上文分析了Handler、MessageQueue、Message及Looper四者的關(guān)系，本文將對(duì)Android中最重要的線程——ActivityThread進(jìn)行說(shuō)明。

二倦畅、ActivityThread的主要工作及實(shí)現(xiàn)機(jī)制

ActivityThread是Android應(yīng)用的主線程（UI線程），說(shuō)起ActivityThread绣的，不得不提到Activity的創(chuàng)建叠赐、啟動(dòng)過(guò)程以及ActivityManagerService欲账，但本文將僅從線程管理的角度來(lái)分析ActivityThread。ActivityManagerService芭概、ActivityStack赛不、ApplicationThread等會(huì)在后續(xù)文章中詳細(xì)分析，敬請(qǐng)期待喔~~不過(guò)為了說(shuō)清楚ActivityThread的由來(lái)罢洲，還是需要簡(jiǎn)單介紹下俄删。

以下引用自羅升陽(yáng)大師的博客：《Android應(yīng)用程序的Activity啟動(dòng)過(guò)程簡(jiǎn)要介紹和學(xué)習(xí)計(jì)劃》

Step 1. 無(wú)論是通過(guò)Launcher來(lái)啟動(dòng)Activity，還是通過(guò)Activity內(nèi)部調(diào)用startActivity接口來(lái)啟動(dòng)新的Activity奏路，都通過(guò)Binder進(jìn)程間通信進(jìn)入到ActivityManagerService進(jìn)程中，并且調(diào)用ActivityManagerService.startActivity接口臊诊；
Step 2. ActivityManagerService調(diào)用ActivityStack.startActivityMayWait來(lái)做準(zhǔn)備要啟動(dòng)的Activity的相關(guān)信息鸽粉；
Step 3. ActivityStack通知ApplicationThread要進(jìn)行Activity啟動(dòng)調(diào)度了，這里的ApplicationThread代表的是調(diào)用ActivityManagerService.startActivity接口的進(jìn)程抓艳，對(duì)于通過(guò)點(diǎn)擊應(yīng)用程序圖標(biāo)的情景來(lái)說(shuō)触机，這個(gè)進(jìn)程就是Launcher了，而對(duì)于通過(guò)在Activity內(nèi)部調(diào)用startActivity的情景來(lái)說(shuō)玷或，這個(gè)進(jìn)程就是這個(gè)Activity所在的進(jìn)程了儡首；
Step 4. ApplicationThread不執(zhí)行真正的啟動(dòng)操作，它通過(guò)調(diào)用ActivityManagerService.activityPaused接口進(jìn)入到ActivityManagerService進(jìn)程中偏友，看看是否需要?jiǎng)?chuàng)建新的進(jìn)程來(lái)啟動(dòng)Activity蔬胯；
Step 5. 對(duì)于通過(guò)點(diǎn)擊應(yīng)用程序圖標(biāo)來(lái)啟動(dòng)Activity的情景來(lái)說(shuō)，ActivityManagerService在這一步中位他，會(huì)調(diào)用startProcessLocked來(lái)創(chuàng)建一個(gè)新的進(jìn)程氛濒，而對(duì)于通過(guò)在Activity內(nèi)部調(diào)用startActivity來(lái)啟動(dòng)新的Activity來(lái)說(shuō)，這一步是不需要執(zhí)行的鹅髓，因?yàn)樾碌腁ctivity就在原來(lái)的Activity所在的進(jìn)程中進(jìn)行啟動(dòng)舞竿；
Step 6. ActivityManagerServic調(diào)用ApplicationThread.scheduleLaunchActivity接口，通知相應(yīng)的進(jìn)程執(zhí)行啟動(dòng)Activity的操作窿冯；
Step 7. ApplicationThread把這個(gè)啟動(dòng)Activity的操作轉(zhuǎn)發(fā)給ActivityThread骗奖，ActivityThread通過(guò)ClassLoader導(dǎo)入相應(yīng)的Activity類，然后把它啟動(dòng)起來(lái)醒串。

大師的這段描述把ActivityManagerService执桌、ActivityStack、ApplicationThread及ActivityThread的調(diào)用關(guān)系講的很清楚厦凤，本文將從ActivityThread的main()方法開始分析其主要工作及實(shí)現(xiàn)機(jī)制鼻吮。（如有需要，可前往ActivityThread源碼）

public static void main(String[] args) {
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
    SamplingProfilerIntegration.start();

