概述
搞Android的不懂Handler消息循环机制，都不好意思说自己是Android工程师。面试的时候一般也都会问这个知识点，但是我相信大多数码农肯定是没有看过相关源码的，顶多也就是网上搜搜，看看别人的文章介绍。学姐不想把那个万能的关系图拿出来讨论。
学姐先从我们平时的使用方法引出这个机制，再结合源码进行分析。
我们平时使用是这样的：
//1. 主线程
Handler handler = new MyHandler();
//2. 非主线程
HandlerThread handlerThread = new HandlerThread("handlerThread");
handlerThread.start();
Handler handler = new Handler(handlerThread.getLooper());
//发送消息
handler.sendMessage(msg);
//接收消息
static class MyHandler extends Handler {
//对于非主线程处理消息需要传Looper，主线程有默认的sMainLooper
public MyHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
}
}那么为什么初始化的时候，我们执行了1或2，后面只需要sendMessage就可处理任务了呢？学姐这里以非主线程为例进行介绍，handlerThread.start()的时候，实际上创建了一个用于消息循环的Looper和消息队列MessageQueue，同时启动了消息循环，并将这个循环传给Handler，这个循环会从MessageQueue中依次取任务出来执行。用户若要执行某项任务，只需要调用handler.sendMessage即可，这里做的事情是将消息添加到MessaeQueue中。对于主线程也类似，只是主线程sMainThread和sMainLooper不需要我们主动去创建，程序启动的时候Application就创建好了，我们只需要创建Handler即可。
我们这里提到了几个概念：
HandlerThread 支持消息循环的线程
Handler 消息处理器
Looper 消息循环对象
MessageQueue 消息队列
Message 消息体
对应关系是：一对多，即（一个）HandlerThread、Looper、MessageQueue －> （多个）Handler、Message
源码解析
1. Looper
（1）创建消息循环
prepare()用于创建Looper消息循环对象。Looper对象通过一个成员变量ThreadLocal进行保存。
（2）获取消息循环对象
myLooper()用于获取当前消息循环对象。Looper对象从成员变量ThreadLocal中获取。
（3）开始消息循环
loop()开始消息循环。循环过程如下：
每次从消息队列MessageQueue中取出一个Message
使用Message对应的Handler处理Message
已处理的Message加到本地消息池，循环复用
循环以上步骤，若没有消息表明消息队列停止，退出循环
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 Looper myLooper() {
return sThreadLocal.get();}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.recycleUnchecked();
}}2. Handler
（1）发送消息
Handler支持2种消息类型，即Runnable和Message。因此发送消息提供了post(Runnable r)和sendMessage(Message msg)两个方法。从下面源码可以看出Runnable赋值给了Message的callback，最终也是封装成Message对象对象。学姐个人认为外部调用不统一使用Message，应该是兼容Java的线程任务，学姐认为这种思想也可以借鉴到平常开发过程中。发送的消息都会入队到MessageQueue队列中。
（2）处理消息
Looper循环过程的时候，是通过dispatchMessage(Message msg)对消息进行处理。处理过程：先看是否是Runnable对象，如果是则调用handleCallback(msg)进行处理，最终调到Runnable.run()方法执行线程；如果不是Runnable对象，再看外部是否传入了Callback处理机制，若有则使用外部Callback进行处理；若既不是Runnable对象也没有外部Callback，则调用handleMessage(msg)，这个也是我们开发过程中最常覆写的方法了。
（3）移除消息
removeCallbacksAndMessages()，移除消息其实也是从MessageQueue中将Message对象移除掉。
public void handleMessage(Message msg) {}
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}}private static void handleCallback(Message message) {
message.callback.run();}public final Message obtainMessage(){
return Message.obtain(this);}public final boolean post(Runnable r){
return sendMessageDelayed(getPostMessage(r), 0);}public final boolean sendMessage(Message msg){
return sendMessageDelayed(msg, 0);}private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;}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);}private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);}public final void removeCallbacksAndMessages(Object token) {
mQueue.removeCallbacksAndMessages(this, token);}3. MessageQueue
（1）消息入队
消息入队方法enqueueMessage(Message msg, long when)。其处理过程如下：
待入队的Message标记为InUse，when赋值
若消息链表mMessages为空为空，或待入队Message执行时间小于mMessage链表头，则待入队Message添加到链表头
若不符合以上条件，则轮询链表，根据when从低到高的顺序，插入链表合适位置
（2）消息轮询
next()依次从MessageQueue中取出Message
（3）移除消息
removeMessages()可以移除消息，做的事情实际上就是将消息从链表移除，同时将移除的消息添加到消息池，提供循环复用。
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("MessageQueue", 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;}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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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("MessageQueue", "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;
}}void removeMessages(Handler h, int what, Object object) {
if (h == null) {
return;
}
synchronized (this) {
Message p = mMessages;
// Remove all messages at front.
while (p != null && p.target == h && p.what == what && (object == null || p.obj == object)) {
Message n = p.next;
mMessages = n;
p.recycleUnchecked();
p = n;
}
// Remove all messages after front.
while (p != null) {
Message n = p.next;
if (n != null) {
if (n.target == h && n.what == what && (object == null || n.obj == object)) {
Message nn = n.next;
n.recycleUnchecked();
p.next = nn;
continue;
}
}
p = n;
}
}}4. Message
（1）消息创建
Message.obtain()创建消息。若消息池链表sPool不为空，则从sPool中获取第一个，flags标记为UnInUse，同时从sPool中移除，sPoolSize减1；若消息池链表sPool为空，则new Message()
（2）消息释放
recycle()将消息释放，从内部实现recycleUnchecked()可知，将flags标记为InUse，其他各种状态清零，同时将Message添加到sPool，且sPoolSize加1
/**
* 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();}/**
* Return a Message instance to the global pool.
* <p>
* You MUST NOT touch the Message after calling this function because it has
* effectively been freed. It is an error to recycle a message that is currently
* enqueued or that is in the process of being delivered to a Handler.
* </p>
*/public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();}/**
* 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) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}}5. HandlerThread
由于Java中的Thread是没有消息循环机制的，run()方法执行完，线程则结束。HandlerThread通过使用Looper实现了消息循环，只要不主动调用HandlerThread或Looper的quit()方法，循环就是一直走下去。
public class HandlerThread extends Thread {int mPriority;int mTid = -1;Looper mLooper;public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;}@Overridepublic 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;}}总结
关键类：HandlerThread、Handler、Looper、MessageQueue、Messaga
MessageQueue数据结构，链表。
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