatcher,
                app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,
                isRestrictedBackupMode || !normalMode, app.persistent,
                new Configuration(mConfiguration), app.compat, getCommonServicesLocked(),
                mCoreSettingsObserver.getCoreSettingsLocked());   //  
        updateLruProcessLocked(app, false, null);
        app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis();

       // ...
}

最终调用客户端的ApplicationThread的bindApplication

 public final void bindApplication(String processName,
                ApplicationInfo appInfo, List<ProviderInfo> providers,
                ComponentName instrumentationName, String profileFile,
                ParcelFileDescriptor profileFd, boolean autoStopProfiler,
                Bundle instrumentationArgs, IInstrumentationWatcher instrumentationWatcher,
                IUiAutomationConnection instrumentationUiConnection, int debugMode,
                boolean enableOpenGlTrace, boolean isRestrictedBackupMode, boolean persistent,
                Configuration config, CompatibilityInfo compatInfo, Map<String, IBinder> services,
                Bundle coreSettings) {

            if (services != null) {
                // Setup the service cache in the ServiceManager
                ServiceManager.initServiceCache(services);
            }

            setCoreSettings(coreSettings);

            AppBindData data = new AppBindData();
            data.processName = processName;
            data.appInfo = appInfo;
            data.providers = providers;
            data.instrumentationName = instrumentationName;
            data.instrumentationArgs = instrumentationArgs;
            data.instrumentationWatcher = instrumentationWatcher;
            data.instrumentationUiAutomationConnection = instrumentationUiConnection;
            data.debugMode = debugMode;
            data.enableOpenGlTrace = enableOpenGlTrace;
            data.restrictedBackupMode = isRestrictedBackupMode;
            data.persistent = persistent;
            data.config = config;
            data.compatInfo = compatInfo;
            data.initProfileFile = profileFile;
            data.initProfileFd = profileFd;
            data.initAutoStopProfiler = false;
            sendMessage(H.BIND_APPLICATION, data); // Handler发送消息
}

回调到mH（Handler）的handleMessage方法，然后调用了 handleBindApplication(data) 方法

ApplicationThread的handleBindApplication

private void handleBindApplication(AppBindData data) {
        // ...

        // Allow disk access during application and provider setup. This could
        // block processing ordered broadcasts, but later processing would
        // probably end up doing the same disk access.
        final StrictMode.ThreadPolicy savedPolicy = StrictMode.allowThreadDiskWrites();
        try {
            // If the app is being launched for full backup or restore, bring it up in
            // a restricted environment with the base application class.
            Application app = data.info.makeApplication(data.restrictedBackupMode, null); //创建一个Application对象
            mInitialApplication = app;

            // don't bring up providers in restricted mode; they may depend on the
            // app's custom Application class
            if (!data.restrictedBackupMode) {
                List providers = data.providers;
                if (providers != null) {
                    installContentProviders(app, providers);
                    // For process that contains content providers, we want to
                    // ensure that the JIT is enabled "at some point".
                    mH.sendEmptyMessageDelayed(H.ENABLE_JIT, 10*1000);
                }
            }

            // Do this after providers, since instrumentation tests generally start their
            // test thread at this point, and we don't want that racing.
            try {
                mInstrumentation.onCreate(data.instrumentationArgs);
            }
            catch (Exception e) {
                throw new RuntimeException(
                    "Exception thrown in onCreate() of "
                    + data.instrumentationName + ": " + e.toString(), e);
            }

            try {
                mInstrumentation.callApplicationOnCreate(app);// app是Application对象
            } catch (Exception e) {
                if (!mInstrumentation.onException(app, e)) {
                    throw new RuntimeException(
                        "Unable to create application " + app.getClass().getName()
                        + ": " + e.toString(), e);
                }
            }
        } finally {
            StrictMode.setThreadPolicy(savedPolicy);
        }
    }

先是创建Application

public Application makeApplication(boolean forceDefaultAppClass,
        Instrumentation instrumentation) {
    if (mApplication != null) {
        return mApplication;
    }

    Application app = null;

    String appClass = mApplicationInfo.className;
    if (forceDefaultAppClass || (appClass == null)) {
        appClass = "android.app.Application";
    }

    try {
        java.lang.ClassLoader cl = getClassLoader();
        ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
        app = mActivityThread.mInstrumentation.newApplication(
                cl, appClass, appContext); // 由instrument通过反射创建Application
        appContext.setOuterContext(app);
    } catch (Exception e) {
        if (!mActivityThread.mInstrumentation.onException(app, e)) {
            throw new RuntimeException(
                "Unable to instantiate application " + appClass
                + ": " + e.toString(), e);
        }
    }
    mActivityThread.mAllApplications.add(app);
    mApplication = app;

    if (instrumentation != null) { //应为上面传过来的参数为null，所以不会执行下面的代码
        try {
            instrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
            if (!instrumentation.onException(app, e)) {
                throw new RuntimeException(
                    "Unable to create application " + app.getClass().getName()
                    + ": " + e.toString(), e);
            }
        }
    }

    return app;
}

终于找到了Application的onCreate方法

/**
 * Perform calling of the application's {@link Application#onCreate}
 * method.  The default implementation simply calls through to that method.
 *
 * 
Note: This method will be called immediately after {@link #onCreate(Bundle)}.
 * Often instrumentation tests start their test thread in onCreate(); you
 * need to be careful of races between these.  (Well between it and
 * everything else, but let's start here.)
 *
 * @param app The application being created.
 */
public void callApplicationOnCreate(Application app) {
    app.onCreate();
}

概括一下就是，在ActivityThread的main方法中，通过IPC机制和远端进行通信，服务端ActivityManagerService收到消息发送消息通知客户端，客户端的 ApplicationThread收到消息后，通过Handler发送消息，调用handleBindApplication方法，最终是通过 mInstrumentation.callApplicationOnCreate(app)回调到Application的onCreate方法.

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