Android的消息機(jī)制

提到消息機(jī)制,想必大家都不陌生吧诵竭,在日常開發(fā)中不可避免要涉及到這方面的內(nèi)容直晨。從開發(fā)的角度來說,Handler是Android的消息機(jī)制的上層接口症虑,這使得在開發(fā)過程中只需要和Handler交互即可缩歪。Handler的使用過程很簡(jiǎn)單,通過它可以輕松地將一個(gè)任務(wù)切換到Handler所在的線程中去執(zhí)行谍憔。由于Android的開發(fā)規(guī)范的限制匪蝙,我們并不能在子線程中訪問UI控件,否則就會(huì)觸發(fā)程序異常习贫,這個(gè)時(shí)候通過Handler就可以將更新的UI的操作切換到主線程中執(zhí)行逛球,因此從本質(zhì)上來來說,Handler并不是專門用于更新UI的苫昌,它只是常被開發(fā)者用來更新UI颤绕。

Android中的消息機(jī)制主要指Handler的運(yùn)行機(jī)制,Handler的運(yùn)行需要底層的MessageQueue和Looper的支撐祟身。MessageQueue翻譯過來就是消息隊(duì)列奥务,它內(nèi)部存儲(chǔ)了一組消息,以隊(duì)列的形式對(duì)外提供插入和刪除的過程袜硫,雖然叫做消息隊(duì)列氯葬,但是它內(nèi)部存儲(chǔ)結(jié)構(gòu)并不是真正的隊(duì)列,而是采用單鏈表的數(shù)據(jù)結(jié)構(gòu)來存儲(chǔ)消息列表婉陷,Looper翻譯過來就是循環(huán)帚称,這里可以理解為消息循環(huán)。由于MessageQueue只是一個(gè)消息的存儲(chǔ)單元憨攒,它不能去處理消息世杀,而Looper填補(bǔ)了這個(gè)功能阀参,Looper會(huì)無限循環(huán)的形式去查找是否有新的消息肝集,如果有的話就處理消息,否則就中一直等待蛛壳。Looper中還有一個(gè)特殊的概念杏瞻,那就是ThreadLocal,Threadlocal并不是線程衙荐,它的作用是可以在每個(gè)線程中存儲(chǔ)數(shù)據(jù)捞挥。

我們知道,Handler創(chuàng)建的時(shí)候會(huì)采用當(dāng)前線程的Looper來構(gòu)造消息循環(huán)系統(tǒng)忧吟,那么Handler內(nèi)部如何獲取到當(dāng)前線程的Looper呢砌函,這就要使用ThreadLocal了,ThreadLocal可以在不同的線程中互不干擾地存儲(chǔ)并提供數(shù)據(jù),通過ThreadLocal可以輕松獲取每個(gè)線程的Looper讹俊。需要注意的是垦沉,線程是默認(rèn)沒有Looper的,如果需要使用Handler就必須為線程創(chuàng)建Looper仍劈,我們經(jīng)常提到的主線程厕倍,也叫UI線程,它就是ActivityThread贩疙,ActivityThread被創(chuàng)建時(shí)就會(huì)初始化Looper讹弯,這也是在主線程中默認(rèn)可以使用Handler的原因。

Android的消息機(jī)制概述

我們知道Handler的主要作用是將一個(gè)任務(wù)切換到某個(gè)指定的線程中去執(zhí)行这溅,那么Android為什么要提供這個(gè)功能呢组民,這是因?yàn)锳ndroid規(guī)定訪問UI只能在主線程中進(jìn)行,如果子線程中訪問UI芍躏,那么程序就會(huì)拋出異常邪乍。

  void checkThread() {
        if (mThread != Thread.currentThread()) {
            throw new CalledFromWrongThreadException(
                    "Only the original thread that created a view hierarchy can touch its views.");
        }
    }

這是ViewRootImpl的checkThread方法,從這段代碼就可以看出对竣,如果不在當(dāng)前線程庇楞,就會(huì)拋出異常。同時(shí)呢否纬,Android不建議在主線程中進(jìn)行耗時(shí)操作吕晌, 否則會(huì)導(dǎo)致程序無法響應(yīng),即ANR临燃。那么系統(tǒng)為什么允許在子線程中訪問UI呢睛驳,這是因?yàn)锳ndroid中的UI控件并不是線程安全,它同時(shí)也延伸了Java系統(tǒng)中默認(rèn)進(jìn)程的話會(huì)產(chǎn)生默認(rèn)的單線程習(xí)慣膜廊,當(dāng)用戶點(diǎn)擊乏沸、滑動(dòng)等事件操作時(shí),UI線程是負(fù)責(zé)分發(fā)的爪瓜,統(tǒng)一管理會(huì)更高效點(diǎn)蹬跃,采取單線程來處理UI操作,對(duì)于開發(fā)者來說也不是很麻煩铆铆,只是需要通過Handler切換下UI訪問的執(zhí)行線程即可蝶缀。

