一文带你看懂Android Application启动流程是怎样的

基于Android11-API30

总览

• 获取applicationThread，AMS这两个Binder2.attach时，将获取applicationThread对象也传递到AMS进程，请求远程调用通知AMS应用进程想要创建Application，此时AMS为服务端
• AMS收到消息，请求调用applicationThread的远程接口，此时AMS为客户端
• applicationThread收到AMS的请求，通过Handler发起创建Application的处理任务，后面就没有远程接口调用了
• 通过反射创建Application的实例，通过Instrumentation启动Application的onCreate方法

详细流程分析

从 ActivityThread.java 的main方法开始看；

public static void main(String[] args) {
    ...
    ActivityThread thread = new ActivityThread();
    thread.attach(system=false, startSeq);//1
    ...
}

进入attach方法；

if(!system){
    final IActivityManager mgr = ActivityManager.getService();
    try {
        mgr.attachApplication(mAppThread, startSeq);//1
    } catch (RemoteException ex) {
        throw ex.rethrowFromSystemServer();
    }
}

非系统应用流程，根据 getSeervice和捕获的RemoteException可以断定，此处在使用Binder进行远程接口调用。
转身看下mAppThread是什么？

final ApplicationThread mAppThread = new ApplicationThread();

private class ApplicationThread extends IApplicationThread.Stub {
    //批量的schedule*接口，比如scheduleReceiver、scheduleCreateService等
    public final void schedule*

    //TODO 关键方法
    public final void bindApplication(some args){}//1

    //一堆dump方法，比如dumpMemory、dumpActivity等

}

可以看到，ApplicationThread是一个实现了远程接口的Binder客户端，内部封装实现了很多远程接口。不过这个客户端什么时候连接的服务器还未可知，没有找到bindService关键字，反正此时应该已经连接上对应的Service了。应该是在RuntimeInit.java类中进行应用进程启动时启动的。

回来看下前一步服务的实例IActivityManager.attachApplication()内部的实现。

先获取AMS的实例，此处获取AMS实例代码跟Activity启动流程中一致

public static IActivityManager getService() {
    return IActivityManagerSingleton.get();
}
private static final Singleton<IActivityManager> IActivityManagerSingleton =
        new Singleton<IActivityManager>() {
            @Override
            protected IActivityManager create() {
                final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                final IActivityManager am = IActivityManager.Stub.asInterface(b);
                return am;
            }
        };

…获取到AMS的Binder后，继续查看ActivityManagerService.java中的attachApplication方法

public final void attachApplication(IApplicationThread thread, long startSeq) {

   	synchronized (this) {
        int callingPid = Binder.getCallingPid();
        final int callingUid = Binder.getCallingUid();
        final long origId = Binder.clearCallingIdentity();
        attachApplicationLocked(thread, callingPid, callingUid, startSeq); //1
        Binder.restoreCallingIdentity(origId);
    }
}

单例获取AMS实例，AMS服务在系统启动就已经注册到ServiceManager了，此处直接去获取Binder实例就行，ServiceManager以Binder池的方式管理注册的Server。

AMS的attachApplication方法中进入到attachApplicationLocked方法，捡能看懂的代码看，跟着thread参数查看代码。

private boolean attachApplicationLocked(@NonNull IApplicationThread thread,
            int pid, int callingUid, long startSeq) {

    try {
        AppDeathRecipient adr = new AppDeathRecipient(
                app, pid, thread);
        thread.asBinder().linkToDeath(adr, 0);//1
        app.deathRecipient = adr;
    } catch (RemoteException e) {
        app.resetPackageList(mProcessStats);
        mProcessList.startProcessLocked(app,
                new HostingRecord("link fail", processName),
                ZYGOTE_POLICY_FLAG_EMPTY);
        return false;
    }

    final ActiveInstrumentation instr2 = app.getActiveInstrumentation();

    if (instr2 != null) {//2
        thread.bindApplication(processName, appInfo, providerList,
                instr2.mClass,
                profilerInfo, instr2.mArguments,
                instr2.mWatcher,
                instr2.mUiAutomationConnection, testMode,
                mBinderTransactionTrackingEnabled, enableTrackAllocation,
                isRestrictedBackupMode || !normalMode, app.isPersistent(),
                new Configuration(app.getWindowProcessController().getConfiguration()),
                app.compat, getCommonServicesLocked(app.isolated),
                mCoreSettingsObserver.getCoreSettingsLocked(),
                buildSerial, autofillOptions, contentCaptureOptions,
                app.mDisabledCompatChanges);
    } else {
        thread.bindApplication(processName, appInfo, providerList, null, profilerInfo,
                null, null, null, testMode,
                mBinderTransactionTrackingEnabled, enableTrackAllocation,
                isRestrictedBackupMode || !normalMode, app.isPersistent(),
                new Configuration(app.getWindowProcessController().getConfiguration()),
                app.compat, getCommonServicesLocked(app.isolated),
                mCoreSettingsObserver.getCoreSettingsLocked(),
                buildSerial, autofillOptions, contentCaptureOptions,
                app.mDisabledCompatChanges);
    }

