读Android源码-冷启动时的ActivityThread

冷启动创建应用时，ActivityThread的主要方法：

main()
-- 1. 开启消息循环
-- 2. 通知ActivityManagerService
-- 3. 添加GCWatcher

handleBindApplication()
-- 1. 创建LoadedApk
-- 2. 创建Instrumentation
-- 3. 创建Application
-- 4. 通过Instrumentation调用Application的onCreate()方法

main()

main方法是一个应用创建的入口，他主要做了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可以通过ApplicationThread控制我们的应用，建立了服务器端对客户端的通信渠道。

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

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

3. 添加GCWatcher

在attach()方法中，添加了监听dialvik内存使用情况得监听者GcWatcher，当内存使用超过总容量的3/4，则打印Log进行记录，并且调用ActivityManagerService的releaseSomeActivities()进行内存释放操作，以防止内存溢出导致应用崩溃。

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:

通过上面AMS.attachApplication()后，AMS获得了控制应用的Binder对象ApplicationThread。

AMS进行了一系列操作后（这里先省略），调用了ApplicationThread的bindApplication()

bindApplication中通过消息机制，sendMessage到ActivityThread，调用了ActivityThread的handleBindApplication()

handleBindApplication是创建用于所需组件的入口，他主要做了4件事：

1. 创建LoadedApk

LoadedApk对象包含应用的所有信息

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

2. 创建Instrumentation

Instrumentation是应用组件的管家，组件的生命周期方法都需要通过它来调用，是客户端与服务器端通信的最后一步。

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. 创建Application

调用了LoadedApk.makeApplication()进行Application的创建

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

先通过ContextImpl.createAppContext(）获得Application的Context
再通过Instrumentation.newApplication()，用ClassLoder创建Applicaition
创建后Applicaition调用自身的attach(Context)方法，将Context与Applicaition绑定，完成创建

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. 通过Instrumentation调用Application的onCreate()方法

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

来源：oschina

链接：https://my.oschina.net/u/920274/blog/3064408

标签

ams

android

Activity Manager