簡(jiǎn)單描述下Handler的工作原理,Handler創(chuàng)建完畢后薄货,這個(gè)時(shí)候內(nèi)部的Looper以及MessageQueue就可以和Handler一起協(xié)同工作翁都,然后通過Handler的post方法將一個(gè)Runnable投遞到Handler內(nèi)部的Looper中去處理,也可以通過Handler的send方法發(fā)送一個(gè)消息谅猾,這個(gè)消息同樣會(huì)在Looper中去處理柄慰。

Android消息機(jī)制分析

先看下整體的架構(gòu)圖:

整體UML圖
  • Looper有一個(gè)MessageQueue消息隊(duì)列
  • MessageQueue有一組待處理的Message
  • Message中有一個(gè)用于處理消息的Handler
  • Handler中有Looper和MessageQueue

Looper的工作原理

Looper在Android的消息機(jī)制扮演著消息循環(huán)的角色鳍悠,具體來說就是它會(huì)不停地從MessageQueue中查看是否有新消息過來,如果有新的消息的就會(huì)立刻處理坐搔,否則就一直阻塞在那里贼涩。首先看下它的構(gòu)造方法:

  private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
  }

在構(gòu)造方法中,它會(huì)創(chuàng)建一個(gè)MessageQueue對(duì)象薯蝎,然后將當(dāng)前線程的對(duì)象給保存起來遥倦。我們知道,Handler的工作需要Looper占锯,沒有Looper線程就會(huì)報(bào)錯(cuò)袒哥,那么如何為一個(gè)線程創(chuàng)建Looper呢,有以下方法:

    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));
    }

    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

從中我們可以看出消略,每個(gè)線程只有一個(gè)Looper堡称,多創(chuàng)建一個(gè)會(huì)報(bào)錯(cuò),然后prepareMainLooper這個(gè)方法主要給主線程也就是ActivityThread創(chuàng)建Looper使用艺演,其本質(zhì)也是通過prepare方法來實(shí)現(xiàn)的却紧。

    public void quit() {
        mQueue.quit(false);
    }

    public void quitSafely() {
        mQueue.quit(true);
    }

Looper提供了quit和quitSafely方法退出一個(gè)Looper,這兩者最主要區(qū)別在于一個(gè)設(shè)定退出標(biāo)記胎撤,一個(gè)是把消息隊(duì)列中的已有消息處理完畢后才安全地退出晓殊。

當(dāng)然還有Looper的loop方法是最核心的。

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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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.recycleUnchecked();
        }
    }

這個(gè)也比較好理解伤提,loop方法是一個(gè)死循環(huán)巫俺,唯一跳出循環(huán)的方式就是MessageQueue的next方法返回了null。Looper就會(huì)調(diào)用MessageQueue的quit或者quitSafely方法來通知消息隊(duì)列退出肿男,當(dāng)消息隊(duì)列被標(biāo)記為退出狀態(tài)時(shí)介汹,它的next方法就會(huì)返回null,也就是說looper必須退出舶沛,否則loop方法就會(huì)無限循序下去嘹承。

MessageQueue工作原理

在Android中MessageQueue主要包含兩個(gè)操作:插入和讀取。讀取操作本身會(huì)伴隨著刪除操作如庭,插入和讀取對(duì)應(yīng)的方法分別為enqueueMessage和next叹卷,其中enqueueMessage的作用是往消息隊(duì)列中插入一條消息,而next的作用是從消息隊(duì)列中取出一條消息并將其從消息隊(duì)列中移除柱彻。在MessageQueue內(nèi)部通過一個(gè)單鏈表的數(shù)據(jù)結(jié)構(gòu)來維護(hù)消息列表豪娜,單鏈表在插入和刪除上比較有優(yōu)勢(shì)餐胀。

看下enqueueMessage代碼:

boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        if (msg.isInUse()) {
            throw new IllegalStateException(msg + " This message is already in use.");
        }

        synchronized (this) {
            if (mQuitting) {
                IllegalStateException e = new IllegalStateException(
                        msg.target + " sending message to a Handler on a dead thread");
                Log.w(TAG, e.getMessage(), e);
                msg.recycle();
                return false;
            }

            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // New head, wake up the event queue if blocked.
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

            // We can assume mPtr != 0 because mQuitting is false.
            if (needWake) {
                nativeWake(mPtr);
            }
        }
        return true;
    }

主要操作其實(shí)就是單鏈表的插入操作哟楷。

看下next代碼:

 Message next() {
        // Return here if the message loop has already quit and been disposed.
        // This can happen if the application tries to restart a looper after quit
        // which is not supported.
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }

            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                if (msg != null && msg.target == null) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

