一张图让你明白Handler、Message、Looper消息机制
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在android开发中，Handler太熟悉了，处处可见。确实挺好用的。它可以轻松的将任务切换到Handler所在的线程中去执行。如下面代码：
//发送message
MyHandler myHandler = new MyHandler(this);
//业务逻辑
...
Message message = new Message();
message.what = 0x001;
myHandler.sendMessage(message);//接收message
public class MyHandler extends Handler {
private Context context;
public MyHandler(Context context) {
this.context = context;
}
@Override
public void handleMessage(Message msg) {
switch (msg.what){
case 0x001:
//业务逻辑
//...
Toast.makeText(context,"收到messge",Toast.LENGTH_SHORT).show();
break;
default:
super.handleMessage(msg);
break;
}
}
}so easy，对吧！很多人认为Handler的作用是更新UI，这没问题。但是更新UI只是Handler的一个特殊使用场景。具体来讲，所有比较耗时的操作一般都是在子线程中完成，否则容易引起ANR，如读写文件、数据插入、网络请求等，完成后需要对UI进行修改的，但是由于android开发规范限制，子线程不能更新UI。所以这时候通过Handler将更新UI的操作切换到主线程中执行。通俗的讲，Handler不是为了UI而生，只是常被大家用来更新UI。Handler的主要作用应该是将一个任务切换到某个指定的线程中去执行。
Android的消息机制主要是指Handler运行机制，关于Handler运行机制，我们不得不提到其底层的MessageQueue与Looper。其关系大概如下图：
没看懂？没关系,后面会更详细的。先说一下MessageQueue与Looper是啥呢!
MessageQueue：消息队列，它内部存储了一组消息，以队列的形式对外提供插入和删除的工作。内部结构不是真正的队列，而是采用单链表的数据结构来存储消息列表。
Looper：可以理解为消息循环。MessageQueue 只是一个消息的存储单元，它不能去处理消息，Looper 填补了这个功能，Looper 会以无限循环的模式去查看Message中是否有新消息，否则就一直等待。
重点：如果需要使用Handler就必须为当前线程创建一个Looper，因为线程默认是没有Looper的。如果线程中没有Looper的话，会抛出异常的。我我们可以一起看看一下Handler的构造源码，它有好几个构造函数：
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}无参数的，接着往下看：
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle messages.
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler(Callback callback) {
this(callback, false);
}还有好几个其它的构造方法，可自行阅读源码。这两个构造方法都指向了this(callback, false)，我们接着看this(callback, false)的具体实现：
public Handler(Callback callback, boolean async) {
...
//获取当前线程的looper对象
mLooper = Looper.myLooper();
//判断Looper是否为null
if (mLooper == null) {
//如果为null则报异常
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
通过源码可以看出，在创建Handler实例的时候，先会通过Looper.myLooper()去获取当前线程的Looper，然后判断looper是否为null。如果为null，会提示通过Looper.prepare()去创建。
我们接着看下Looper.prepare()的源码：
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
prepare()是一个空构造函数，其内部是调用了prepare(true)，我们接着看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));
}最后一行代码很很清楚的显示了创建了一个Looper对象。先判断当前线程时候是否有Looper，如果有的话就会抛异常提示每个线程只能有一个Looper，没有的话则创建。
子线程中Handler的初始化Looper的代码：
new Thread("Thread#1"){
@Override
public void run() {
//初始化Looper对象
Looper.prepare();
Handler handler = new Handler();
//开启消息循环
Looper.loop();
}
}.start();细心的同学可能发现了，怎么最开始的示例代码中没有见着去调用 Looper.prepare()创建Looper对象啊！
其实UI线程，也就是ActivityThread，在被创建时就会初始化Looper，这也是我们在主线程中不需要调用Looper.prepare()能直接使用Handler的原因。
Handler创建完毕后，可以通过send方法发送消息，也可以通过post将一个Runnable对象投递到Looper中去处理。看一下post方式源码：
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*/
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}我们其实会发先post本质上也是同过send方式来完成的。Handler通过send方式发送消息，它会调用调用MessageQueue中的enqueueMessage方法将消息插入到队列中，然后Looper调用MessageQueue中next方法发现有新的消息来了，就开始处理这个消息，然后消息中的Runnable或者Handler中的handleMessage就会被调用处理消息。这样子消息就从子线程切换到了主线程中。大概流程如下图所示：
Handler中send方式发送消息的方法有三种：
- sendMessage(Message msg)
- sendMessageDelayed(Message msg, long delayMillis )
- sendMessageAtTime(message msg , long uptimeMillis)。
具体用法就不说了，看方法命名就能知道。我们进一步看看其源码，首先是sendMessage(Message msg)：
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}很简单，调用了sendMessageDelayed，我们接着看：