}

先给ApplicationThread这个Binder上个死亡代理，根据这个死亡代理应该可以找到对应的Service是如何重新启动的，感兴趣可以继续深入，咱们继续往下走。
此处调用到thread.bindApplication接口，前面咱们查看ApplicationThread时有看到，直接切入。

private class ApplicationThread extends IApplicationThread.Stub {
    //批量的schedule*接口，比如scheduleReceiver、scheduleCreateService等
    public final void schedule*

    //TODO 关键方法
    public final void bindApplication(some args){
        AppBindData data = new AppBindData();
        ...一堆参数
        sendMessage(H.BIND_APPLICATION, data);//1
    }

    //一堆dump方法，比如dumpMemory、dumpActivity等

}

到达咱们Android开发工程师比较熟悉的点了，封装了一堆参数后，通过H这个Handler对象发了一条BIND_APPLICATION消息，咱们看看这条消息去哪了，直接跟进BIND_APPLICATION这个消息的捕捉位置。

//消息分发
class H extends Handler{
    public void handleMessage(Message msg){
        swich(msg.what){
            case BIND_APPLICATION:
                AppBindData data = (AppBindData)msg.obj;
                handleBindApplication(data);//1
                break;
            ...省略
        }
    }
}

进入消息分发处理方法，这个方法比较长，注意阅读能看懂的代码，不求甚解，跟踪data的处理。

private void handleBindApplication(AppBindData data) {
    //各种初始化，比如进程名，应用名，AsyncTask线程池的配置，时区，网络发现

    //Context的初始化
    final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);

    try {
        final ClassLoader cl = instrContext.getClassLoader();
        mInstrumentation = (Instrumentation)//1
            cl.loadClass(data.instrumentationName.getClassName()).newInstance();
    } catch (Exception e) {
        throw new RuntimeException(
            "Unable to instantiate instrumentation "
            + data.instrumentationName + ": " + e.toString(), e);
    }

    final ComponentName component = new ComponentName(ii.packageName, ii.name);
    mInstrumentation.init(this, instrContext, appContext, component,//1
            data.instrumentationWatcher, data.instrumentationUiAutomationConnection);

    ...
    Application app;
    app = data.info.makeApplication(data.restrictedBackupMode, null);//2

    mInstrumentation.onCreate(data.instrumentationArgs);
    mInstrumentation.callApplicationOnCreate(app);//3
}

通过反射实例化mInstrumentation对象，该对象为Android系统组件的管家，目前看可以控制Application和Activity的生命周期。

创建Application对象，进去看下创建的代码

//LoadApk.java #makeApplication
public Application makeApplication(boolean forceDefaultAppClass,
    Instrumentation instrumentation){
    ...
    app = mActivityThread.mInstrumentation.newApplication(cl, appClass, appContext);//1
    appContext.setOuterContext(app);
    ...
}

//Instrumentation.java #newApplication
public Application newApplication(ClassLoader cl, String className, Context context)
        throws InstantiationException, IllegalAccessException,
        ClassNotFoundException {
    Application app = getFactory(context.getPackageName())
            .instantiateApplication(cl, className);//2
    app.attach(context);//首先回调attachBaseContext方法
    return app;
}

//AppComponentFactory #instantiateApplication
public @NonNull Application instantiateApplication(@NonNull ClassLoader cl,
        @NonNull String className)
        throws InstantiationException, IllegalAccessException, ClassNotFoundException {
    return (Application) cl.loadClass(className).newInstance();//3
}

可以看出最后还是通过反射初始化了Application。

最后通过mInstrumentation对象完成Application类的onCreate方法的调用。

mInstrumentation.callApplicationOnCreate(app);//1

//Instrumentation.java #callApplicationOnCreate
public void callApplicationOnCreate(Application app) {
    app.onCreate();
}

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