                // Process the quit message now that all pending messages have been handled.
                if (mQuitting) {
                    dispose();
                    return null;
                }

                // If first time idle, then get the number of idlers to run.
                // Idle handles only run if the queue is empty or if the first message
                // in the queue (possibly a barrier) is due to be handled in the future.
                if (pendingIdleHandlerCount < 0
                        && (mMessages == null || now < mMessages.when)) {
                    pendingIdleHandlerCount = mIdleHandlers.size();
                }
                if (pendingIdleHandlerCount <= 0) {
                    // No idle handlers to run.  Loop and wait some more.
                    mBlocked = true;
                    continue;
                }

                if (mPendingIdleHandlers == null) {
                    mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
                }
                mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
            }

            // Run the idle handlers.
            // We only ever reach this code block during the first iteration.
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }

            // Reset the idle handler count to 0 so we do not run them again.
            pendingIdleHandlerCount = 0;

            // While calling an idle handler, a new message could have been delivered
            // so go back and look again for a pending message without waiting.
            nextPollTimeoutMillis = 0;
        }
    }

可以發(fā)現(xiàn)next方法就是一個(gè)無限循環(huán)的方法,如果消息隊(duì)列中沒有消息否灾,那么next方法就會(huì)一直阻塞在這里卖擅,當(dāng)有新消息到來時(shí),next方法會(huì)返回這條消息并將其從單鏈表中移除。

Message

每個(gè)消息用Message表示惩阶,Message主要包含以下內(nèi)容:

數(shù)據(jù)類型 成員變量 解釋
int what 消息類別
long when 消息觸發(fā)時(shí)間
int arg1 參數(shù)1
int arg2 參數(shù)2
Object obj 消息內(nèi)容
Handler target 消息響應(yīng)方
Runnable callback 回調(diào)方法

創(chuàng)建消息的過程挎狸,就是填充消息的上述內(nèi)容的一項(xiàng)或多項(xiàng)。

消息池

在代碼中断楷,可能經(jīng)诚谴遥看到recycle()方法,咋一看冬筒,可能是在做虛擬機(jī)的gc()相關(guān)的工作恐锣,其實(shí)不然,這是用于把消息加入到消息池的作用舞痰。這樣的好處是土榴,當(dāng)消息池不為空時(shí),可以直接從消息池中獲取Message對(duì)象响牛,而不是直接創(chuàng)建,提高效率。

靜態(tài)變量sPool的數(shù)據(jù)類型為Message背蟆,通過next成員變量豌研,維護(hù)一個(gè)消息池;靜態(tài)變量MAX_POOL_SIZE代表消息池的可用大斜岽浴坯台;消息池的默認(rèn)大小為50。

消息池常用的操作方法是obtain()和recycle()瘫寝。

public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null; //從sPool中取出一個(gè)Message對(duì)象蜒蕾,并消息鏈表斷開
            m.flags = 0; // 清除in-use flag
            sPoolSize--; //消息池的可用大小進(jìn)行減1操作
            return m;
        }
    }
    return new Message(); // 當(dāng)消息池為空時(shí),直接創(chuàng)建Message對(duì)象
}

obtain()焕阿,從消息池取Message咪啡,都是把消息池表頭的Message取走,再把表頭指向next暮屡。

public void recycle() {
    if (isInUse()) { //判斷消息是否正在使用
        if (gCheckRecycle) { //Android 5.0以后的版本默認(rèn)為true,之前的版本默認(rèn)為false.
            throw new IllegalStateException("This message cannot be recycled because it is still in use.");
        }
        return;
    }
    recycleUnchecked();
}

//對(duì)于不再使用的消息撤摸,加入到消息池
void recycleUnchecked() {
    //將消息標(biāo)示位置為IN_USE,并清空消息所有的參數(shù)褒纲。
    flags = FLAG_IN_USE;
    what = 0;
    arg1 = 0;
    arg2 = 0;
    obj = null;
    replyTo = null;
    sendingUid = -1;
    when = 0;
    target = null;
    callback = null;
    data = null;
    synchronized (sPoolSync) {
        if (sPoolSize < MAX_POOL_SIZE) { //當(dāng)消息池沒有滿時(shí)准夷,將Message對(duì)象加入消息池
            next = sPool;
            sPool = this;
            sPoolSize++; //消息池的可用大小進(jìn)行加1操作
        }
    }
}

recycle(),將Message加入到消息池的過程莺掠,都是把Message加到鏈表的表頭衫嵌。

Handler工作原理

Handler的工作主要包含消息的發(fā)送和接收過程。消息發(fā)送可以通過post的一系列的方法以及send的一系列方法來實(shí)現(xiàn)彻秆,post其實(shí)也是通過send的方法來實(shí)現(xiàn)的楔绞。

