# Handler Looper Message Thread

首先要闡述幾者之間的關系。 Thread 可以擁有多個handler對象； Thread 只能擁有一個Looper 和一個MessageQueue。

Looper 只能屬于一個Thread， 并且只能和MessageQueue 一一對應。 looper的在幾者中的作用是什么呢！ Looper的作用就是起到 發動機的原理，當然它不是讓車跑起來，而是讓MessageQueue里的message被執行。 那么 Message被誰執行呢？ 后文即會提到。

MessageQueue 也僅是和一個looper綁定，在出生的時候即決定了這件事，后面在代碼中會解釋為什么！ MessageQueue里面存放就是 Message。

Looper

首先需要關注的是該方法。

public static void prepare() {prepare(true); } 復制代碼

參數為是否允許退出，答案是肯定的 true; 只有一種情況即主線程調用prepare時傳遞false,因為主線程不允許退出。 該方法即為 預熱發動機的入口。讓 Looper這臺機器進行啟動之前的準備工作。

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)); } 復制代碼

分析一下 是如何判斷已經prepare的呢？ sThreadLocal.get() != null 那就需要看一下set是什么東東。就是準備的是什么呢？

/*** Sets the current thread's copy of this thread-local variable* to the specified value. Most subclasses will have no need to* override this method, relying solely on the {@link #initialValue}* method to set the values of thread-locals.** @param value the value to be stored in the current thread's copy of* this thread-local.*/ public void set(T value) {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value); } 復制代碼

這個value就是上文提到的 new Looper(quitAllowed) createMap創建的是一個ThreadLocalMap。 每一個線程僅有一個ThreadLocalMap， 在該map中存儲內容為該線程本地變量的副本。ThreadLocalMap使用及注意事項以后單獨開講。

void createMap(Thread t, T firstValue) {t.threadLocals = new ThreadLocalMap(this, firstValue); } 復制代碼

當第一次sThreadLocal.get()時，會返回setInitialValue=null;

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

當一個線程只能有一個Looper之后也就意味著只能有一個MessageQueue.class

Looper.loop即為啟動發動機的入口，啟動之后開始進行消息輪詢，并且注釋說明一定要調用quit()退出輪詢。 Looper一直把MesageQueue所有的message執行完。 每執行完一個后即通過next拿到下一個message.

public static void loop() { --- for (;;) {Message msg = queue.next(); // might blockif (msg == null) {// No message indicates that the message queue is quitting.return;} //當隊列中沒有消息之后 即退出。// This must be in a local variable, in case a UI event sets the loggerfinal 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); //msg.target即為執行message工具。} 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();} } 復制代碼

MessageQueue MessageQueue和Looper之間有個緊密的聯系就是通過 MessageQueue.next()方法。以next方法為切入點介紹MessageQueue.class

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 iterationint nextPollTimeoutMillis = 0;for (;;) {if (nextPollTimeoutMillis != 0) {Binder.flushPendingCommands();}//natvie層進行阻塞,后文在Looper.c中介紹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.// 當因為有 "同步分隔欄" 引起停滯后， 將要找到下一個異步消息， // 同步分隔欄后面的同步消息并不會執行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.//如果當前的msg沒有準備好，那么就下次輪詢進入到等待。//計算等待時間nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);} else {// Got a message.//標記當次輪詢不會被wait,不需要被喚醒mBlocked = false;//當在鏈表隊列中找到可執行msg,把當前message調出，并修復原鏈接if (prevMsg != null) {prevMsg.next = msg.next;} else {mMessages = msg.next;}msg.next = null;if (DEBUG) Log.v(TAG, "Returning message: " + msg);//標記當前msg被使用狀態msg.markInUse();return msg;}} else {// No more messages.nextPollTimeoutMillis = -1;}// Process the quit message now that all pending messages have been handled.//如果looper調用了quit, messagequeue也進行退出操作。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.// 引入了另外一個messagequeue的功能, idle handles的處理，// 當隊列為空的時候或沒有任務可執行的時候，執行idle handles內容。if (pendingIdleHandlerCount < 0&& (mMessages == null || now < mMessages.when)) {pendingIdleHandlerCount = mIdleHandlers.size();}if (pendingIdleHandlerCount <= 0) {// No idle handlers to run. Loop and wait some more.// 既沒有idle handlers 和message可以處理那么就需要阻塞,入隊時候就需要喚醒。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 handlerboolean keep = false;try {//執行idler中的回調，并且有返回值，true意味著想要保持這個idle下次繼續執行，//false則會從隊列中移除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;} } 復制代碼

