Message：線程間通訊的消息體
Handler: 主要是負(fù)責(zé)發(fā)送消息，和接收消息
MessageQueue：負(fù)責(zé)以隊(duì)列的方式存儲(chǔ)消息
Looper： 就是一直輪詢的從MessageQueue中取消息，獲取到消息就通過(guò)dispatchMessage()將消息發(fā)送給Handler去處理。
舉例理解一下：
平常生活中，從網(wǎng)上購(gòu)物，商家把一個(gè)商品打包好后，將郵件投遞給了快遞公司，快遞公司就從投遞的網(wǎng)點(diǎn)取出來(lái)郵件，然后根據(jù)郵件上的地址將郵件發(fā)送給收件人，那么這里的郵件就是Message , 而快遞公司的收寄網(wǎng)點(diǎn)就像是MessageQueue ,然后快遞公司取到郵件后按照地址發(fā)送郵件，就類似Looper , 最后收件人接收快遞。
這樣理解，就可以明白這其實(shí)就是一種生產(chǎn)者消費(fèi)者的模式。
Handler原理.png
一、Message消息
1、Message創(chuàng)建
下面三種獲取消息的方式基本上類似：
從全局池中獲取新的消息實(shí)例，這樣就可以達(dá)到復(fù)用用過(guò)的消息，避免創(chuàng)建銷毀消息對(duì)象，這樣性能和內(nèi)存上都比直接new一個(gè)消息要好。
//Message.java
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
/**
* Same as {@link #obtain()}, but copies the values of an existing
* message (including its target) into the new one.
* @param orig Original message to copy.
* @return A Message object from the global pool.
*/
public static Message obtain(Message orig) {
Message m = obtain();
m.what = orig.what;
m.arg1 = orig.arg1;
m.arg2 = orig.arg2;
m.obj = orig.obj;
m.replyTo = orig.replyTo;
m.sendingUid = orig.sendingUid;
m.workSourceUid = orig.workSourceUid;
if (orig.data != null) {
m.data = new Bundle(orig.data);
}
m.target = orig.target;
m.callback = orig.callback;
return m;
}
/**
* Same as {@link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
//這里也可以給message設(shè)置一個(gè)處理message的callback ，在Handler處理消息的時(shí)候使用
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
二、Handler消息發(fā)送與消費(fèi)流程
1、發(fā)送消息
//Handler.java
public final boolean sendMessage(@NonNull Message msg) {
//注意這里傳入的第二個(gè)參數(shù)delayMillis 為 0
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
//繼續(xù)進(jìn)入sendMessageAtTime()
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
//這里為什么要講成員變量mQueue賦值給一個(gè)局部變量呢？筆者猜想這樣如果出現(xiàn)對(duì)mQueue的多線程操作，就不會(huì)導(dǎo)致阻塞；
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
//這里傳入enqueueMessage的第三個(gè)參數(shù)不在是0 ，已經(jīng)修改了的
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;//這里的this就是當(dāng)前的Handler,這樣msg就和當(dāng)前的Handler綁定在一起了
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//這里就會(huì)進(jìn)入到MessageQueue的enqueueMessage()方法中去
return queue.enqueueMessage(msg, uptimeMillis);
}
至此，Handler通過(guò)sendMessage 一步一步將msg 加入到MessageQueue中，這樣就完成了消息的發(fā)送。
2、消費(fèi)消息
根據(jù)上面的Handler運(yùn)行機(jī)制我們可以知道，從消息隊(duì)列獲取消息是在Looper的loop中通過(guò)一個(gè)無(wú)限循環(huán)來(lái)完成，然后通過(guò)獲取到的消息target（也就是在發(fā)送消息時(shí)傳入的Handler）來(lái)講消息dispatchMessage()分發(fā)出去。關(guān)于Looper中獲取消息會(huì)在第四部分分析，這里就只從Handler的dispatchMessage（）開(kāi)始。
//Handler.java
/**
* Handle system messages here.