看下Handler的構(gòu)造方法结闸。

    public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

從中可以看到關(guān)聯(lián)MessageQueue、Looper酒朵,所以在Handler之前Looper要prepare先桦锄,如果沒有Looper的話,就會(huì)拋出“Can't create handler inside thread that has not called Looper.prepare()”這句話蔫耽。

    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

    public final boolean sendEmptyMessage(int what)
    {
        return sendEmptyMessageDelayed(what, 0);
    }

    public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageDelayed(msg, delayMillis);
     }

     public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
        Message msg = Message.obtain();
        msg.what = what;
        return sendMessageAtTime(msg, uptimeMillis);
      }

     public final boolean sendMessageDelayed(Message msg, long delayMillis)
     {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
     }

     public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
      }

     public final boolean sendMessageAtFrontOfQueue(Message msg) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, 0);
      }

從中可以看出结耀,最終都是調(diào)用sendMessageAtTime/sendMessageAtFrontOfQueue方法,進(jìn)而執(zhí)行enqueueMessage方法匙铡,最終把消息發(fā)送到MessageQueue隊(duì)列中饼记。

相關(guān)消息發(fā)送方式

那么消息又是如何在Handler處理的呢?

    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

通過dispatchMessage來處理消息的慰枕。

ThreadLocal工作原理

ThreadLocal是一個(gè) 線程內(nèi)部的數(shù)據(jù)存儲(chǔ)類具则,通過它可以在指定的線程中存儲(chǔ)數(shù)據(jù),數(shù)據(jù)存儲(chǔ)以后具帮,只有在指定線程中可以獲取到存儲(chǔ)的數(shù)據(jù)博肋,對(duì)于其他線程來說是無法獲取到數(shù)據(jù)。在日常開發(fā)中用到ThreadLocal的場(chǎng)景很少蜂厅,但是在某些特殊的場(chǎng)景下匪凡,通過ThreadLocal可以輕松地實(shí)現(xiàn)一些看起來很復(fù)雜的功能,這一點(diǎn)在Android源碼中也有所體現(xiàn)掘猿,比如Looper病游、ActivityThread以及AMS中都用到ThreadLocal。

ThreadLocal.set(T value):將value存儲(chǔ)到當(dāng)前線程的TLS區(qū)域稠通。

  public void set(T value) {
      Thread currentThread = Thread.currentThread(); //獲取當(dāng)前線程
      Values values = values(currentThread); //查找當(dāng)前線程的本地儲(chǔ)存區(qū)
      if (values == null) {
          //當(dāng)線程本地存儲(chǔ)區(qū)衬衬,尚未存儲(chǔ)該線程相關(guān)信息時(shí),則創(chuàng)建Values對(duì)象
          values = initializeValues(currentThread);
      }
      //保存數(shù)據(jù)value到當(dāng)前線程this
      values.put(this, value);
  }

在set方法中改橘,首先會(huì)通過values方法來獲取當(dāng)前線程的ThreadLocal數(shù)據(jù)滋尉,通過put方式去獲取。

ThreadLocal.get():獲取當(dāng)前線程TLS區(qū)域的數(shù)據(jù)飞主。

public T get() {
      Thread currentThread = Thread.currentThread(); //獲取當(dāng)前線程
      Values values = values(currentThread); //查找當(dāng)前線程的本地儲(chǔ)存區(qū)
      if (values != null) {
          Object[] table = values.table;
          int index = hash & values.mask;
          if (this.reference == table[index]) {
              return (T) table[index + 1]; //返回當(dāng)前線程儲(chǔ)存區(qū)中的數(shù)據(jù)
          }
      } else {
          //創(chuàng)建Values對(duì)象
          values = initializeValues(currentThread);
      }
      return (T) values.getAfterMiss(this); //從目標(biāo)線程存儲(chǔ)區(qū)沒有查詢是則返回null
  }

get方法同樣是取出當(dāng)前線程的localValues對(duì)象狮惜,如果這個(gè)對(duì)象為null,那么就返回初始值碌识。

在Looper源碼中碾篡,有這么一句:

   // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

從ThreadLocal的set和get方法可以看出,它們所操作的對(duì)象都是當(dāng)前線程的localValues對(duì)象的table數(shù)組筏餐,因此在不同線程中訪問同一個(gè)ThreadLocal的set和get方法开泽,它們對(duì)ThreadLocal所做的讀/寫操作僅限于各自線程的內(nèi)部,這也就是為什么ThreadLocal可以在多個(gè)線程中互不干擾地存儲(chǔ)和修改數(shù)據(jù)胖烛。

所以眼姐,整體來說,Handler佩番、Looper众旗、MessageQueue、Message這三者之間的關(guān)系如下:


消息機(jī)制關(guān)系圖

主線程的消息循環(huán)

