Handler作为Android应用层开发，进程通信一大重点，可以说是使用最频繁的一个机制，不管是IntentService,ThreadHandler都绕不开它。本文详解Handler机制的内部源码
深入剖析Handler，没有看错，比别人更深更精准！
看本文可以回答你这几个问题：
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UI线程的Looper在哪里创建？
-
MessageQueue真的是个队列吗？
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延迟处理机制的原理？
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Handler中的Message同步和MessageQueue同步？
一、Handler介绍
Handler在Android os包下，当我们创建Handler时，它会绑定一个线程，并且创建一个消息队列，通过发送Message或者Runnable对象到队列并轮询取出，实现关联。
我们常用的Handler功能是，定时执行Runnable或者处理不同线程通信的问题，比如UI线程和子线程等。
由此可见Handler内部机制中的几大元素：Handler,Message,MessageQueue,Looper,ThreadLocal等，接下来，分别查看它的内部源码。
二、Handler源码剖析
Handler作为封装对外的处理器，我们来看看它对外的接口内部是做了哪些操作。
1. Handler构造函数：
它的构造函数,我归纳为三种方式，分别是：
1.传入自定义Looper对象，
2.继承Handler实现Callback接口模式，
3.默认创建的Looper模式，
其中2，3是我们常用的，当然1和2也能同时使用，callback接口中实现handleMessage，用于我们自定义Handler是实现回调用的。还有个被hide隐藏的传参，async是否同步，默认是不同步，且不支持设置同步模式。
可以传入自定义的Looper，Callback接口
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
常规的构造方法如下:
-
其中FIND_POTENTIAL_LEAKS标签是检查“继承类是否为非静态内部类”标签，我们知道，非静态内部类持有对象，容易导致内存泄漏的问题，可以查看我的《Android内存优化分析篇》
-
mAsynchronous可以看到一直是false
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; }
2. 创建Looper对象和mQueue消息队列
由上构造函数中调用Looper.myLooper()；创建了Looper对象，并取用了新创建Looper对象内部的mQueue队列，详解下Looper分析
3. sendMessage
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其中sendEmptyMessage通过obtion新获取了一个Message对象
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) { Message msg = Message.obtain(); msg.what = what; return sendMessageDelayed(msg, delayMillis); } -
发送消息：sendMessageDelayed—>sendMessageAtTime—>enqueueMessage
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注意到，在调用sendMessageAtTime时，传入的时间值： 系统时钟+delayMillis
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其中将 msg.target标记为当前Handler对象
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最终调用了MessageQueue的enqueueMessage，看后面MessageQueue分析
//----------1 public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } //----------2 public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); } //---------3 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); } //----------4 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
4. removeMessages
从队列删除
5. post(Runnable r)
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在getPostMessage中讲Runnable封装成了Message对象
public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0); } private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m; }
6. dispatchMessage和handlerMessage
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我们看到dispatchMessage调用了callback和handlerMessage分发Message结果
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那么，前面我们看到了经常调用的sendMessage，那么回调是在什么时候调用的呢？
-
让我们接下来一起看看Looper类吧。
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } public void handleMessage(Message msg) {}
三、Looper源码剖析
看looper做了什么，首先看mylooper方法，还记得吗，在Handler初始化时创建Looper对象调用的方法
1. myLooper方法
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调用sThreadLocal取出一个looper对象
public static @Nullable Looper myLooper() { return sThreadLocal.get(); } // sThreadLocal.get() will return null unless you've called prepare(). static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
2. Looper.prepare()创建对象
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上面看到mylooper从sThreadLocal取出，但是什么时候存的呢，looper又是如何创建？
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由下看出Looper通过prepare创建并存入sThreadLocal，在构造同时创建MessageQueue
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标记成员mThread为当前线程
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quitAllowed标识能否安全退出
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)); } private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
3. UI线程调用Handler，Looper怎么创建
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prepareMainLooper：在当前线程初始化looper，在ActivityThread调用，也就是我们创建Activity时已经创建了Looper了
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prepare(false)：由于在ActivityThread创建，是不能安全退出的
