Android 的Handler、Looper和MessageQueue的关系和实现原理

最近比较闲，看了一些关于android消息机制的书籍和文摘，写个文档总结下。

一、基本概念

Message：消息，其中包含了消息ID，消息处理对象以及处理的数据等，由MessageQueue统一列队，终由Handler处理。

Handler：处理者，负责Message的发送及处理。使用Handler时，需要实现handleMessage(Message
msg)方法来对特定的Message进行处理，例如更新UI等。

MessageQueue：消息队列，用来存放Handler发送过来的消息，将Message以链表的方式串联起来的，等待Looper的抽取。

Looper：消息泵，不断地从MessageQueue中抽取Message执行。因此，一个MessageQueue需要一个Looper。

Thread：线程，负责调度整个消息循环，即消息循环的执行场所。
二、Handler、looper和消息队列的关系

2.1、从一个简单的程序说起

代码段1：

</pre><pre name="code" class="java">public class MainActivity extends Activity{
private TextView mText;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

mText = (TextView)findViewById(R.id.myText);
}

public void click(View view)
{
new Thread(new Runnable() {
@Override
public void run() {
// 耗时的操作......

// 设置UI
mText.setText("padmatek");
}
}).start();
}
}

布局文件：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.padmatek.liang.myhandler.MainActivity">

<TextView
android:id="@+id/myText"
android:text="0"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:textSize="30sp"/>

<Button
android:id="@+id/myBtn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="测试"
android:layout_below="@id/myText"
android:layout_centerHorizontal="true"
android:onClick="click"/>
</RelativeLayout>

运行并点击“测试”按钮，app崩溃，报错如下；原因是因为android中UI控件不是线程安全的，所以我们不能在工作线程（非UI）中访问UI控件







但是如果我们确实有在工作线程运行的过程中去处理UI的需求该怎么办呢？

我们将代码段1稍作修改：

代码段2：

public class MainActivity extends Activity{
private TextView mText;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

mText = (TextView)findViewById(R.id.myText);
}

private final static int MSG_SET_TEXT = 0;
private Handler handler = new Handler(){
public void handleMessage(Message msg)
{
switch (msg.what)
{
case MSG_SET_TEXT:
mText.setText("padmatek");
break;
default:
break;
}
}
};

public void click(View view)
{
new Thread(new Runnable() {
@Override
public void run() {
// 耗时的操作......

// 设置UI
handler.sendEmptyMessage(MSG_SET_TEXT);
}
}).start();
}
}

在该代码段中，我们创建了一个handler,并将按钮处理函数的设置Text的语句改为了使用handler发送一个消息；
此时我们再运行并点击按钮，发现文本设置成功。

那么问题来了:

问题1:handler.sendEmptyMessage做了什么？

2.2、解析Handler的消息传递（问题1）

首先我们进入sendEmptyMessage，并依次找到被调用的函数，代码如下:

public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}

public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}

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);
}

从函数sendMessageAtTime里面蹦出来个mQueue

且当queue == null时，会弹出以“Looper”作为Tag的异常信息；

如果queue != null时，通过queue.enqueueMessage将消息交给了消息队列。

上面我们解答了问题1,此时我们又有以下问题:
2、Looper是什么东西，有什么作用？
3、这个消息队列是怎么来的？

2.3、Handler与Looper的关系
Handler有两个主要的构造函数，在这里贴上默认构造函数对应的代码:

public Handler() {
this(null, false);
}

其中this对应的实现如下:

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();
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;
}

从以上代码可以看出，消息队列mQueue 来自于mLooper（问题3） ，而mLooper 又来自于Looper.myLooper()；

Looper.myLooper()的实现如下:

public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}

看到这里，是不是有点“拔出萝卜带出泥”的感觉......

