做过android开发的同学，相信对android的消息处理都不陌生。在我们开发应用中也是无时不刻不用到它，相信大家对handler的消息处理都能够熟练使用了，那么本文将从源码角度带领读者完成Handler，MessageQueue，Looper，ThreadLocal之间的联系以及以及从消息发送到完成接收，系统为我们做了些什么。
handler相信大家都不陌生了，要完成线程间的消息通信一定会用到它，而使用handler无非是用到他的sendMessage或post方法，以及handleMessage回调。其实，在整个消息流中也确实是只有这些处理了。主要依赖的还是MessageQueue,Looper与handler之间的联系。
在消息发送与消息处理完成，MessageQueue对于我们一直是不可见的，但我们却必须用到它。MessageQueue作为消息队列，顾名思义是用来存储消息的。而MessageQueue作为消息队列，主要完成消息的插入与删除操作。即enqueueMessage与next方法：
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;
}
通过enqueueMessage方法，将message插入到合适的位置，从这里我们可以看到，MessageQueue并非使用队列作为消息存储的，其实使用了单向链表的结构。
接下来我们看MessageQueue是如何获取消息与删除消息的：
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;
}
}
可以看到在next方法中，一开始就进入了一个死循环 for (;;)，在循环中主要完成对message的查找并返回的操作，当MessageQueue为空时，那么就会一直处于block状态，直到MessageQueue不为空或mQuitting为true.
分析完MessageQueue我们接下来看Looper，其实Looper在消息通信中主要扮演了消息循环的角色，这一点可以从Looper.loop方法中看出：
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();
}
}
在loop中仍然是进行了死循环操作 for (;;)，在循环中不断从获取MessageQueue中获取消息：
Message msg = queue.next();
唯一跳出循环的条件是msg == null。其实当我们调用Looper.quit()时，就是改变MessageQueue的mQuitting为true，使其返回null退出的。
当获取到新消息时，会调用到
msg.target.dispatchMessage(msg);
而msg.target又是一个handler对象，咦，是不是有些熟悉呢？其实一点也没错，这里的handler就是我们一开始在代码里面定义的handler,这一点可以在handler中找到：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
不管你是通过send还是post方式，最终都会调用到这里的，不信自己可以去跟踪下代码。
接下来我们继续跟踪下msg.target.dispatchMessage(msg)；方法
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
到这里我们又回到了handler方法中，又回到了最初的起点：这里会进行条件判断，如果msg.callback不为null,处理handleCallback(msg)，而msg.callback是我们通过handler的post方法传进来的Runnable对象，这一点源码中可以看到：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
继续跟踪getPostMessage(r)方法：
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
可以看到，通过getPostMessage方法，将Runnable对象赋给了Message的callback回调：
而handleCallback最终对调用到Runnalbe的run方法：
private static void handleCallback(Message message) {
message.callback.run();
}
而第二个判断mCallback != null是作为构造参数传给handler，是我们自己定义实现的callback回调；最后一种实现是handleMessage(msg);是一个空实现，是由我们自己继承handleMessage(msg)来实现的。
到这里我们从消息发送到消息接收并处理整个流程都跑通了，是不是该结束了呢？
不，因为还有一个核心的东西我们还没有提到，而它正是我们可以完成跨线程通信的核心机制，就是我们开始说到的ThreadLocal，ThreadLocal并不是本地线程，甚至连一个新线程也不算，为什么要叫ThreadLocal？他跟Thread有什么关系？这里我们要从Looper的生命周期开始看起：
在我们创建handler时，如果我们没有在构造中传入Looper，那么handler会在构造方法中初始化Looper.
mLooper = Looper.myLooper();
在Looper.myLooper()中：
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
这里调用到了sThreadLocal.get()方法，而sThreadLocal正是ThreadLocal对象，而在sThreadLocal.get()方法中：
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
这里用到了ThreadLocalMap类，主要用作数据存储的，而在getMap(t)方法中，
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
返回了Thread的threadLocals对象，也是ThreadLocalMap的对象，其实ThreadLocalMap是用作数据存储的，而ThreadLocalMap既是ThreadLocal的内部类，又是Thread的成员变量，不过通过ThreadLocal的set和get方法我们可以得知其实ThreadLocal操作的就是当前线程的ThreadLocalMap对象，通过调用Looper.parpare()为当前线程的ThreadLocalMap对象设置looper对象：
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));
}
在sThreadLocal.set(new Looper(quitAllowed))方法中：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
当第一次创建Map时，
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
将Looper对象最终传给了当前线程的ThreadLocalMap对象并缓存，这就是为什么Looper和线程为什么可以绑定了，因为每一个Thread对应了一个Looper对象，当我们通过调用当前线程的Looper.parpare时，其实就已经将 Looper与当前线程绑定在了一起，而Looper持有了MessageQueue的引用，所以接下来的操作都绑定在了Looper所关联的线程中了。所以handler处理消息回调时所在的线程并不一定是在handler的创建线程。
不信我们可以试试在子线程中创建handler，但在handler构造中传入主线程的Looper,并在回调中打印当前线程名。
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