Android的主線程就是ActivityThread趟畏,主線程的入口方法在main贡歧,在main方法中系統(tǒng)會(huì)通過Looper.prepareMainLooper方法來創(chuàng)建主線程的Looper以及MessageQueue,并通過Looper.loop方法來開啟主線程的消息循環(huán)赋秀。

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());

        // 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");
    }

主線程的消息循環(huán)開始以后利朵,ActivityThread還需要一個(gè)Handler來和消息隊(duì)列進(jìn)行交互,這個(gè)Handler就是ActivityThread.H猎莲,它內(nèi)部定義了一組消息類型绍弟,主要包括了四大組件的啟動(dòng)和停止等過程。


    private class H extends Handler {
        public static final int LAUNCH_ACTIVITY         = 100;
        public static final int PAUSE_ACTIVITY          = 101;
        public static final int PAUSE_ACTIVITY_FINISHING= 102;
        public static final int STOP_ACTIVITY_SHOW      = 103;
        public static final int STOP_ACTIVITY_HIDE      = 104;
        public static final int SHOW_WINDOW             = 105;
        public static final int HIDE_WINDOW             = 106;
        public static final int RESUME_ACTIVITY         = 107;
        public static final int SEND_RESULT             = 108;
        public static final int DESTROY_ACTIVITY        = 109;
        public static final int BIND_APPLICATION        = 110;
        public static final int EXIT_APPLICATION        = 111;
        public static final int NEW_INTENT              = 112;
        public static final int RECEIVER                = 113;
        public static final int CREATE_SERVICE          = 114;
        public static final int SERVICE_ARGS            = 115;
        public static final int STOP_SERVICE            = 116;

        public static final int CONFIGURATION_CHANGED   = 118;
        public static final int CLEAN_UP_CONTEXT        = 119;
        public static final int GC_WHEN_IDLE            = 120;
        public static final int BIND_SERVICE            = 121;
        public static final int UNBIND_SERVICE          = 122;
        public static final int DUMP_SERVICE            = 123;
        public static final int LOW_MEMORY              = 124;
        public static final int ACTIVITY_CONFIGURATION_CHANGED = 125;
        public static final int RELAUNCH_ACTIVITY       = 126;
        public static final int PROFILER_CONTROL        = 127;
        public static final int CREATE_BACKUP_AGENT     = 128;
        public static final int DESTROY_BACKUP_AGENT    = 129;
        public static final int SUICIDE                 = 130;
        public static final int REMOVE_PROVIDER         = 131;
        public static final int ENABLE_JIT              = 132;
        public static final int DISPATCH_PACKAGE_BROADCAST = 133;
        public static final int SCHEDULE_CRASH          = 134;
        public static final int DUMP_HEAP               = 135;
        public static final int DUMP_ACTIVITY           = 136;
        public static final int SLEEPING                = 137;
        public static final int SET_CORE_SETTINGS       = 138;
        public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139;
        public static final int TRIM_MEMORY             = 140;
        public static final int DUMP_PROVIDER           = 141;
        public static final int UNSTABLE_PROVIDER_DIED  = 142;
        public static final int REQUEST_ASSIST_CONTEXT_EXTRAS = 143;
        public static final int TRANSLUCENT_CONVERSION_COMPLETE = 144;
        public static final int INSTALL_PROVIDER        = 145;
        public static final int ON_NEW_ACTIVITY_OPTIONS = 146;
        public static final int CANCEL_VISIBLE_BEHIND = 147;
        public static final int BACKGROUND_VISIBLE_BEHIND_CHANGED = 148;
        public static final int ENTER_ANIMATION_COMPLETE = 149;
        public static final int START_BINDER_TRACKING = 150;
        public static final int STOP_BINDER_TRACKING_AND_DUMP = 151;
        public static final int MULTI_WINDOW_MODE_CHANGED = 152;
        public static final int PICTURE_IN_PICTURE_MODE_CHANGED = 153;
        public static final int LOCAL_VOICE_INTERACTION_STARTED = 154;