2.4、浅尝[b]sThreadLocal[/b]
既然到了这里，我们再来看一下sThreadLocal是个什么玩意，其定义如下:

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;

而其get函数的定义如下:

public T get() {
// Optimized for the fast path.
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
Object[] table = values.table;
int index = hash & values.mask;
if (this.reference == table[index]) {
return (T) table[index + 1];
}
} else {
values = initializeValues(currentThread);
}

return (T) values.getAfterMiss(this);
}

由于sThreadLocal为类静态变量，说明他是全局有效的，说明我们的所有的looper应该都保存在sThreadLocal中，所以get的实现原理大概就是通过线程句柄来获取对应的looper；

好了，sThreadLocal的探讨就先到这里，我们还是回到原来的地方；

2.4、Handler、looper和消息队列的关系
通过2.3我们大概知道了Handler和looper的关系，而且也知道消息队列来自于looper，为了肯定这一点，我们查看了源码，如下:

private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}

到现在为止，我们说了这么多，是不是有点小晕了，反正我是有点，那么放下脚步小结一下：

一句话:handler通过looper获取消息队列
由于主线程中默认有looper，而子线程没有，我们接下来在子线程更加详细的说明3者的关系

三、Looper的原理（问题2）
3.1、looper.prepare的作用
修改代码片段1的click函数如下:

public void click(View view)
{
new Thread(new Runnable() {
@Override
public void run() {
// 耗时的操作......

// 设置UI
handler.sendEmptyMessage(MSG_SET_TEXT);
Handler handlerSub = new Handler();
}
}).start();
}

只是添加了一行代码，运行并点击按钮，会报错，错误信息如下:



意思就是，没有调用Looper.prepare(),这个是干神马的呢？看源码如下:

/** 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);
}

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));
}

通过源码知道，prepare创建了looper，并且将其保存在sThreadLocal中，通过注释理解到创建完成后要调用loop函数；
注意:主线程创建Handler并不需要手动创建looper，是因为主线程在初始化的main函数中调用Looper.prepareMainLooper()生成了looper；
3.2、Looper.loop的作用
贴源代码:

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();
}
}

由以上源代码可知，loop方法是一个死循环，不断地从消息队列中取消息，将取出的消息交给msg.target的

dispatchMessage处理，其中msg.target就是我们创建的handler. 如果消息队列返回为null，则跳出loop，终止获取消息.

那么问题来了
问题4：以上源代码可知、不发送消息，loop调用后是不是循环就立马终止呢？
我们先来看看dispatchMessage的源代码:

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

如果msg带的callback和Handler的们Callback都为空，则调用handleMessage来处理消息，也就是代码片段1handleMessage函数；

3.3、正常创建消息循环

说了这么多，那么我们就看如何在工作线程中创建消息循环吧

public void click(View view)
{
new Thread(new Runnable() {
@Override
public void run() {
// 耗时的操作......

// 设置UI
handler.sendEmptyMessage(MSG_SET_TEXT);
Looper.prepare();
Handler handlerSub = new Handler();
Looper.loop();
}
}).start();
}

这样子，就可以在子线程中通handler来发送和处理消息了，自己去实现吧。

四、消息队列机制
从2.2的最好一个片段的代码，我们知道，handler最终将消息放入到了消息，队列中，那么消息队列又是怎样存储消息的呢？
4.1、消息存储机制
MessageQueue的enqueueMessage源码如下:

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;
}

从源码可知，消息按照其delay的时间从小到大保存在一个链表中，并且消息保存时线程安全的；

4.2、消息获取机制
从3.2中的loop函数，我们可以得知，消息队列调用类next函数来获取消息，MessageQueue的next源码如下:

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;
}
}

由此可知，当mQuitting为false时，不管有没有消息，for循环都不会结束，如果消息队列中有消息，则返回消息，如果没有一直循环，不返回；
那么3.2节中的问题4的答案就出来了，当没有消息，且没有调用Looper.quit时，消息队列的next函数会一直阻塞，那么loop函数也会阻塞；

参考的资料：

1、http://blog.csdn.net/liuhe688/article/details/6407225