下面繼續介紹enqueueMessage，入隊操作由Handler.class執行。后文提到其中幾種入隊操作。

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) {//如果looper已經條用quit，那么就放棄入隊。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.// 如果messagequeue中沒有message或者需要立即執行或者插入message時間優于對頭// message所需要執行時間，那么就把msg插入到對頭。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.//通常情況下將目標message插入到隊里中間時，是不需要喚醒隊列的，//除非有一個"同步分隔欄"在對頭或者目標message是最早需要執行的異步message。needWake = mBlocked && p.target == null && msg.isAsynchronous();Message prev;for (;;) {prev = p;p = p.next;if (p == null || when < p.when) {//找到最后一個位置，或者時間排序上晚于目標message的位置break;}//當需要喚醒，但是 要插入目標message的前面所有位置的message//只要有異步消息的話既不需要喚醒。if (needWake && p.isAsynchronous()) {needWake = false;}}// 將目標message插入到理想位置，修復整個鏈接msg.next = p; // invariant: p == prev.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {//此處為喚醒 LoopernativeWake(mPtr);}}return true; } 復制代碼

上面介紹了MessageQueue的兩個主要方法next()和enqueueMessage()，其中涉及到了兩個native層的本地方法分別為： nativePollOnce(ptr, nextPollTimeoutMillis); nativeWake(mPtr); 那么下面介紹一下這兩個方法。 方法在/frameworks/base/core/jni/android_os_MessageQueue.cpp中進行了定義。

static JNINativeMethod gMessageQueueMethods[] = {/* name, signature, funcPtr */{ "nativeInit", "()V", (void*)android_os_MessageQueue_nativeInit },{ "nativeDestroy", "()V", (void*)android_os_MessageQueue_nativeDestroy },{ "nativePollOnce", "(II)V", (void*)android_os_MessageQueue_nativePollOnce },{ "nativeWake", "(I)V", (void*)android_os_MessageQueue_nativeWake } }; 復制代碼static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,jint ptr, jint timeoutMillis) {NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);nativeMessageQueue->pollOnce(timeoutMillis); } 復制代碼

最終調用到Looper::pollOnce====>Looper::pollInner

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData); int Looper::pollInner(int timeoutMillis); 復制代碼int Looper::pollInner(int timeoutMillis) {--- #ifdef LOOPER_USES_EPOLLstruct epoll_event eventItems[EPOLL_MAX_EVENTS];//通過Epoll進行阻塞int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis); #else// Wait for wakeAndLock() waiters to run then set mPolling to true.mLock.lock();while (mWaiters != 0) {mResume.wait(mLock);}mPolling = true;mLock.unlock();size_t requestedCount = mRequestedFds.size();int eventCount = poll(mRequestedFds.editArray(), requestedCount, timeoutMillis); #endif--- }復制代碼

其中最終運用epoll進行控制（epoll不再本文討論感興趣讀者可自行查詢！ ）。下面引入《深入理解Android：卷II》對pollOnce解釋：

其中四個參數： timeoutMillis參數為超時等待時間。如果值為–1，則表示無限等待，直到有事件發生為止。如果值為0，則無須等待立即返回。 outFd用來存儲發生事件的那個文件描述符。 outEvents用來存儲在該文件描述符上發生了哪些事件，目前支持可讀、可寫、錯誤和中斷4個事件。這4個事件其實是從epoll事件轉化而來的。后面我們會介紹大名鼎鼎的epoll。 outData用于存儲上下文數據，這個上下文數據是由用戶在添加監聽句柄時傳遞的，它的作用和pthread_create函數最后一個參數param一樣，用來傳遞用戶自定義的數據。 另外，pollOnce函數的返回值也具有特殊的意義，具體如下： 當返回值為ALOOPER_POLL_WAKE時，表示這次返回是由wake函數觸發的，也就是管道寫端的那次寫事件觸發的。 返回值為ALOOPER_POLL_TIMEOUT表示等待超時。 返回值為ALOOPER_POLL_ERROR表示等待過程中發生錯誤。 返回值為ALOOPER_POLL_CALLBACK表示某個被監聽的句柄因某種原因被觸發。這時，outFd參數用于存儲發生事件的文件句柄，outEvents用于存儲所發生的事件。

