Android消息机制Handler解读
本文基于Android源码8.1java
概述
在Android系统中使用了不少种通讯的方式，好比进程间通讯使用的Socket,Binder机制等，可是在相同进程不一样线程之间通讯再使用这些方式就显得杀鸡用牛了，因而Android使用了一种新的Handler消息机制，有了它咱们能够很方便的进行不一样线程之间的通讯，固然了Handler消息机制也只能限定在同一个进程中。可是，Android系统中关于Handler消息机制的使用却不限于此，好比:android的四大组件，事件机制等都和Handler消息机制密切相关。咱们所说的Handler消息机制是由Looper、MessageQueue、Message、Handler等类共同组成的，接下来就经过源码研究一下handler消息机制的原理。话很少说，先上张图，下图虽然简单，可是它体现了handler消息机制最核心的运行流程，在接下来枯燥而乏味的源码解读中，你们能够结合这张图去看，思路可能会更清晰些。android
Handler实例
下面先看一个咱们平时使用handler的例子，经过这个这个例子，咱们一步一步去探究handler机制的整个运行的流程。这个例子就是怎么在一个线程中建立Handler,能够简单归纳为下面的步骤:git
- 调用Looper.prepare()方法
- 建立Handler对象
- 调用Looper.loop()方法
- 调用Looper的quit方法结束loop
private void handlerTest(){
mLooperThread = new LooperThread("xray");
mLooperThread.start();
findViewById(R.id.btn_send_msg).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
mLooperThread.mHandler.sendEmptyMessage(10);
}
});
}
class LooperThread extends Thread{
public Handler mHandler;
public LooperThread(String name) {
super(name);
}
@Override
public void run() {
super.run();
Looper.prepare();
mHandler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Log.d(TAG, "looperThread thread id = " + Thread.currentThread().getId());
}
};
Looper.loop();
}
}
@Override
protected void onDestroy() {
if(mLooperThread != null){
mLooperThread.mHandler.getLooper().quit();
}
super.onDestroy();
}
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Looper
经过上面的例子，咱们在一个线程中建立handler的时候，首先得调用Looper.prepare()方法。github
//Looper.java
/** 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));
}
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除了prepare()方法，还有一个同名的带参数的方法，这个参数判断咱们是否能够主动退出loop()循环，等一会咱们讲到 loop()方法的时候会对这个参数有更深的理解。而后prepare方法建立了Looper对象,并将其实例保存在了sThreadLocal这个成员变量中。关于ThreadLocal我会单独写篇文章介绍，这里只要知道ThreadLocal会保存当前线程中，而且多个线程之间不会互相干扰。缓存
Looper的构造方法：bash
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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咱们发如今Looper的构造方法中，建立了MessageQueue对象,俗称消息队列，这个是handler消息机制的另一个主人公。咱们稍后会着重对它进行介绍。总结一下Looper.prepare()所作的工做:数据结构
- 建立Looper对象，并将其保存在ThreadLocal中
- 在Looper中建立了MessageQueue对象
如今咱们回到上面的例子，先无论Handler的建立,看Looper.loop()方法.app
//Looper.java
/**
* 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 (;;) {
Message msg = queue.next(); // 可能会阻塞在这里
if (msg == null) {
// No message indicates that the message queue is quitting.
//若是没有message说明消息队列正在退出,好比调用了quit方法时
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 {
//从消息队列中获得消息后，要将消息进行分发处理，这个target大多数就是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); } //消息进行回收 msg.recycleUnchecked();//todo，介绍消息回收的方式 } } 复制代码
代码比较长，重要的地方我作了注释，这个方法被调用后Looper就启动了，归纳一下该方法作的主要工做：less
- 有个for循环，在循环中不断的从消息队列中获取消息，而且获取的方法可能会被阻塞。
- 将获取到的Message分发出去， msg.target.dispatchMessage(msg)，这个target大多数状况就是Handler.
- 将Message进行回收，以即可以复用，下文会详细介绍。
mylooper()
从ThreadLocal中获取当前线程的Looper对象async
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
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quit()和quitSafely()
退出loop循环的方法，其实他真正的实如今MessageQueue中。说一下二者的区别，quit方法是将MessageQueue中全部的消息所有清除，而后退出loop, quitSafely方法是将此时此刻，尚未到执行时间的消息清除，可是已经达到执行时间了，可是还没来得及执行的消息会保留,等执行完了再退出loop.
/**
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
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MessageQueue
在上面讲Handler的时候，咱们屡次提到了MessageQueue，下面就介绍一下MessageQueue的原理。
next()
next方法的做用就是从消息队列中取出Message,固然具体不是一句话这么简单，下面看看其内部的实现。
//MessageQueue.java
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.
