Android消息机制
Android中的消息机制主要是指Handler机制、消息队列、消息循环处理的一种工作方式，通过该消息机制可以实现进程间通信、任务在线程间的切换。涉及到的类主要有Handler、Looper、ThreadLocal（线程相关）、MessageQueue，这四个类进行协同工作，来处理消息。
Handler
Handler的作用是将一个任务切换到指定的某个线程中去执行，通过它的一系列send方法和post方法将消息发送到消息队列中，然后通过Looper循环去取出这些消息并处理，这一系列发送消息的方法最终会调用到sendMessageAtTime方法，sendMessageAtTime的源码：
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);
}
//往消息队列（MessageQueue ）中插入消息
return queue.enqueueMessage(msg, uptimeMillis);
}Handler的创建会依赖于当前线程的Looper，如果当前线程没有Looper那么就会抛出异常。Handler的构造方法：
public Handler(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();
//mLooper为null就会抛出异常
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}Looper
Looper是一个无限循环，循环取消息队列中的消息进行处理，如果消息队列中没有消息，那么Looper就会阻塞，直到有新消息的到来才会被唤醒。每一个线程都对应一个自己的Looper，线程默认是没有Looper的，但是在应用启动的时候ActivityThread会初始化主线程的Looper，所以主线程可以认为默认是有Looper的，这也就是为什么主线程中可以直接使用Handler的原因。子线程是没有Looper的，如果要在子线程中使用Looper就需要手动创建Looper并开启循环，创建Looper的方法是prepare方法，开启循环是loop方法，这两个方法源码：
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
//创建Looper对象
sThreadLocal.set(new Looper(quitAllowed));
}
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 (;;) {//无条件的死循环
//注意这行代码，是从消息队列中取消息，如果没有消息，这行代码会阻塞
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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
//msg.target指代的是发送消息的Handler
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}通过分析上面的源码可知，在prepare方法中会创建Looper的对象，在loop方法中通过一个无条件的for循环来实现无限循环，通过MessageQueue的next方法获取消息，如果没有消息就通过MessageQueue的next方法实现阻塞等待新消息到来。
msg.target.dispatchMessage(msg)：这行代码是消息的分发处理，msg.target指代的是发送该消息的Handler，这个时候该消息就切换到发送消息的Handler中去处理了，也就意味着该任务切换到该Handler所在的线程中处理了。比如，在线程1中的run方法里创建一个Handler，然后在线程2中调用该Handler发送消息到Handler对应的消息队列中，然后在由Handler对应的Looper取出该消息交给该Handler的dispatchMessage（在Handler对应的线程里面执行的，也就是线程1）方法分发处理该消息，这样任务就从线程2切换到线程1中处理了。
Looper可以调用quit（立刻退出）和quitSafely（消息执行完才退出）来退出，如果是自己在子线程中手动开启的looper，消息执行完的时候一定要退出Looper（当前线程才能终止），否则这个线程就一直不会结束（Looper一直在循环）。
ThreadLocal
上面介绍Looper的时候我们知道，Looper是跟线程相绑定的，也就是一个线程对应一个自己私有的独立的Looper。那么如何实现这个功能呢，ThreadLocal可以解决这个问题。
ThreadLocal意思是线程本地存储，ThreadLocal里面存储的数据是仅当前线程自己所私有的，其他线程无权访问，多个线程访问同一个ThreadLoacl对象，通过ThreadLoacl的set和get方法的操作，每个线程是相互不影响的。那么就看下ThreadLoacl的set和get方法的源码是如何实现的：
ThreadLocal的set方法：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}由上面的set方法可知，调用ThreadLocal的set方法的时候，会先获取到当前调用的线程，然后会获取线程自己的ThreadLocalMap （t.threadLocals，线程内部的一个变量，每个线程都有一份自己的），然后将值存储到ThreadLocalMap 里面，所以可以知道，每个线程的ThreadLocalMap是完全独立的、相互没有影响的，只有当前线程所拥有，也只能当前线程能操作。
ThreadLocal的get方法：
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}上面的get方法同样是，先根据当前线程获取自己的ThreadLocalMap ，然后在从ThreadLocalMap 里面获取之前set的值，因为ThreadLocalMap 每个线程都有自己的单独的一份，所以获取的值，也是当前线程独有的，不受其他线程的影响。
通过上面的分析可以知道，ThreadLocal确实能实现跟线程绑定的功能，我们在分析Looper的时候发现，Looper的存储和获取是通过ThreadLocal的set、get方法实现的，所以Looper是跟线程进行绑定的，一个线程对应一个自己的单独的Looper。
MessageQueue
消息队列MessageQueue是单链表结构，MessageQueue通过里面的enqueueMessage和next两个方法实现消息的插入和读取，enqueueMessage方法是往消息队列中插入消息，next方法是从消息队列中取出消息返回并从消息队列中删除该消息。next方法里面有个for（；；）死循环，如果没有消息，next方法会阻塞一直等待新消息的到来。MessageQueue的创建是在Looper的构造方法中创建的。下面是相关源码：
Looper构造方法：
private Looper(boolean quitAllowed) {
//创建一个MessageQueue对象
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}enqueueMessage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.
//如果之前next方法是阻塞状态，那么有新消息到来时会进行唤醒
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}next方法：
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();
}
//这行代码很重要，它会出现阻塞
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 {
//msg为null（没有消息）
// 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;
}
}在next方法中调用了nativePollOnce(ptr, nextPollTimeoutMillis)方法，这个方法是一个Native方法，这个方法会出现阻塞，nextPollTimeoutMillis这个参数有三种值：
- 如果nextPollTimeoutMillis=-1，一直阻塞不会超时。
- 如果nextPollTimeoutMillis=0，不会阻塞，立即返回。
- 如果nextPollTimeoutMillis>0，最长阻塞nextPollTimeoutMillis毫秒(超时)，如果期间有程序唤醒会立即返回。
上面的代码中显示，如果 msg==null，也就是没有消息的时候，nextPollTimeoutMillis = -1，这个时候next会阻塞
在enqueueMessage方法中调用了nativeWake(mPtr)方法，如果之前消息循环在阻塞状态下，有新消息到来时，会调用该方法进行唤醒。
上面介绍了消息机制相关的一些类，至于Handler机制中消息传递的整体流程，放到下篇文章中进行总结，文章链接： https://blog.csdn.net/huideveloper/article/details/80655357