之前写过一篇文章，概述了Android应用程序消息处理机制。本文在此文基础上，在源码级别上展开进行概述
简单用例
Handler的使用方法如下所示：
Handler myHandler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) {
...
}
}
};
class myThread implements Runnable {
public void run() {
while (!Thread.currentThread().isInterrupted()) {
Message message = Message.obtain();
message.what = TestHandler.GUIUPDATEIDENTIFIER;
TestHandler.this.myHandler.sendMessage(message);
message.recycle();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
}
}
或者:
mHandler=new Handler();
mHandler.post(new Runnable(){
void run(){
...
}
});
又或者：
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
源码解析
首先看其构造函数：
new Handler()
...
public Handler() {
this(null, false);
}
...
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
...
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) { // 默认为false，若为true则会检测当前handler是否是静态类
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());
}
}
// 1. 获得当前线程的looper
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
//2. 获得looper上的message queue
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
由此引入了两个关键对象Looper和MessageQueue。
先看 mLooper = Looper.myLooper(); 这一句发生了什么：
public static Looper myLooper() {
return sThreadLocal.get();
}
可以看到，该方法返回一个sThreadLocal对象中保存的Looper。关于ThreadLocal类，请参考这里，本文不展开。
如果尚未在当前线程上运行过Looper.prepare()的话，myLooper会返回null。接下来看看Looper.prepare()的实现：
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));
}
可以看到该方法只是简单地新建了一个Looper对象，并将其保存在sThreadLocal中。接下来看一下Looper的构造函数。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
调用完Looper.prepare()后，需调用Looper.loop()才能使消息循环运作起来，其源码如下所示：
public static void loop() {
final Looper me = myLooper(); //1. 取出looper对象
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue; //2. 取出looper绑定的message queue
// 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();
// loop time.
long tm = 0;
...
for (;;) {
Message msg = queue.next(); // 3. 堵塞式在message queue中取数据
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
msg.target.dispatchMessage(msg); 4. 分发message到指定的target handler
...
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
...
}
msg.recycleUnchecked(); // 5. 回收message对象
...
}
}
可以简单地将Looper.loop()理解成一个不断检测message queue是否有数据，若有即取出并执行回调的死循环。 接下来看一下Message类：
public final class Message implements Parcelable {
public int what;
public int arg1;
public int arg2;
public Object obj;
...
/*package*/ int flags;
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
/*package*/ Message next;
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
}
what、arg1、arg2这些属性本文不作介绍，我们把目光集中在next、sPoolSync、sPool、sPoolSize这四个静态属性上。
当我们调用Message.obtain()时，返回了一个Message对象。Message对象使用完毕后，调用recycle()方法将其回收。其中obtain方法的代码如下所示：
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();
}
可以看到，obtain方法被调用时，首先检测sPool对象是否为空，若否则将其当做新的message对象返回，并“指向"message对象的next属性，sPoolSize自减。可以看出message对象通过next属性串成了一个链表，sPool为“头指针”。再来看看recycle方法的实现：
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it is still in use.");
}
return;
}
recycleUnchecked();
}
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
如果message对象不是处于正在被使用的状态，则会被回收。其属性全部恢复到原始状态后，放在了链表的头部。sPool对象“指向”它，sPoolSize自增。
综上可以看出，通过obtain和recycle方法可以重用message对象。通过操作next、sPoolSync、sPool、sPoolSize这四个属性，实现了一个类似栈的对象池。
msg.target为handler类型，即向handler成员的dispatchMessage方法传入msg参数，其实现如下所示：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
这里可以看到回调了各种接口。
到目前为止，我们知道了如何处理在消息队列里面的msg对象，但仍不知道msg对象是如何放到消息队列里面的。通常来说，我们通过Handler的sendMessage(msg)方法来发送消息，其源码如下所示：
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
...
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);
}
可知sendMessage最终会调用queue.enqueueMessage(msg, uptimeMillis)将msg对象保存至message queue中，uptimeMillis表示msg执行回调的时刻。 我们来看一下MessageQueue类的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("MessageQueue", e.getMessage(), e);
msg.recycle();
return false;
}
// 1.设置当前msg的状态
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
// 2.检测当前头指针是否为空（队列为空）或者没有设置when 或者设置的when比头指针的when要前
if (p == null || when == 0 || when < p.when) {
// 3. 插入队列头部，并且唤醒线程处理msg
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//4. 几种情况要唤醒线程处理消息：1）队列是堵塞的 2)barrier，头部结点无target 3）当前msg是堵塞的
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;
}
}
// 5. 将当前msg插入第一个比其when值大的结点前。
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;
}
结合注释，我们可以了解到msg push到queue中时，queue的状态的变化和处理队列的逻辑。
前文中Looper对象的loop方法中：
for (;;) {
...
Message msg = queue.next(); // 3. 堵塞式在message queue中取数据
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
msg.target.dispatchMessage(msg); 4. 分发message到指定的target handler
...
}
可以看出，message queue的next方法被调用时，可能会发生堵塞。我们来看一看message queue的next方法：
Message next() {
// 1. 判断当前loop是否已经使用过，下文会解释这个mPtr
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
// 2. 进入死循环，直到获取到合法的msg对象为止。
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands(); // 这个是什么？
}
// 3. 进入等待，nextPollTimeoutMillis为等待超时值
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// 4. 获取下一个msg
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// 当前节点为barrier，所以要找到第一个asynchronous节点
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// 当前队列里最早的节点比当前时间还要晚，所以要进入堵塞状态，超时值为nextPollTimeoutMillis
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 删除当前节点，并返回
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
return msg;
}
} else {
// 头结点指向null
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// 5. 如果当前状态为idle（就绪），则进入idle handle的代码块
// 进入idle的情况有：队列为空；队列头元素blocking;
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// 6. 本轮唤醒（next被调用）时没处理任何东西，故再次进入等待。
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// 在一次next调用中，这个代码块只会执行一次
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 {
// 如果返回true，则idler被保留，下次next的idle时会被调用。
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf("MessageQueue", "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;
}
}
代码执行流程见注释。其中IdleHandler是一个接口：
public static interface IdleHandler {
boolean queueIdle();
}
IdleHandler提供了一个在MessageQueue进入idle时的一个hook point。更多时与barrier机制一起使用，使message queue遇到barrier时产生一个回调。
总结
前面涉及到的几个主要的类Handler、Looper、MessageQueue和Message的关系如下所述：
Handler负责将Looper绑定到线程，初始化Looper和提供对外API。
Looper负责消息循环和操作MessageQueue对象。
MessageQueue实现了一个堵塞队列。
Message是一次业务中所有参数的载体。
框架图如下所示：
+------------------+
| Handler |
+----+--------^----+
| |
send | | dispatch
| |
v |
+----- <---+
| |
| Looper |
| |
| |
+---> -----+
| ^
enqueue| | next
| |
+--------v------+----------+
| MessageQueue |
+--------+------^----------+
| |
nativePollOnce | | nativeWake
| |
+-----------------v------+---------------------+
Lower Layer
最后，留意到MessageQueue中有4个native方法：
// 初始化和销毁
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
// 等待和唤醒
private native static void nativePollOnce(long ptr, int timeoutMillis);
private native static void nativeWake(long ptr);
// 判断native层的状态
private native static boolean nativeIsIdling(long ptr);
将会在后续文章中进行介绍。