    // CloseGuard defaults to true and can be quite spammy.  We
    // disable it here, but selectively enable it later (via
    // StrictMode) on debug builds, but using DropBox, not logs.
    CloseGuard.setEnabled(false);

    Environment.initForCurrentUser();

    // Set the reporter for event logging in libcore
    EventLogger.setReporter(new EventLoggingReporter());

    AndroidKeyStoreProvider.install();

    // Make sure TrustedCertificateStore looks in the right place for CA certificates
    final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
    TrustedCertificateStore.setDefaultUserDirectory(configDir);

    Process.setArgV0("<pre-initialized>");

    Looper.prepareMainLooper();

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

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

    if (false) {
        Looper.myLooper().setMessageLogging(new
                LogPrinter(Log.DEBUG, "ActivityThread"));
    }

    // End of event ActivityThreadMain.
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    Looper.loop();

    throw new RuntimeException("Main thread loop unexpectedly exited");
}

上述代碼中较鼓，從Looper.prepareMainLooper()到Looper.loop()的代碼主要用于環(huán)境初始化椎木、AndroidKeyStoreProvider安裝等违柏，這里不做重點(diǎn)說(shuō)明。紅色部分的代碼主要分為兩個(gè)功能塊：1）綁定應(yīng)用進(jìn)程到ActivityManagerService香椎；2）主線程Handler消息處理漱竖。
關(guān)于線程通信機(jī)制，Handler畜伐、MessageQueue馍惹、Message及Looper四者的關(guān)系請(qǐng)參考上一篇文章《Android線程管理——線程通信》。

2.1 應(yīng)用進(jìn)程綁定

main()方法通過(guò)thread.attach(false)綁定應(yīng)用進(jìn)程玛界。ActivityManagerNative通過(guò)getDefault()方法返回ActivityManagerService實(shí)例万矾，ActivityManagerService通過(guò)attachApplication將ApplicationThread對(duì)象綁定到ActivityManagerService，而ApplicationThread作為Binder實(shí)現(xiàn)ActivityManagerService對(duì)應(yīng)用進(jìn)程的通信和控制慎框。

private void attach(boolean system) {
    sCurrentActivityThread = this;
    mSystemThread = system;
    if (!system) {
        ……            
        RuntimeInit.setApplicationObject(mAppThread.asBinder());
        final IActivityManager mgr = ActivityManagerNative.getDefault();
        try {
            mgr.attachApplication(mAppThread);
        } catch (RemoteException ex) {
            // Ignore
        }
        ……        
    } else {}
}

在ActivityManagerService內(nèi)部良狈，attachApplication實(shí)際是通過(guò)調(diào)用attachApplicationLocked實(shí)現(xiàn)的，這里采用了synchronized關(guān)鍵字保證同步笨枯。

@Override
public final void attachApplication(IApplicationThread thread) {
    synchronized (this) {
        int callingPid = Binder.getCallingPid();
        final long origId = Binder.clearCallingIdentity();
        attachApplicationLocked(thread, callingPid);
        Binder.restoreCallingIdentity(origId);
    }
}

attachApplicationLocked的實(shí)現(xiàn)較為復(fù)雜薪丁，其主要功能分為兩部分：

• thread.bindApplication
• mStackSupervisor.attachApplicationLocked(app)

private final boolean attachApplicationLocked(IApplicationThread thread,
        int pid) {

    // Find the application record that is being attached...  either via
    // the pid if we are running in multiple processes, or just pull the
    // next app record if we are emulating process with anonymous threads.
    ProcessRecord app;
    if (pid != MY_PID && pid >= 0) {
        synchronized (mPidsSelfLocked) {
            app = mPidsSelfLocked.get(pid);
        }
    } else {
        app = null;
    }
   // ……
    try {
       // ……
        thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
                profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
                app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,
                enableTrackAllocation, isRestrictedBackupMode || !normalMode, app.persistent,
                new Configuration(mConfiguration), app.compat,
                getCommonServicesLocked(app.isolated),
                mCoreSettingsObserver.getCoreSettingsLocked());
        updateLruProcessLocked(app, false, null);
        app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis();
    } catch (Exception e) {
        // todo: Yikes!  What should we do?  For now we will try to
        // start another process, but that could easily get us in
        // an infinite loop of restarting processes...
        Slog.wtf(TAG, "Exception thrown during bind of " + app, e);

        app.resetPackageList(mProcessStats);
        app.unlinkDeathRecipient();
        startProcessLocked(app, "bind fail", processName);
        return false;
    }