        String codeToString(int code) {
            if (DEBUG_MESSAGES) {
                switch (code) {
                    case LAUNCH_ACTIVITY: return "LAUNCH_ACTIVITY";
                    case PAUSE_ACTIVITY: return "PAUSE_ACTIVITY";
                    case PAUSE_ACTIVITY_FINISHING: return "PAUSE_ACTIVITY_FINISHING";
                    case STOP_ACTIVITY_SHOW: return "STOP_ACTIVITY_SHOW";
                    case STOP_ACTIVITY_HIDE: return "STOP_ACTIVITY_HIDE";
                    case SHOW_WINDOW: return "SHOW_WINDOW";
                    case HIDE_WINDOW: return "HIDE_WINDOW";
                    case RESUME_ACTIVITY: return "RESUME_ACTIVITY";
                    case SEND_RESULT: return "SEND_RESULT";
                    case DESTROY_ACTIVITY: return "DESTROY_ACTIVITY";
                    case BIND_APPLICATION: return "BIND_APPLICATION";
                    case EXIT_APPLICATION: return "EXIT_APPLICATION";
                    case NEW_INTENT: return "NEW_INTENT";
                    case RECEIVER: return "RECEIVER";
                    case CREATE_SERVICE: return "CREATE_SERVICE";
                    case SERVICE_ARGS: return "SERVICE_ARGS";
                    case STOP_SERVICE: return "STOP_SERVICE";
                    case CONFIGURATION_CHANGED: return "CONFIGURATION_CHANGED";
                    case CLEAN_UP_CONTEXT: return "CLEAN_UP_CONTEXT";
                    case GC_WHEN_IDLE: return "GC_WHEN_IDLE";
                    case BIND_SERVICE: return "BIND_SERVICE";
                    case UNBIND_SERVICE: return "UNBIND_SERVICE";
                    case DUMP_SERVICE: return "DUMP_SERVICE";
                    case LOW_MEMORY: return "LOW_MEMORY";
                    case ACTIVITY_CONFIGURATION_CHANGED: return "ACTIVITY_CONFIGURATION_CHANGED";
                    case RELAUNCH_ACTIVITY: return "RELAUNCH_ACTIVITY";
                    case PROFILER_CONTROL: return "PROFILER_CONTROL";
                    case CREATE_BACKUP_AGENT: return "CREATE_BACKUP_AGENT";
                    case DESTROY_BACKUP_AGENT: return "DESTROY_BACKUP_AGENT";
                    case SUICIDE: return "SUICIDE";
                    case REMOVE_PROVIDER: return "REMOVE_PROVIDER";
                    case ENABLE_JIT: return "ENABLE_JIT";
                    case DISPATCH_PACKAGE_BROADCAST: return "DISPATCH_PACKAGE_BROADCAST";
                    case SCHEDULE_CRASH: return "SCHEDULE_CRASH";
                    case DUMP_HEAP: return "DUMP_HEAP";
                    case DUMP_ACTIVITY: return "DUMP_ACTIVITY";
                    case SLEEPING: return "SLEEPING";
                    case SET_CORE_SETTINGS: return "SET_CORE_SETTINGS";
                    case UPDATE_PACKAGE_COMPATIBILITY_INFO: return "UPDATE_PACKAGE_COMPATIBILITY_INFO";
                    case TRIM_MEMORY: return "TRIM_MEMORY";
                    case DUMP_PROVIDER: return "DUMP_PROVIDER";
                    case UNSTABLE_PROVIDER_DIED: return "UNSTABLE_PROVIDER_DIED";
                    case REQUEST_ASSIST_CONTEXT_EXTRAS: return "REQUEST_ASSIST_CONTEXT_EXTRAS";
                    case TRANSLUCENT_CONVERSION_COMPLETE: return "TRANSLUCENT_CONVERSION_COMPLETE";
                    case INSTALL_PROVIDER: return "INSTALL_PROVIDER";
                    case ON_NEW_ACTIVITY_OPTIONS: return "ON_NEW_ACTIVITY_OPTIONS";
                    case CANCEL_VISIBLE_BEHIND: return "CANCEL_VISIBLE_BEHIND";
                    case BACKGROUND_VISIBLE_BEHIND_CHANGED: return "BACKGROUND_VISIBLE_BEHIND_CHANGED";
                    case ENTER_ANIMATION_COMPLETE: return "ENTER_ANIMATION_COMPLETE";
                    case MULTI_WINDOW_MODE_CHANGED: return "MULTI_WINDOW_MODE_CHANGED";
                    case PICTURE_IN_PICTURE_MODE_CHANGED: return "PICTURE_IN_PICTURE_MODE_CHANGED";
                    case LOCAL_VOICE_INTERACTION_STARTED: return "LOCAL_VOICE_INTERACTION_STARTED";
                }
            }
            return Integer.toString(code);
        }
        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, "LAUNCH_ACTIVITY");
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case RELAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;
                    handleRelaunchActivity(r);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case PAUSE_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handlePauseActivity((IBinder) args.arg1, false,
                            (args.argi1 & USER_LEAVING) != 0, args.argi2,
                            (args.argi1 & DONT_REPORT) != 0, args.argi3);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case PAUSE_ACTIVITY_FINISHING: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handlePauseActivity((IBinder) args.arg1, true, (args.argi1 & USER_LEAVING) != 0,
                            args.argi2, (args.argi1 & DONT_REPORT) != 0, args.argi3);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case STOP_ACTIVITY_SHOW: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handleStopActivity((IBinder) args.arg1, true, args.argi2, args.argi3);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case STOP_ACTIVITY_HIDE: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handleStopActivity((IBinder) args.arg1, false, args.argi2, args.argi3);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case SHOW_WINDOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityShowWindow");
                    handleWindowVisibility((IBinder)msg.obj, true);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case HIDE_WINDOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityHideWindow");
                    handleWindowVisibility((IBinder)msg.obj, false);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case RESUME_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handleResumeActivity((IBinder) args.arg1, true, args.argi1 != 0, true,
                            args.argi3, "RESUME_ACTIVITY");
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SEND_RESULT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDeliverResult");
                    handleSendResult((ResultData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case DESTROY_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDestroy");
                    handleDestroyActivity((IBinder)msg.obj, msg.arg1 != 0,
                            msg.arg2, false);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case BIND_APPLICATION:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
                    AppBindData data = (AppBindData)msg.obj;
                    handleBindApplication(data);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case EXIT_APPLICATION:
                    if (mInitialApplication != null) {
                        mInitialApplication.onTerminate();
                    }
                    Looper.myLooper().quit();
                    break;
                case NEW_INTENT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityNewIntent");
                    handleNewIntent((NewIntentData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case RECEIVER:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveComp");