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}实质是调用了sendMessageAtTime，继续：
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);
}可以看到在其方法内部，先获取了消息队列，然后判断是否为null，接着调用了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);
}通过Handler中enqueueMessage方法内部调用了MessageQueue中的enqueueMessage方法，虽然方法名一样，却是两个不同类中的不同方法。
MessageQueue即Handler中的消息队列，主要包含两个操作：插入和读取。分别对应的方法是：
- enqueueMessage
- next
其中enqueueMessage作用是往消息队列中插入一条消息，而next的作用是从消息队列中取出一条消息并将其从消息队列中移除。
继续看MessageQueue中的enqueueMessage：
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;
}通过其实现来看，它的主要操作是消息队列的单链表的插入操作。MessageQueue尽管叫消息队列，但是它的内部是通过一个单链表的数据结构来维护消息列表，单链表在插入和删除上比较有优势。
我们接着看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();
}
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;
}
}我们可以看到next内部其实是一个无限循环的方法，如果消息队列中没有消息，那么next方法就一直阻塞在哪里。当有新消息来，next方法就会返回这条消息并将其从单链表中移除。
看完了MessageQueue，接着我们来看Looper：
Looper 在消息机制中作用是消息循环，它会不停的从MessageQueue中查看时候有新消息到达，如果有新消息会立刻处理，如果没有新消息，则一直阻塞在哪里。
Looper中最重要的一个方法是loop，只有调用了loop后，消息循环才真正的开始工作。查看其源码：
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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 (;;) {
//在这里没有消息的话就一直阻塞在这里
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.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
}
}首先是通过myLooper()获取当前线程的Looper对象，接着获取消息队列。继续往下我们有看了一个无限循环，当没有消息存在时，queue.next()就会一直阻塞在这里。接着往下看，当有消息存在时，msg.target.dispatchMessage(msg)就会处理消息。msg.target是什么呢？在前面我们分析的Handler中enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)源码中我们可以看到：
msg.target = this;即msg.target就是Handler对象，这样消息从Handler发出，最后又交给自己的dispatchMessage处理了，这里dispatchMessage方法是在创建Handler时所使用的Looper中执行。这样就成功的将代码逻辑切换到指定的线程中去执行了。
MessageQueue与Looper的分析到这里基本结束了。
我们接着回到Handler中dispatchMessage方法：
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}我们接着分析，先判断msg.callback是否为空，否则调用handleCallback(msg)方法。那msg.callback是什么呢？在前面我们说了Handler的发送消息有post和send两种方式。看下post方式源码：
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*/
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}很简单，接着看看getPostMessage(r):
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}OK，终于找到m.callbac了，其实就是Handler的post方法中所传递的Runnable参数。我们回到dispatchMessage方法继续，当Runnable对象不为null时，调用handleCallback方法：
private static void handleCallback(Message message) {
message.callback.run();
}是不是一下子很清楚了，就是Runnable中的run()方法。我们接着看，然后是判断mCallback是否为null，不为null则调用handleMessage()方法。Callback其实是一个接口：
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}那么这个Callback是什么时候传入的呢？前面我们有说过，Handler的构造方法有好几个，我们看看其中的另几个：
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Callback callback) {
this(callback, false);
} /**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
} public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}一下清晰了，Callback可以作为构造函数的参数传入。我们接着看，当mCallback为null时，执行handleMessage(msg):
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}这个方法是不是异常熟悉，就是我们平常创建Handler时派生一个其子类并重写其handleMessage方法来处理具体的消息。同时，Callback给我们提供了另外一种可以不用派生子类使用Handler的方式。
到这里，Handler的源码分析基本结束！
有人可能大喊一声，说好的一张图就让我明白的Handler消息机制呢？coder与coder之间的信任在哪里？
好吧，综合上述，可以用这样的一张图来表示其相互之间的关系及调用逻辑：
图有点…我也很忧伤，大家放大了看看吧！将它们之间的联系描述得非常清晰
呃..然后对第一张图中出现的ThreadLocal说明一下，在获取Looper的prepare方法中，我们其实可以看到其实是通过ThreadLocal获取到的Looper对象。ThreadLocal是一个线程内部的数据存储类，它可以在不同的线程互不干扰的存储并提供数据。
基本上是这样子了。
如果有不明白的，可以留言；如果某些地方有误，非常欢迎指出，谢谢！