Message：线程间通讯的消息体
Handler: 主要是负责发送消息，和接收消息
MessageQueue：负责以队列的方式存储消息
Looper： 就是一直轮询的从MessageQueue中取消息，获取到消息就通过dispatchMessage()将消息发送给Handler去处理。
举例理解一下：
平常生活中，从网上购物，商家把一个商品打包好后，将邮件投递给了快递公司，快递公司就从投递的网点取出来邮件，然后根据邮件上的地址将邮件发送给收件人，那么这里的邮件就是Message , 而快递公司的收寄网点就像是MessageQueue ,然后快递公司取到邮件后按照地址发送邮件，就类似Looper , 最后收件人接收快递。
这样理解，就可以明白这其实就是一种生产者消费者的模式。
Handler原理.png
一、Message消息
1、Message创建
下面三种获取消息的方式基本上类似：
从全局池中获取新的消息实例，这样就可以达到复用用过的消息，避免创建销毁消息对象，这样性能和内存上都比直接new一个消息要好。
//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 target member on the Message returned.
* @param h Handler to assign to the returned Message object's target member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
//这里也可以给message设置一个处理message的callback ，在Handler处理消息的时候使用
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
二、Handler消息发送与消费流程
1、发送消息
//Handler.java
public final boolean sendMessage(@NonNull Message msg) {
//注意这里传入的第二个参数delayMillis 为 0
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
//继续进入sendMessageAtTime()
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
//这里为什么要讲成员变量mQueue赋值给一个局部变量呢？笔者猜想这样如果出现对mQueue的多线程操作，就不会导致阻塞；
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
//这里传入enqueueMessage的第三个参数不在是0 ，已经修改了的
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;//这里的this就是当前的Handler,这样msg就和当前的Handler绑定在一起了
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
//这里就会进入到MessageQueue的enqueueMessage()方法中去
return queue.enqueueMessage(msg, uptimeMillis);
}
至此，Handler通过sendMessage 一步一步将msg 加入到MessageQueue中，这样就完成了消息的发送。
2、消费消息
根据上面的Handler运行机制我们可以知道，从消息队列获取消息是在Looper的loop中通过一个无限循环来完成，然后通过获取到的消息target（也就是在发送消息时传入的Handler）来讲消息dispatchMessage()分发出去。关于Looper中获取消息会在第四部分分析，这里就只从Handler的dispatchMessage（）开始。
//Handler.java
/**
* Handle system messages here.
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
//1）处理消息1：如果在msg中设置了callback ,那么就会从这里进行消息的处理
handleCallback(msg);
} else {
//2）处理消息2：这里的mCallback是一个接口类型，也就说需要将该接口的实例对象传递进来就会走这里的分支，调用其handleMessage()方法
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//3）处理消息1：重写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. 消息创建时传入callback ，就会使用消息自带的callback来处理消息
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
//2. 给Handler设置callback
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class 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;
}
消费消息有三种方式：
- 在创建Message设置自己的callback
- 在创建Handler消息是，给Handler对象设置callback(注意这类构造是hide)
- 重写Handler的handleMessage()方法（这种也是最常见的）
Handler的工作流程：
Handler通过发送一个消息，将消息加入到消息队列中，然后Looper从消息队列中取出消息后通过Handler的dispatchMessage()将消息分发出去，消息的处理可以有Message自带的callback 、handler的callback 或者重写的handler的handleMessage()来完成消息的消费。
三、MessageQueue分析
MessageQueue作为一个存储消息的队列容器，那么他的核心就是消息的存储和取出。
1、消息入队(enqueueMessage)
- 在入队时使用了Synchronized锁，锁的是this，也就是说对同一个MessageQueue对象的所有调用者来说，都是互斥的，他们必须等到上一个调用者释放了锁，后面调用者才能执行锁中的代码；
- 在消息加入队列的时候，会按照消息执行的时间顺序进行队列的排序；
//MessageQueue.java
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
//这里的synchronized锁需要注意一下
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、消息出队(next)
消息出队主要就是将队列的首部取出，因为在入队的时候已经按照时间进行了排序；
在取消息时也使用了synchronized锁，这个锁是用来针对调用者enqueueMessage、next只能执行一个操作，不能同时进行，这样就可以保证消息的不同线程访问的时候有序的进行。
//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);
}//这里就synchronized就结束了
// 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
根据上面的内容，消息的发送是同步的（即按照排队顺序一条一条执行），如果有一条紧急的消息需要处理时，按照常规就需要继续排队，等到满足了自己执行的条件再处理该消息，那么这时候同步屏障就来了，同步屏障说白了就是开通的消息的绿色通道。
例如：在高速上遇到堵车的情况，常规就应该排队等待，高速的应急车道就可以理解成是同步屏障，这样可以处理一些紧急的事情。
3.3.1 设置同步屏障：
//MessageQueue.java
public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}
//注意下下面并没有给msg设置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取消息时处理同步屏障
- 由于在设置同步屏障时并没有给msg设置target ，所有就会进入下面的do while里；
- while的条件（isAsynchronous 默认是false ，msg 也不为Null）,所以会循环执行
- msg 每一次循环都会获取下一条消息，也就是会变量消息队列中的所有消息执行；
- 屏障在执行，下面的同步代码块就不会执行，需要等待；
- 需要溢出同步屏障后方可执行后面的同步代码块；
//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) {
// 这里就是处理同步屏障消息的
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
//.......此处省略了很多代码
}
}
}
//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的创建
- 创建Looper时，给mQueue构造方法传入了是否允许退出的值为true ,是因为Message的quit()方法，如果传入的是false ，那么调用quit()会抛出 ' Main thread not allowed to quit. ' 的异常
- 创建主线程Looper其实和prepare基本上是一致的，只是传入的quitAllowed的值为false
//Looper.java
public static void prepare() {
//这里默认传入的是否允许退出为：true
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//这里进入Looper的构造函数，构造的参数传入的是false
//把创建好的Looper对象加入到ThreadLocal中
sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
//这里创建了一个消息队列
mQueue = new MessageQueue(quitAllowed);
//获取当前线程
mThread = Thread.currentThread();
}
//》》》》》》》》》》》》》》》》》》》》》》》》》》》》
//准备一个主线程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开始工作
- 获取创建好的Looper对象
- 从Looper对象中获取消息队列
- 通过一个死循环从消息队列中去消息，如果消息队列中有消息，就调用消息体中的Target进行DispatchMessage()
//Looper.java
public static void loop() {
//这里取出来之前创建好的Looper对象
final Looper me = myLooper();
if (me == null) {
//如果没有Looper对象，就会抛出需要调用prepare()的异常
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
for (;;) {
//这里是一个死循环，从消息队列中取消息
Message msg = queue.next(); // might block
if (msg == null) {
return;
}
try {
//这里通过消息体中的target调用dispatchMessage()来分发消息
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结束工作
//Looper.java
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
总结：
一个线程对应一个Looper, 一个Looper包含一个MessageQueue,这样多个Handler在发送消息时，通过MessageQueue中的同步锁来达到线程同步的目的，消息队列采用链表的结构来对消息进行排序，Looper通过一个无限循环从消息队列中取消息。然后再通过Message的Target进行dispatchMessage,然后进入Handler的hanleMessage()处理消息。