详解Android中的ActivityThread和APP启动过程

//ActivityThread的main方法
public static void main(String[] args) {
    ...
    Looper.prepareMainLooper();
    ActivityThread thread = new ActivityThread();
    //在attach方法中会完成Application对象的初始化，然后调用Application的onCreate()方法
    thread.attach(false);

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }
    ...
    Looper.loop();
    throw new RuntimeException("Main thread loop unexpectedly exited");
}

//主线程Looper的初始化
public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

//普通线程Looper的初始化
public static void prepare() {
    prepare(true);
}

private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

//Looper的退时会判断quitAllowed
void quit(boolean safe) {
    if (!mQuitAllowed) {
        throw new IllegalStateException("Main thread not allowed to quit.");
    }
    synchronized (this) {
        ...
    }
}

public static Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}

ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
    sMainThreadHandler = thread.getHandler();
}

//与之相关的代码如下：
//ActivityThread的成员变量
final H mH = new H();

final Handler getHandler() {
    return mH;
}

//ActivityThread的attach方法：
private void attach(boolean system) {
    ...
    if (!system) {
        final IActivityManager mgr = ActivityManager.getService();
        try {
            mgr.attachApplication(mAppThread);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }else{
            ...
        }
    }
}

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

//AMS中的方法，主要功能有以下两步
private final boolean attachApplicationLocked(IApplicationThread thread, int pid) {
    ...
    //主要用于创建Application，用调用onCreate方法
    thread.bindApplication(...);
    ...
    //主要用于创建Activity
    if (mStackSupervisor.attachApplicationLocked(app)) {
        ...
    }
}

//ActivityThread中的bindApplication方法
public final void bindApplication(...) {
    ...
    // 该消息的处理，会调用handleBindApplication方法
    sendMessage(H.BIND_APPLICATION, data);
}
//ActivityThread中的handleBindApplication方法
private void handleBindApplication(AppBindData data) {
    ...
    try {
        Application app = data.info.makeApplication(data.restrictedBackupMode, null);
        mInitialApplication = app;
        ...
        try {
            mInstrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
        }
    } finally {
    }
}
    
//LoadedApk中的方法，用于创建Application
public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) {
    //如果存在Application的实例，则直接返回，这也说明Application是个单例
    if (mApplication != null) {
        return mApplication;
    }

    Application app = null;
    //...这里通过反射初始化Application

    if (instrumentation != null) {
        try {
            //调用Application的onCreate方法
            instrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
        }
    }
    return app;
}

boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
    ...
    if (realStartActivityLocked(hr, app, true, true)) {
        ...
    }          
    ...
}

final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
    boolean andResume, boolean checkConfig) throws RemoteException {
    ...
    try {
        //调用ApplicationThread的scheduleLaunchActivity用于启动一个Activity
        app.thread.scheduleLaunchActivity(...);
    } catch (RemoteException e) {

    }
}

//主线程Handler接收到创建Activity的消息LAUNCH_ACTIVITY后，最终会调用performLaunchActivity方法
//performLaunchActivity方法会通过反射去创建一个Activity，然后会调用Activity的各个生命周期方法
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ...
    ContextImpl appContext = createBaseContextForActivity(r);
    Activity activity = null;
    try {
        //这里是反射创建Activity
        java.lang.ClassLoader cl = appContext.getClassLoader();
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    }

    try {
        //这里注意，又调用了一次Application的创建方法，但是前面分析过，Application是个单例，所以这里的实际上是获取Application实例，但是这里为什么会再次调用创建Application的方法呢？
        
        Application app = r.packageInfo.makeApplication(false, mInstrumentation);
        ...
    } 
    ...
    return activity;
}

//创建Application的方法
public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) {
    //如果存在Application的实例，则直接返回，这也说明Application是个单例
    if (mApplication != null) {
        return mApplication;
    }

    Application app = null;
    //...创建Application
    return app;
}