    // See if the top visible activity is waiting to run in this process...
    if (normalMode) {
        try {
            if (mStackSupervisor.attachApplicationLocked(app)) {
                didSomething = true;
            }
        } catch (Exception e) {
            Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
            badApp = true;
        }
    }
// ……
}

thread對(duì)象其實(shí)是ActivityThread里ApplicationThread對(duì)象在ActivityManagerService的代理對(duì)象，故此執(zhí)行thread.bindApplication馅精，最終會(huì)調(diào)用ApplicationThread的bindApplication方法严嗜。該bindApplication方法的實(shí)質(zhì)是通過(guò)向ActivityThread的消息隊(duì)列發(fā)送BIND_APPLICATION消息，消息的處理調(diào)用handleBindApplication方法洲敢，handleBindApplication方法比較重要的是會(huì)調(diào)用如下方法：

mInstrumentation.callApplicationOnCreate(app);

callApplicationOnCreate即調(diào)用應(yīng)用程序Application的onCreate()方法漫玄，說(shuō)明Application的onCreate()方法會(huì)比所有activity的onCreate()方法先調(diào)用。

mStackSupervisor為ActivityManagerService的成員變量压彭，類型為ActivityStackSupervisor称近。

/** Run all ActivityStacks through this */
ActivityStackSupervisor mStackSupervisor;

從注釋可以看出，mStackSupervisor為Activity堆棧管理輔助類實(shí)例哮塞。ActivityStackSupervisor的attachApplicationLocked()方法的調(diào)用了realStartActivityLocked()方法刨秆，在realStartActivityLocked()方法中，會(huì)調(diào)用scheduleLaunchActivity()方法：

final boolean realStartActivityLocked(ActivityRecord r,
        ProcessRecord app, boolean andResume, boolean checkConfig)
        throws RemoteException {
 
    //...  
    try {
        //...
        app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                System.identityHashCode(r), r.info,
                new Configuration(mService.mConfiguration),
                r.compat, r.icicle, results, newIntents, !andResume,
                mService.isNextTransitionForward(), profileFile, profileFd,
                profileAutoStop);
 
        //...
 
    } catch (RemoteException e) {
        //...
    }
    //...    
    return true;
}

app.thread也是ApplicationThread對(duì)象在ActivityManagerService的一個(gè)代理對(duì)象忆畅，最終會(huì)調(diào)用ApplicationThread的scheduleLaunchActivity方法衡未。

// we use token to identify this activity without having to send the
// activity itself back to the activity manager. (matters more with ipc)
@Override
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
    ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
    CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
    int procState, Bundle state, PersistableBundle persistentState,
    List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
    boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {

        updateProcessState(procState, false);

        ActivityClientRecord r = new ActivityClientRecord();

        ……
        sendMessage(H.LAUNCH_ACTIVITY, r);
}

同bindApplication()方法，最終是通過(guò)向ActivityThread的消息隊(duì)列發(fā)送消息家凯，在ActivityThread完成實(shí)際的LAUNCH_ACTIVITY的操作缓醋。

public void handleMessage(Message msg) {
    if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
    switch (msg.what) {
        case LAUNCH_ACTIVITY: {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
            final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

            r.packageInfo = getPackageInfoNoCheck(
                r.activityInfo.applicationInfo, r.compatInfo);
            handleLaunchActivity(r, null);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            } break;
    ……
}

handleLaunchActivity()用于啟動(dòng)Activity。具體的啟動(dòng)流程不在這里詳述了绊诲，這里重點(diǎn)說(shuō)明ApplicationThread及ActivityThread的線程通信機(jī)制送粱。