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
//1）處理消息1：如果在msg中設(shè)置了callback ,那么就會(huì)從這里進(jìn)行消息的處理
handleCallback(msg);
} else {
//2）處理消息2：這里的mCallback是一個(gè)接口類型，也就說(shuō)需要將該接口的實(shí)例對(duì)象傳遞進(jìn)來(lái)就會(huì)走這里的分支，調(diào)用其handleMessage()方法
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//3）處理消息1：重寫handleMessage
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(@NonNull Message msg) {
}
//>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
//1. 消息創(chuàng)建時(shí)傳入callback ，就會(huì)使用消息自帶的callback來(lái)處理消息
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
//2. 給Handler設(shè)置callback
public Handler(@Nullable 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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
消費(fèi)消息有三種方式：
- 在創(chuàng)建Message設(shè)置自己的callback
- 在創(chuàng)建Handler消息是，給Handler對(duì)象設(shè)置callback(注意這類構(gòu)造是hide)
- 重寫Handler的handleMessage()方法（這種也是最常見(jiàn)的）
Handler的工作流程：
Handler通過(guò)發(fā)送一個(gè)消息，將消息加入到消息隊(duì)列中，然后Looper從消息隊(duì)列中取出消息后通過(guò)Handler的dispatchMessage()將消息分發(fā)出去，消息的處理可以有Message自帶的callback 、handler的callback 或者重寫的handler的handleMessage()來(lái)完成消息的消費(fèi)。
三、MessageQueue分析
MessageQueue作為一個(gè)存儲(chǔ)消息的隊(duì)列容器，那么他的核心就是消息的存儲(chǔ)和取出。
1、消息入隊(duì)(enqueueMessage)
- 在入隊(duì)時(shí)使用了Synchronized鎖，鎖的是this，也就是說(shuō)對(duì)同一個(gè)MessageQueue對(duì)象的所有調(diào)用者來(lái)說(shuō)，都是互斥的，他們必須等到上一個(gè)調(diào)用者釋放了鎖，后面調(diào)用者才能執(zhí)行鎖中的代碼；
- 在消息加入隊(duì)列的時(shí)候，會(huì)按照消息執(zhí)行的時(shí)間順序進(jìn)行隊(duì)列的排序；
//MessageQueue.java
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//這里的synchronized鎖需要注意一下
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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;
}
2、消息出隊(duì)(next)
消息出隊(duì)主要就是將隊(duì)列的首部取出，因?yàn)樵谌腙?duì)的時(shí)候已經(jīng)按照時(shí)間進(jìn)行了排序；
在取消息時(shí)也使用了synchronized鎖，這個(gè)鎖是用來(lái)針對(duì)調(diào)用者enqueueMessage、next只能執(zhí)行一個(gè)操作，不能同時(shí)進(jìn)行，這樣就可以保證消息的不同線程訪問(wèn)的時(shí)候有序的進(jìn)行。
//MessageQueue.java
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);
}//這里就synchronized就結(jié)束了
// 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;
}
}
3、同步屏障
根據(jù)上面的內(nèi)容，消息的發(fā)送是同步的（即按照排隊(duì)順序一條一條執(zhí)行），如果有一條緊急的消息需要處理時(shí)，按照常規(guī)就需要繼續(xù)排隊(duì)，等到滿足了自己執(zhí)行的條件再處理該消息，那么這時(shí)候同步屏障就來(lái)了，同步屏障說(shuō)白了就是開(kāi)通的消息的綠色通道。
例如：在高速上遇到堵車的情況，常規(guī)就應(yīng)該排隊(duì)等待，高速的應(yīng)急車道就可以理解成是同步屏障，這樣可以處理一些緊急的事情。
3.3.1 設(shè)置同步屏障：
//MessageQueue.java
public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}
//注意下下面并沒(méi)有給msg設(shè)置target
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.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
3.3.2 Looper取消息時(shí)處理同步屏障
- 由于在設(shè)置同步屏障時(shí)并沒(méi)有給msg設(shè)置target ，所有就會(huì)進(jìn)入下面的do while里；
- while的條件（isAsynchronous 默認(rèn)是false ，msg 也不為Null）,所以會(huì)循環(huán)執(zhí)行
- msg 每一次循環(huán)都會(huì)獲取下一條消息，也就是會(huì)變量消息隊(duì)列中的所有消息執(zhí)行；
- 屏障在執(zhí)行，下面的同步代碼塊就不會(huì)執(zhí)行，需要等待；
- 需要溢出同步屏障后方可執(zhí)行后面的同步代碼塊；
//MessageQueue.java
Message next() {
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) {
// 這里就是處理同步屏障消息的
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
//.......此處省略了很多代碼
}
}
}
//移除同步屏障
public void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
synchronized (this) {
Message prev = null;
Message p = mMessages;
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
final boolean needWake;
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;
}
p.recycleUnchecked();
// If the loop is quitting then it is already awake.
