从源码角度分析java层Handler机制
在Android中，当要更新ui的时候，咱们必需要在主线程中进行更新，缘由时当主线程被阻塞了5s以上就会出现anr异常，会致使程序崩溃。因此一些耗时的操做必需要放在子线程中，可是在子线程中又不能作更新ui的操做，因此为了解决这个问题，Android设计了handler机制，handler的出现创建起了主线程与子进程之间的通讯桥梁，使得ui更新问题获得改善，下面就来剖析一下handler。ActivityThread启动了应用程序的主线程，在ActivityThread的main方法中：java
public static final void main(String[] args) {
SamplingProfilerIntegration.start();
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
......
}
从上述代码能够看出，首先要执行Looper.prepareMainLooper();操做，而后进入loop进行循环。在prepareMainLooper中，调用prepare方法使用sThreadLocal给当前线程设置一个Looper，若是当前线程中没有，就初始化一个Looper，在Looper的构造方法中顺便建立了一个MessageQueue。细心的读者可能会注意到prepareMainLooper和prepare方法都是static的，sThreadLocal也是个静态变量，首先不考虑子线程存在的状况，只考虑主线程，因此不管咱们在应用程序的哪一个地方调用Looper.prepareMainLooper();经过sThreadLocal.get()获得的都是同一个looper对象，这样就能够保证一个线程中只有一个Looper对象，那么也就意味着一个线程中只有一个MessageQueue。web
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
执行完 Looper.prepareMainLooper()以后，就是开始Looper.loop()进行消息的循环读取而且进行分发，这个稍后分析完Handler后再分析。async
下面咱们再分析一下Handler。
在代码中咱们常常的这样用：ide
private Handler handler = new Handler(){
public void handleMessage(Message msg) {
// process incoming messages here
}
}
public Handler(Callback callback, boolean async) {
......
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;
}
在Handler的构造方法中，经过Looper.myLooper()获取本线程中惟一的一个Looper对象，而且初始化hanlder中的消息队列，这个消息队列和Looper中的一开始初始化的消息队列是同一个。
当调用handler.sendMessage或者sendEmpty方法时，最终要走的方法都是sendMessageAtTime方法：svg
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);
}
在enqueueMessage方法中，给当前要加入消息队列的msg设置一个target为this，这个this也就是当前的handler对象，主要是为了后面的looper循环出消息后，方便知道这个msg向何处分发，该由哪一个handler进行处理。接着就调用MessageQueue的enqueueMessage方法将msg加入队列中。oop
boolean enqueueMessage(Message msg, long when) {
......
synchronized (this) {
......
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 {
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;
}
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
当消息对列中没有任何msg的时候，当前加入的msg就应该是队列的队头，而且从else语句咱们能够看出，整个消息对列是个循环对列。此时消息对列中已经有了msg，那么这个msg应该被接受并进行分发处理，在ActivityThread中调用了Looper.loop()方法进行消息的轮询。ui
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) {
return;
}
Printer logging = me.mLogging;
.......
msg.target.dispatchMessage(msg);
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
}
msg.recycle();
}
}
其中的for循环为死循环，有人可能纳闷了，looper.loop()是运行在主线程中的，而其中有是个死循环，不是说好的主线程中不能作超时的操做吗？呵呵，由于在循环中轮询消息队列中的消息时候，若是没有消息，则会被阻塞。因此这里不用担忧anr的问题。经过queue.next()获取出msg后，经过msg.target.dispatchMessage(msg)处理这个消息，这个msg.target就是要处理消息的handle，这也就是为啥在handler中要重写dispatchMessage方法的缘由。最后调用recycle释放消息，之因此要recycle一下，是由于Message能够不用new的方式，也能够经过Message.obtain方法从消息池中获取一个，由于消息池中的消息个数有限，若是用完消息后，不及时的recycle的 话，就会形成msg对象不能重复利用。this
接下来具体的分析下queue.next()这个方法，在注释中咱们看到，这个方法有可能会被阻塞，阻塞的缘由是消息队列中没有消息。spa
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// We can assume mPtr != 0 because the loop is obviously still running.
// The looper will not call this method after the loop quits.
nativePollOnce(mPtr, 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 (false) Log.v("MessageQueue", "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("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;
}
}
nativePollOnce(mPtr, nextPollTimeoutMillis)当轮询没有消息时，会进行阻塞。消息唤醒和阻塞机制将会在下一篇文章进行介绍，请你们关注。.net
最后对Handler作一下总结。从消息的分发一直到消息的处理，前后接触到的几个名词有Looper、MessageQueue、Thread、Handler、Message。
- Looper：负责初始化消息队列，并不断的从消息队列中轮询消息。
- MessageQueue ： 是消息队列，存放在handler发送过来的消息。
- Thread：当前消息队列和looper所操做的场所或者说运行的环境。
- Handler：负责消息的发送和处理。
- Message：一个更新UI的消息，由handler发出，由MessageQueue列队。
消息处理机制大概的一个处理过程以下：
关于轮询的时候，阻塞和唤醒机制请看下一篇文章。
从源码角度分析native层消息机制与java层消息机制的关联
补充：在子线程中要更新ui的时候，能够这样处理
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();
}
}