2.2 主線程消息處理

在《Android線程管理——線程通信》中談到了普通線程中Handler、MessageQueue掂之、Message及Looper四者的關(guān)系抗俄，那么脆丁，ActivityThread中的線程通信又有什么不同呢？不同之處主要表現(xiàn)為兩點(diǎn)：1）Looper的初始化方式动雹；2）Handler生成槽卫。

首先，ActivityThread通過(guò)Looper.prepareMainLooper()初始化Looper胰蝠，為了直觀比較ActivityThread與普通線程初始化Looper的區(qū)別歼培，把兩種初始化方法放在一起：

/** Initialize the current thread as a looper.
  * This gives you a chance to create handlers that then reference
  * this looper, before actually starting the loop. Be sure to call
  * {@link #loop()} after calling this method, and end it by calling
  * {@link #quit()}.
  */
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));
}

/**
 * Initialize the current thread as a looper, marking it as an
 * application's main looper. The main looper for your application
 * is created by the Android environment, so you should never need
 * to call this function yourself.  See also: {@link #prepare()}
 */
public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

• 普通線程的prepare()方法默認(rèn)quitAllowed參數(shù)為true，表示允許退出茸塞，ActivityThread在prepareMainLooper()方法中調(diào)用prepare()方法躲庄，參數(shù)為false，表示主線程不允許退出钾虐。
• 普通線程只調(diào)用prepare()方法读跷，ActivityThread在調(diào)用完prepare()方法之后，會(huì)通過(guò)myLooper()方法將本地線程<ThreadLocal>的Looper對(duì)象的引用交給sMainLooper禾唁。myLooper()其實(shí)就是調(diào)用sThreadLocal的get()方法實(shí)現(xiàn)的。

/**
 * Return the Looper object associated with the current thread.  Returns
 * null if the calling thread is not associated with a Looper.
 */
public static Looper myLooper() {
    return sThreadLocal.get();
}

• 之所以要通過(guò)sMainLooper指向ActivityThread的Looper對(duì)象无切，就是希望通過(guò)getMainLooper()方法將主線程的Looper對(duì)象開放給其他線程荡短。

/** Returns the application's main looper, which lives in the main thread of the application.*/
public static Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}

其次，ActivityThread與普通線程的Handler生成方式也不一樣哆键。普通線程生成一個(gè)與Looper綁定的Handler即可掘托，ActivityThread通過(guò)sMainThreadHandler指向getHandler()的返回值，而getHandler()方法返回的其實(shí)是一個(gè)繼承Handler的H對(duì)象籍嘹。

private class H extends Handler {
    ……
}

final H mH = new H();

final Handler getHandler() {
    return mH;
}

真正實(shí)現(xiàn)消息機(jī)制“通”信的其實(shí)是Looper的loop()方法闪盔，loop()方法的核心實(shí)現(xiàn)如下：

/**
 * Run the message queue in this thread. Be sure to call
 * {@link #quit()} to end the loop.
 */
public static void loop() {
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;

    // Make sure the identity of this thread is that of the local process,
    // and keep track of what that identity token actually is.
    Binder.clearCallingIdentity();
    final long ident = Binder.clearCallingIdentity();

    for (;;) {
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }

        // This must be in a local variable, in case a UI event sets the logger
        Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }

        msg.target.dispatchMessage(msg);

        if (logging != null) {
            logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
        }

        // Make sure that during the course of dispatching the
        // identity of the thread wasn't corrupted.
        final long newIdent = Binder.clearCallingIdentity();
        if (ident != newIdent) {
            Log.wtf(TAG, "Thread identity changed from 0x"
                    + Long.toHexString(ident) + " to 0x"
                    + Long.toHexString(newIdent) + " while dispatching to "
                    + msg.target.getClass().getName() + " "
                    + msg.callback + " what=" + msg.what);
        }

        msg.recycle();
    }
}

大致流程如下：

• 首先通過(guò)上述myLooper()方法獲取Looper對(duì)象，取出Looper持有的MessageQueue辱士；
• 然后從MessageQueue取出Message泪掀，如果Message為null，說(shuō)明線程正在退出颂碘；
• Message不為空异赫，則調(diào)用Message的target handler對(duì)該Message進(jìn)行分發(fā)，具體分發(fā)头岔、處理流程可參考《Android線程管理——線程通信》塔拳；
• 消息處理完畢，調(diào)用recycle()方法進(jìn)行回收峡竣。