/** * Initialize the current thread as a looper, marking it as an * application's main looper. The main looper for your application * is created by the Android environment, so you should never need * to call this function yourself. See also: {@link #prepare()} */ public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } } //-------->ActivityThread: Main: public static void main(String[] args) { --- Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); }
4. Looper.loop()
- UI线程创建Looper，上ActivityThread中，在调用prepare后接着调用Looper.loop
- loop通过 for (;;)死循环，从queue中取下一则消息
- 其中 msg.target.dispatchMessage(msg);，在上面Handler中将handler对象传给了looper
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;
//--------------确保同一进程
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;
}
//--------------打印日志
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
//--------------从队列中获取分发消息延时
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
//--------------Trace标记，用于记录message分发完成
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);
}
}
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);
}
//--------------充值message对象参数
msg.recycleUnchecked();
}
}
四、MessageQueue源码剖析
MessageQueue主要分析插入和取出，由下enqueueMessage插入方法看出，它名字带着Queue,但其实并不是，它实际是个单链表结构，通过native操作指针，去进行msg的读取操作。当然，这更加快捷的实施取出，删除和插入操作。
1. enqueueMessage
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msg.markInUse();标记当前msg正在使用
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其中mMessages是可以理解为即将执行的Message对象
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将当前mMessages与新传入的Msg的设置触发时间对比，如果新的Msg设置时间早，则将2者位置对调，将新的排前面，与之对比的mMessages排到其后。反之，则与mMessages后一个对比时间，依次类比，插入到队列中
-
其中，如果msg事Asynchronous同步的，那么它只能等到上一个同步msg执行完，才能被唤醒执行。
boolean enqueueMessage(Message msg, long when) { ... synchronized (this) { if (mQuitting) { //------------->抛出一个IllegalStateException Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } //------------->标记当前msg正在使用 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; }
2. next取出
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可以看出enqueueMessage和next都是同步的
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通过循环，把mMessages当前msg
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比较当前时间和Msg标记时间，如果早的话就设置一段指针查找超时时间
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将msg标记为使用，并取出消息返回
Message next() { //----->当消息轮询退出时，mPtr指针找不到地址，返回空取不到对象 final long ptr = mPtr; if (ptr == 0) { return null; } //----->同步指针查找的时间，根据超时时间计算，比如当前未到msg的时间，指针会在一段计算好的超时时间后去查询 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; //----->如果target为null,寻找下一个带“同步”标签的msg 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) { //----->比较当前时间和Msg标记时间，如果早的话就设置一段指针查找超时时间 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; } }
3. quit操作
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前面标记是否能安全退出，否则报错
-
退出后唤醒指针，接触msg的锁
void quit(boolean safe) { if (!mQuitAllowed) { throw new IllegalStateException("Main thread not allowed to quit."); } synchronized (this) { if (mQuitting) { return; } mQuitting = true; if (safe) { removeAllFutureMessagesLocked(); } else { removeAllMessagesLocked(); } // We can assume mPtr != 0 because mQuitting was previously false. nativeWake(mPtr); } }
五、Message源码剖析
Message主要是一个Parcelable序列号对象，封装了不分信息和操作，它构造了一个对象池，这也是为什么我们一直发送msg，不会内存爆炸的原因，来看看实现
1. obtain()
-
维持一个大小为50的同步线程池
-
这里可以看出Message是个链表结构，obtain将sPool取出return Message，并对象池下一个msg标记为sPool
private static final int MAX_POOL_SIZE = 50; ... 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(); }
2.recycleUnchecked 回收消息
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回收初始化当前msg
-
如果当前对象池大小小于MAX_POOL_SIZE，则将初始化后的msg放到表头sPool，sPoolSize++。
-
由此可以看出，如果每次new新的Message传入Handler，必然增加内存消耗，通过obtain服用才是正确的做法
/** * Recycles a Message that may be in-use. * Used internally by the MessageQueue and Looper when disposing of queued Messages. */ 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++; } } }
3. Message标签：是否使用，同步标签
public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
/*package*/ boolean isInUse() {
return ((flags & FLAG_IN_USE) == FLAG_IN_USE);
}
/*package*/ void markInUse() {
flags |= FLAG_IN_USE;
}
六、总结
废了小半天功夫，整理了对Handler源码的阅读总结，虽然东西很多也很繁琐，不过如果认真去看，是不是发现越深入就越有趣，也越发发现Android源码的严谨性。平常至少简单一用，只要深入了解才能更好地去使用它理解它，比如Message对象池的应用，这不就是享元模式吗，希望大家都能有所体悟。