//若是下一条message被延时，设置一个延时，等时间到了再去返回该Message
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;
//若是没有idle handlers须要执行，Loop将输入等待状态，也就是，next方法处于阻塞的状态，此处执行调到下一次循环，
//直到有新的消息，或者loop被终止，或则有idle handlers 须要执行
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.
//重置idle handler的个数为0, 须要等下次再没有可执行的Message执行时，idle handler才能继续执行
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.
//须要重置这个过时时间，由于有可能有新的message须要执行，因此须要的检查
nextPollTimeoutMillis = 0;
}
}
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经过上面的代码，咱们知道MessageQueue中维护了一个链表，在从队列中获取消息时，是根据消息真正的执行时间来取出的，若是这段时间空闲，也就是获取Message处于阻塞状态的时候，会回调IdleHandler,假使咱们设置了它，若是当前的Message的执行时间没到，又没有IdleHandler须要处理，那么程序就会阻塞在这里。看到这里若是你们够细心的话，必定能推测出MessageQueue中的Message必定是按时间排好序的，不然Message的分发顺序就会有问题，排序的逻辑就在enqueueMessage方法中。
enqueueMessage
enqueueMessage方法的做用是往消息队列中添加消息，而且在插入的时候会以消息执行的时间进行排序。下面咱们看看具体的代码实现，其实仍是对链表的操做。
//MessageQueue.java
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();//回收message todo
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; //按时间去排序，将message插入到队列相应的位置 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; } 复制代码
总结一下这段代码，在往链表中插入消息时，会先对Message执行的时间进行对比，对于延时的消息,MessageQueue会遍历整个链表，直到找到合适的插入的位置。
Message
Message顾名思义是handler消息机制中的那个消息，Handler发送和处理的实体就是这个它.
obtain()
当咱们平时须要Message实例时，能够直接new Message(),也能够调用Message.obtain()方法，可是更推荐 使用后者，由于Message中有个Message的缓存池，这个缓存池的大小是50（从MAX_POOL_SIZE这个常量值能够获得）， 而obtain()方法会先从缓存池中获取，这个缓存池也是用链表实现的。若是obtain()获取不到Message实例，才会从新new
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
//message pool也是用链表实现的
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
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recycle()和recycleUnchecked()
这两个方法的做用是将使用完的Message对象进行回收，从新放入都Message缓存池中，以便下次使用，其实代码很简单， 仍是对链表的操做，有没有发现链表这种数据结构真的使用的不少。
/**
* Return a Message instance to the global pool.
* <p>
* You MUST NOT touch the Message after calling this function because it has
* effectively been freed. It is an error to recycle a message that is currently
* enqueued or that is in the process of being delivered to a Handler.
* </p>
*/
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}
/**
* 该方法可能回收还在使用的Message
*/
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) {//Message缓存池大小为50
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
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Handler
咱们讲Handler消息机制，如今终于轮到Handler了，它在整个流程中就是对Message进行发送和处理。
Handler构造方法
public Handler(Callback callback, boolean async) {
//是否检测内存泄漏的风险
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();//从当前的线程中获取Looper, todo
if (mLooper == null) {//在线程中建立handle时，须要先调用Looper.prepare()
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue; //Looper中建立的消息队列
mCallback = callback;//处理message的回调
mAsynchronous = async;
}
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发送Message
发送消息其实最终就是将根据Message的执行时间，将其插入到MessageQueue中。
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);
}
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private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
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分发消息
Looper在调用loop()方法的时候，当遇到符合条件的Message，就会调用Handler的dispatchMessage方法， 用来分发Message，这样咱们就能够在Handler中处理Message了。
/**
* 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);
}
}
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IdleHandler
前面在讲MessageQueue的next的方法的时候见到过IdleHandler，当咱们取消息处于阻塞状态的时候，若是添加了IdleHandler，就会处理它，因此咱们能够把一些不那么重要的操做放到IdleHandler中执行，这样能够显著的提升性能。好比著名的内存泄漏检测库leakarary中关于内存泄漏检测的操做就放到了IdleHandler中执行。
/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
public static interface IdleHandler {
/**
* 在该方法中执行咱们须要执行的任务，若是该任务是一次性的则返回false,若是该任务须要屡次
* 执行则返回true
*/
boolean queueIdle();
}
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总结
到这里Java层的Handler机制就讲完了，限于篇幅的缘由和做者的水平，有些地方没有很深刻的讲解，在此说声抱歉，可是大体的流程应该是有的，建议读者去仔细的读一下这块的源码，相信收获会不小。前面说了这只是Java层面的Handler消息机制，其实在Native层，也有一套C++的实现，有兴趣的小伙伴也能够看看这个的内容。
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