                    handleReceiver((ReceiverData)msg.obj);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CREATE_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
                    handleCreateService((CreateServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case BIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
                    handleBindService((BindServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UNBIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceUnbind");
                    handleUnbindService((BindServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SERVICE_ARGS:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceStart: " + String.valueOf(msg.obj)));
                    handleServiceArgs((ServiceArgsData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStop");
                    handleStopService((IBinder)msg.obj);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CONFIGURATION_CHANGED:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "configChanged");
                    mCurDefaultDisplayDpi = ((Configuration)msg.obj).densityDpi;
                    mUpdatingSystemConfig = true;
                    handleConfigurationChanged((Configuration)msg.obj, null);
                    mUpdatingSystemConfig = false;
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CLEAN_UP_CONTEXT:
                    ContextCleanupInfo cci = (ContextCleanupInfo)msg.obj;
                    cci.context.performFinalCleanup(cci.who, cci.what);
                    break;
                case GC_WHEN_IDLE:
                    scheduleGcIdler();
                    break;
                case DUMP_SERVICE:
                    handleDumpService((DumpComponentInfo)msg.obj);
                    break;
                case LOW_MEMORY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "lowMemory");
                    handleLowMemory();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case ACTIVITY_CONFIGURATION_CHANGED:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityConfigChanged");
                    handleActivityConfigurationChanged((ActivityConfigChangeData) msg.obj,
                            msg.arg1 == 1 ? REPORT_TO_ACTIVITY : !REPORT_TO_ACTIVITY);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case PROFILER_CONTROL:
                    handleProfilerControl(msg.arg1 != 0, (ProfilerInfo)msg.obj, msg.arg2);
                    break;
                case CREATE_BACKUP_AGENT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "backupCreateAgent");
                    handleCreateBackupAgent((CreateBackupAgentData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case DESTROY_BACKUP_AGENT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "backupDestroyAgent");
                    handleDestroyBackupAgent((CreateBackupAgentData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SUICIDE:
                    Process.killProcess(Process.myPid());
                    break;
                case REMOVE_PROVIDER:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "providerRemove");
                    completeRemoveProvider((ProviderRefCount)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case ENABLE_JIT:
                    ensureJitEnabled();
                    break;
                case DISPATCH_PACKAGE_BROADCAST:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastPackage");
                    handleDispatchPackageBroadcast(msg.arg1, (String[])msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SCHEDULE_CRASH:
                    throw new RemoteServiceException((String)msg.obj);
                case DUMP_HEAP:
                    handleDumpHeap(msg.arg1 != 0, (DumpHeapData)msg.obj);
                    break;
                case DUMP_ACTIVITY:
                    handleDumpActivity((DumpComponentInfo)msg.obj);
                    break;
                case DUMP_PROVIDER:
                    handleDumpProvider((DumpComponentInfo)msg.obj);
                    break;
                case SLEEPING:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "sleeping");
                    handleSleeping((IBinder)msg.obj, msg.arg1 != 0);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SET_CORE_SETTINGS:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "setCoreSettings");
                    handleSetCoreSettings((Bundle) msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UPDATE_PACKAGE_COMPATIBILITY_INFO:
                    handleUpdatePackageCompatibilityInfo((UpdateCompatibilityData)msg.obj);
                    break;
                case TRIM_MEMORY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "trimMemory");
                    handleTrimMemory(msg.arg1);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UNSTABLE_PROVIDER_DIED:
                    handleUnstableProviderDied((IBinder)msg.obj, false);
                    break;
                case REQUEST_ASSIST_CONTEXT_EXTRAS:
                    handleRequestAssistContextExtras((RequestAssistContextExtras)msg.obj);
                    break;
                case TRANSLUCENT_CONVERSION_COMPLETE:
                    handleTranslucentConversionComplete((IBinder)msg.obj, msg.arg1 == 1);
                    break;
                case INSTALL_PROVIDER:
                    handleInstallProvider((ProviderInfo) msg.obj);
                    break;
                case ON_NEW_ACTIVITY_OPTIONS:
                    Pair<IBinder, ActivityOptions> pair = (Pair<IBinder, ActivityOptions>) msg.obj;
                    onNewActivityOptions(pair.first, pair.second);
                    break;
                case CANCEL_VISIBLE_BEHIND:
                    handleCancelVisibleBehind((IBinder) msg.obj);
                    break;
                case BACKGROUND_VISIBLE_BEHIND_CHANGED:
                    handleOnBackgroundVisibleBehindChanged((IBinder) msg.obj, msg.arg1 > 0);
                    break;
                case ENTER_ANIMATION_COMPLETE:
                    handleEnterAnimationComplete((IBinder) msg.obj);
                    break;
                case START_BINDER_TRACKING:
                    handleStartBinderTracking();
                    break;
                case STOP_BINDER_TRACKING_AND_DUMP:
                    handleStopBinderTrackingAndDump((ParcelFileDescriptor) msg.obj);
                    break;
                case MULTI_WINDOW_MODE_CHANGED:
                    handleMultiWindowModeChanged((IBinder) msg.obj, msg.arg1 == 1);
                    break;
                case PICTURE_IN_PICTURE_MODE_CHANGED:
                    handlePictureInPictureModeChanged((IBinder) msg.obj, msg.arg1 == 1);
                    break;
                case LOCAL_VOICE_INTERACTION_STARTED:
                    handleLocalVoiceInteractionStarted((IBinder) ((SomeArgs) msg.obj).arg1,
                            (IVoiceInteractor) ((SomeArgs) msg.obj).arg2);
                    break;
            }
            Object obj = msg.obj;
            if (obj instanceof SomeArgs) {
                ((SomeArgs) obj).recycle();
            }
            if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
        }