MessageQueue還有其他公開方法：

用來添加IdleHandler，當沒有message需要立即處理時就會處理IdleHandler。

void addIdleHandler(@NonNull IdleHandler handler); void removeIdleHandler(@NonNull IdleHandler handler); 復制代碼

用來添加需要監聽的文件描述符fd

void addOnFileDescriptorEventListener(@NonNull FileDescriptor fd,@OnFileDescriptorEventListener.Events int events,@NonNull OnFileDescriptorEventListener listener); void removeOnFileDescriptorEventListener(@NonNull FileDescriptor fd); 復制代碼

Message.class 主要是handler要處理的信使，主要功能攜帶參數。下面主要介紹handler參數。

/*** User-defined message code so that the recipient can identify * what this message is about. Each {@link Handler} has its own name-space* for message codes, so you do not need to worry about yours conflicting* with other handlers.*/ public int what; //定義在handler中要執行的事件/*** arg1 and arg2 are lower-cost alternatives to using* {@link #setData(Bundle) setData()} if you only need to store a* few integer values.*/ public int arg1; //如果要存儲簡單的參數，使用arg1和arg2就可以/*** arg1 and arg2 are lower-cost alternatives to using* {@link #setData(Bundle) setData()} if you only need to store a* few integer values.*/ public int arg2;/*** An arbitrary object to send to the recipient. When using* {@link Messenger} to send the message across processes this can only* be non-null if it contains a Parcelable of a framework class (not one* implemented by the application). For other data transfer use* {@link #setData}.* * <p>Note that Parcelable objects here are not supported prior to* the {@link android.os.Build.VERSION_CODES#FROYO} release.*/ public Object obj; //可存儲任意類型參數/*** Optional Messenger where replies to this message can be sent. The* semantics of exactly how this is used are up to the sender and* receiver.*/ public Messenger replyTo; //可實現跨進程通信，后面會獨立章節進行講解。/*** Optional field indicating the uid that sent the message. This is* only valid for messages posted by a {@link Messenger}; otherwise,* it will be -1.*/ public int sendingUid = -1; //與Messenger 配合使用/*package*/ int flags; //0x00 非使用， 0x01被使用：當入隊和被回收的時候會設置為1 //0x10 表示為異步/*package*/ long when; //延遲執行時間/*package*/ Bundle data; //存儲一些復雜數據/*package*/ Handler target; //執行該message的handler/*package*/ Runnable callback; //hanlder執行該message時，如果有callback即執行該callback// sometimes we store linked lists of these things /*package*/ Message next; //保存鏈表 復制代碼

主要解析一下該函數

/** * Return a new Message instance from the global pool. Allows us to* avoid allocating new objects in many cases.*/ public static Message obtain() {//sPoolSync 同步鎖synchronized (sPoolSync) {//sPool指向鏈表的頭if (sPool != null) {Message m = sPool;sPool = m.next;m.next = null;m.flags = 0; // clear in-use flagsPoolSize--;//將sPool取出，并斷鏈return m;}}//如果鏈中沒有元素，重新分配return new Message(); }/*** Recycles a Message that may be in-use.* Used internally by the MessageQueue and Looper when disposing of queued Messages.*/ void recycleUnchecked() {// Mark the message as in use while it remains in the recycled object pool.// Clear out all other details.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) {//鏈表不會無限增長if (sPoolSize < MAX_POOL_SIZE) {next = sPool;sPool = this;sPoolSize++;//將該message插入頭部}} } 復制代碼

Handler 先比較前幾個Class， Handler比較簡單，成員只有以下幾個：

final Looper mLooper; final MessageQueue mQueue; final Callback mCallback; final boolean mAsynchronous; IMessenger mMessenger; 復制代碼

先看幾個比較重要的構造方法：

//常用的為無參構造形式 public Handler() {this(null, false); }//這是無參構造方法調用的真正構造方法， public Handler(Callback callback, boolean async) {//FIND_POTENTIAL_LEAKS//將此標志設置為true以檢測擴展的Handler類, 擴展的handler類如果不是靜態的匿名,本地或成員類, //則會產生泄漏。我們常見構造時的警告說明!至于消除警告方法一般是設置成靜態或弱引用。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是來自于sThreadLocal中ThreadLocalMap中 通過調用線程ID存儲的looper,唯一mLooper = Looper.myLooper();if (mLooper == null) {throw new RuntimeException("Can't create handler inside thread that has not called Looper.prepare()");}//mqueue來自looper， 也唯一mQueue = mLooper.mQueue;mCallback = callback;//標示該handler發送的數據是否為異步數據。mAsynchronous = async; } 復制代碼

