读Android源码-冷启动时的ActivityThread

 

冷启动建立应用时，ActivityThread的主要方法：

1. main()
   -- 1. 开启消息循环
   -- 2. 通知ActivityManagerService
   -- 3. 添加GCWatcher
2. handleBindApplication()
   -- 1. 建立LoadedApk
   -- 2. 建立Instrumentation
   -- 3. 建立Application
   -- 4. 经过Instrumentation调用Application的onCreate()方法

main()

main方法是一个应用建立的入口，他主要作了3件事

1. 开启消息循环

调用Looper.prepareLoop() Looper.loop()，开启主线程的消息循环，以便于ApplicationThread调用ActivityThread中的生命周期方法。

public static void main(String[] args) {
    ...
    Looper.prepareMainLooper();

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

    if (sMainThreadHandler == null) {
        sMainThreadHandler = thread.getHandler();
    }
    ...
    Looper.loop();
}

2. 通知ActivityManagerService

调用ActivityThread.attach()方法，attach()方法在调用了attachApplication()将ApplicationThread这个Binder交给了ActivityManagerService，意味着ActivityManagerService能够经过ApplicationThread控制咱们的应用，创建了服务器端对客户端的通讯渠道。

private void attach(boolean system){
    ...
    final IActivityManager mgr = ActivityManager.getService();
    try {
        mgr.attachApplication(mAppThread);
    } catch (RemoteException ex) {
        throw ex.rethrowFromSystemServer();
    }
    ...
}

SDK26之后，去除了Native和Proxy，而是直接从ActivityManager(ServiceManager)中获取ActivityManagerService的Binder对象进行通讯

3. 添加GCWatcher

在attach()方法中，添加了监听dialvik内存使用状况得监听者GcWatcher，当内存使用超过总容量的3/4，则打印Log进行记录，而且调用ActivityManagerService的releaseSomeActivities()进行内存释放操做，以防止内存溢出致使应用崩溃。

private void attach(boolean system){
    ...
    BinderInternal.addGcWatcher(new Runnable() {
        @Override public void run() {
            if (!mSomeActivitiesChanged) {
                return;
            }
            Runtime runtime = Runtime.getRuntime();
            long dalvikMax = runtime.maxMemory();
            long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
            if (dalvikUsed > ((3*dalvikMax)/4)) {
                if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                        + " total=" + (runtime.totalMemory()/1024)
                        + " used=" + (dalvikUsed/1024));
                mSomeActivitiesChanged = false;
                try {
                    mgr.releaseSomeActivities(mAppThread);
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
        }
    });
    ...
}

---

handleBindApplication()

ActivityManagerService=AMS
handleBindApplication的被调用时机是:

1. 经过上面AMS.attachApplication()后，AMS得到了控制应用的Binder对象ApplicationThread。
2. AMS进行了一系列操做后（这里先省略），调用了ApplicationThread的bindApplication()
3. bindApplication中经过消息机制，sendMessage到ActivityThread，调用了ActivityThread的handleBindApplication()

handleBindApplication是建立用于所需组件的入口，他主要作了4件事：

1. 建立LoadedApk

LoadedApk对象包含应用的全部信息

private void handleBindApplication(AppBindData data){
    ...
    final LoadedApk pi = getPackageInfo(instrApp, data.compatInfo,
                    appContext.getClassLoader(), false, true, false);
    ...
}

2. 建立Instrumentation

Instrumentation是应用组件的管家，组件的生命周期方法都须要经过它来调用，是客户端与服务器端通讯的最后一步。

private void handleBindApplication(AppBindData data){
    ...
    final ContextImpl instrContext = ContextImpl.createAppContext(this, pi);
    try {
        final ClassLoader cl = instrContext.getClassLoader();
        mInstrumentation = (Instrumentation)
            cl.loadClass(data.instrumentationName.getClassName()).newInstance();
    } catch (Exception e) {
        throw new RuntimeException(
            "Unable to instantiate instrumentation "
            + data.instrumentationName + ": " + e.toString(), e);
    }
    ...
}

3. 建立Application

调用了LoadedApk.makeApplication()进行Application的建立

private void handleBindApplication(AppBindData data){
    ...
    app = data.info.makeApplication(data.restrictedBackupMode, null);
    ...
}

• 先经过ContextImpl.createAppContext(）得到Application的Context
• 再经过Instrumentation.newApplication()，用ClassLoder建立Applicaition
• 建立后Applicaition调用自身的attach(Context)方法，将Context与Applicaition绑定，完成建立

LoadedApk.class
public Application makeApplication(boolean forceDefaultAppClass,
            Instrumentation instrumentation) {
    ...
    Application app = null;
    ...
    ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
    app = mActivityThread.mInstrumentation.newApplication(
            cl, appClass, appContext);
    ...
    return app;
}

4. 经过Instrumentation调用Application的onCreate()方法

private void handleBindApplication(AppBindData data){
    ...
    mInstrumentation.callApplicationOnCreate(app);
    ...
}