ActivityThread通過ApplicationThread和AMS進(jìn)行進(jìn)程間通信著洼,AMS以進(jìn)程間通信的方式完成ActivityThread的請(qǐng)求回調(diào)ApplicationThread中Binder方法然后ApplicationThread向H發(fā)送消息樟遣,H收到消息后會(huì)將ApplicationThread的邏輯切換到ActivityThread中去執(zhí)行,即切換到主線程中去執(zhí)行身笤,整個(gè)過程就是主線程的消息循環(huán)模型豹悬。

HandlerThread

HandlerThread類的源碼:

public class HandlerThread extends Thread {
    int mPriority;
    int mTid = -1;
    Looper mLooper;

    public HandlerThread(String name) {
        super(name);
        mPriority = Process.THREAD_PRIORITY_DEFAULT;
    }
   
    public HandlerThread(String name, int priority) {
        super(name);
        mPriority = priority;
    }
  
    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }
    
    public Looper getLooper() {
        if (!isAlive()) {
            return null;
        }
        
        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

    public boolean quit() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quit();
            return true;
        }
        return false;
    }
  
    public boolean quitSafely() {
        Looper looper = getLooper();
        if (looper != null) {
            looper.quitSafely();
            return true;
        }
        return false;
    }

    /**
     * Returns the identifier of this thread. See Process.myTid().
     */
    public int getThreadId() {
        return mTid;
    }
}

可以看到HandlerThread繼承于Thread類,在獲取Looper對(duì)象時(shí)候液荸,當(dāng)線程已經(jīng)啟動(dòng)瞻佛,則等待直到looper創(chuàng)建完成才能獲取,從本質(zhì)上看HandlerThread是對(duì)Thread的封裝娇钱,主要用途在于多個(gè)線程的通信伤柄,會(huì)有同步的問題,那么Android對(duì)此直接提供了HandlerThread類文搂。

HandlerThread實(shí)戰(zhàn)

在HandlerThread線程中運(yùn)行Loop()方法响迂,在其他線程中通過Handler發(fā)送消息到HandlerThread線程。通過wait/notifyAll的方式细疚,有效地解決了多線程的同步問題蔗彤。從源碼中我們也可以看到當(dāng)looper沒獲取成功就會(huì)阻塞,然后有運(yùn)行完就會(huì)去喚醒所有阻塞的線程疯兼。

// Step 1: 創(chuàng)建并啟動(dòng)HandlerThread線程然遏,內(nèi)部包含Looper
HandlerThread handlerThread = new HandlerThread("test");
handlerThread.start();

// Step 2: 創(chuàng)建Handler
Handler handler = new Handler(handlerThread.getLooper());

// Step 3: 發(fā)送消息
handler.post(new Runnable() {

        @Override
        public void run() {
            System.out.println("thread id="+Thread.currentThread().getId());
        }
    });
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