// We can assume mPtr != 0 when mQuitting is false.
if (needWake && !mQuitting) {
nativeWake(mPtr);
}
}
}
四、Looper 分析
1、Looper的創(chuàng)建
- 創(chuàng)建Looper時(shí)，給mQueue構(gòu)造方法傳入了是否允許退出的值為true ,是因?yàn)镸essage的quit()方法，如果傳入的是false ，那么調(diào)用quit()會(huì)拋出 ' Main thread not allowed to quit. ' 的異常
- 創(chuàng)建主線程Looper其實(shí)和prepare基本上是一致的，只是傳入的quitAllowed的值為false
//Looper.java
public static void prepare() {
//這里默認(rèn)傳入的是否允許退出為：true
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//這里進(jìn)入Looper的構(gòu)造函數(shù)，構(gòu)造的參數(shù)傳入的是false
//把創(chuàng)建好的Looper對(duì)象加入到ThreadLocal中
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
//這里創(chuàng)建了一個(gè)消息隊(duì)列
mQueue = new MessageQueue(quitAllowed);
//獲取當(dāng)前線程
mThread = Thread.currentThread();
}
//》》》》》》》》》》》》》》》》》》》》》》》》》》》》
//準(zhǔn)備一個(gè)主線程Looper
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
2、Looper開(kāi)始工作
- 獲取創(chuàng)建好的Looper對(duì)象
- 從Looper對(duì)象中獲取消息隊(duì)列
- 通過(guò)一個(gè)死循環(huán)從消息隊(duì)列中去消息，如果消息隊(duì)列中有消息，就調(diào)用消息體中的Target進(jìn)行DispatchMessage()
//Looper.java
public static void loop() {
//這里取出來(lái)之前創(chuàng)建好的Looper對(duì)象
final Looper me = myLooper();
if (me == null) {
//如果沒(méi)有Looper對(duì)象，就會(huì)拋出需要調(diào)用prepare()的異常
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
for (;;) {
//這里是一個(gè)死循環(huán)，從消息隊(duì)列中取消息
Message msg = queue.next(); // might block
if (msg == null) {
return;
}
try {
//這里通過(guò)消息體中的target調(diào)用dispatchMessage()來(lái)分發(fā)消息
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
}
}
}
3、Looper結(jié)束工作
//Looper.java
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
總結(jié)：
一個(gè)線程對(duì)應(yīng)一個(gè)Looper, 一個(gè)Looper包含一個(gè)MessageQueue,這樣多個(gè)Handler在發(fā)送消息時(shí)，通過(guò)MessageQueue中的同步鎖來(lái)達(dá)到線程同步的目的，消息隊(duì)列采用鏈表的結(jié)構(gòu)來(lái)對(duì)消息進(jìn)行排序，Looper通過(guò)一個(gè)無(wú)限循環(huán)從消息隊(duì)列中取消息。然后再通過(guò)Message的Target進(jìn)行dispatchMessage,然后進(jìn)入Handler的hanleMessage()處理消息。