通過分析構造方法可驗證前文提到的 handler 僅對應一個looper MessageQueue,，翻過來不成立，也就是說會有多個handler綁定在同一個Looper中。

通過調用post(Runnable r)； postDelayed(Runnable r, long delayMillis);sendMessage(Message msg)；等方法發送的時間，最終調用下面的方法。

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); }private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {msg.target = this;//如果為異步，則對每一個message進行設置。if (mAsynchronous) {msg.setAsynchronous(true);}//調用enqueueMessage 進行入隊Messagereturn queue.enqueueMessage(msg, uptimeMillis); } 復制代碼

還有另外一種入隊方法，需要介紹：

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;}//與enqueueMessage差別為uptimeMillis=0. 在messagequeue中當遇到when=0時，//會將該message放在對頭進行處理return enqueueMessage(queue, msg, 0); } 復制代碼

在需要注意下，這個remove方法，當傳入null時可將MessageQueue中的所有數據remove掉。

public final void removeCallbacksAndMessages(Object token) {mQueue.removeCallbacksAndMessages(this, token);} 復制代碼

回過頭來說一下上面的 同步分隔欄，

在Api 23 之， 通過MessageQueue 進行調用

/*** Posts a synchronization barrier to the Looper's message queue.** Message processing occurs as usual until the message queue encounters the* synchronization barrier that has been posted. When the barrier is encountered,* later synchronous messages in the queue are stalled (prevented from being executed)* until the barrier is released by calling {@link #removeSyncBarrier} and specifying* the token that identifies the synchronization barrier.** This method is used to immediately postpone execution of all subsequently posted* synchronous messages until a condition is met that releases the barrier.* Asynchronous messages (see {@link Message#isAsynchronous} are exempt from the barrier* and continue to be processed as usual.** This call must be always matched by a call to {@link #removeSyncBarrier} with* the same token to ensure that the message queue resumes normal operation.* Otherwise the application will probably hang!** @return A token that uniquely identifies the barrier. This token must be* passed to {@link #removeSyncBarrier} to release the barrier.** @hide*/// 該方法為hide, 正常寫代碼是調用不到的。 public int postSyncBarrier() {return postSyncBarrier(SystemClock.uptimeMillis()); }// The next barrier token. // Barriers are indicated by messages with a null target whose arg1 field carries the token. // 同步分隔欄消息沒有target, 并且arg1用來記錄token private int mNextBarrierToken;private int postSyncBarrier(long when) {// Enqueue a new sync barrier token.// We don't need to wake the queue because the purpose of a barrier is to stall it.synchronized (this) {final int token = mNextBarrierToken++;final Message msg = Message.obtain();msg.markInUse();msg.when = when;msg.arg1 = token;Message prev = null;Message p = mMessages;if (when != 0) {while (p != null && p.when <= when) {prev = p;p = p.next;}}if (prev != null) { // invariant: p == prev.nextmsg.next = p;prev.next = msg;} else {msg.next = p;mMessages = msg;}return token;} } 復制代碼

這個同步分隔message有什么作用呢？ 對開發者沒有明顯的提供，那么就是在系統及別使用。在ViewRootImpl.java中進行了使用。

void scheduleTraversals() {if (!mTraversalScheduled) {mTraversalScheduled = true;mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);if (!mUnbufferedInputDispatch) {scheduleConsumeBatchedInput();}notifyRendererOfFramePending();pokeDrawLockIfNeeded();} } 復制代碼

為了讓View能夠有快速的布局和繪制，ViewRootImpl在開始measure和layout ViewTree時，會向主線程的Handler添加同步分隔message,這樣后續的消息隊列中的同步的消息將不會被執行，以免會影響到UI繪制，但是只有異步消息才能被執行。如果想要使用postSyncBarrier() 那么就需要使用反射進行使用。

總結 Looper、MessageQueue 和 Thread 一一對應。 Handler 需要綁定到一個Looper中， 一個Looper可以有多個Handler。

這是第一文章，以后會多多寫的。歡迎各位指正問題！謝謝。 sy_dqs@163.com

轉載于:https://juejin.im/post/5ae841a451882567336a84a9

總結

以上是生活随笔為你收集整理的小葵花妈妈课堂开课了：《Handler Looper Message 浅析》的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。