Android啟動 - Application的啟動流程

基于Android11-API30

總覽

1. 獲取applicationThread，AMS這兩個Binder
   2.attach時，將獲取applicationThread物件也傳遞到AMS行程，請求遠程呼叫通知AMS應用行程想要創建Application，此時AMS為服務端
2. AMS收到訊息，請求呼叫applicationThread的遠程介面，此時AMS為客戶端
3. applicationThread收到AMS的請求，通過Handler發起創建Application的處理任務，后面就沒有遠程介面呼叫了
4. 通過反射創建Application的實體，通過Instrumentation啟動Application的onCreate方法

詳細流程分析

從 ActivityThread.java 的main方法開始看；

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

進入attach方法；

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

非系統應用流程，根據 getSeervice和捕獲的RemoteException可以斷定，此處在使用Binder進行遠程介面呼叫，
轉身看下mAppThread是什么？

final ApplicationThread mAppThread = new ApplicationThread();

private class ApplicationThread extends IApplicationThread.Stub {
    //批量的schedule*介面，比如scheduleReceiver、scheduleCreateService等
    public final void schedule*
    
    //TODO 關鍵方法
    public final void bindApplication(some args){}//1
    
    //一堆dump方法，比如dumpMemory、dumpActivity等
    
}

可以看到，ApplicationThread是一個實作了遠程介面的Binder客戶端，內部封裝實作了很多遠程介面，不過這個客戶端什么時候連接的服務器還未可知，沒有找到bindService關鍵字，反正此時應該已經連接上對應的Service了，應該是在RuntimeInit.java類中進行應用行程啟動時啟動的，

回來看下前一步服務的實體IActivityManager.attachApplication()內部的實作，

1. 先獲取AMS的實體，此處獲取AMS實體代碼跟Activity啟動流程中一致

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;
            }
        };

…獲取到AMS的Binder后，繼續查看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);
    }
}

單例獲取AMS實體，AMS服務在系統啟動就已經注冊到ServiceManager了，此處直接去獲取Binder實體就行，ServiceManager以Binder池的方式管理注冊的Server，

AMS的attachApplication方法中進入到attachApplicationLocked方法，撿能看懂的代碼看，跟著thread引數查看代碼，

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);
    }

}

先給ApplicationThread這個Binder上個死亡代理，根據這個死亡代理應該可以找到對應的Service是如何重新啟動的，感興趣可以繼續深入，咱們繼續往下走，
此處呼叫到thread.bindApplication介面，前面咱們查看ApplicationThread時有看到，直接切入，

private class ApplicationThread extends IApplicationThread.Stub {
    //批量的schedule*介面，比如scheduleReceiver、scheduleCreateService等
    public final void schedule*
    
    //TODO 關鍵方法
    public final void bindApplication(some args){
        AppBindData data = new AppBindData();
        ...一堆引數
        sendMessage(H.BIND_APPLICATION, data);//1
    }
    
    //一堆dump方法，比如dumpMemory、dumpActivity等
    
}

到達咱們Android開發工程師比較熟悉的點了，封裝了一堆引數后，通過H這個Handler物件發了一條BIND_APPLICATION訊息，咱們看看這條訊息去哪了，直接跟進BIND_APPLICATION這個訊息的捕捉位置，

//訊息分發
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
        }
    }
}

進入訊息分發處理方法，這個方法比較長，注意閱讀能看懂的代碼，不求甚解，跟蹤data的處理，

private void handleBindApplication(AppBindData data) {
    //各種初始化，比如行程名，應用名，AsyncTask執行緒池的配置，時區，網路發現
    
    //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
}

通過反射實體化mInstrumentation物件，該物件為Android系統組件的管家，目前看可以控制Application和Activity的生命周期，

創建Application物件，進去看下創建的代碼

//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);//首先回呼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
}

可以看出最后還是通過反射初始化了Application，

最后通過mInstrumentation物件完成Application類的onCreate方法的呼叫，

mInstrumentation.callApplicationOnCreate(app);//1

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