android之handler messagequene looper threadlocal 之间的详细解读

说到android的handler，他只是消息机制的上层接口，为什么这么说呢，消息机制真正的实现是在messagequene和looper中进行的

首先我们应该遇到过在一些子线程中new handler会报如下错误

Can't create handler inside thread that has not called Looper.prepare().

这是为什么呢，因为handler的创建，我们看下handler的初始化过程

/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages.  Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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;
}

这里插个题外话

Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());

当你在handler初始化的时候最好用static标示，否则会提示这个警告，有可能会造成oom，

下面我们来分析下源码

首先调用Looper.mylooper().得到一个looper对象，我们点进去看mylooper()的实现如下

/**
* Return the Looper object associated with the current thread.  Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}

是通过ThreadLocal的get方法得到looper对象，对于ThreadLocal的详细解读可以在这我的上篇博客ThreadLocal详细解读，而Threadlocal的set()是在Looper.prepare()中初始化的

/** 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));
}

这样就是为什么我们在用之前要调Looper.prepare()，如果没调用这个方法，则looper为空，再回到handler中发现如果looper则会抛运行时异常

if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}

但是这个方法只能调用一次，否则会抛出

if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}

至于主线程，在ActivityThread中已经调用了looper.prepare(),

/**
* 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();
}
}

/** Returns the application's main looper, which lives in the main thread of the application.
*/
public static Looper getMainLooper() {
synchronized (Looper.class) {
return sMainLooper;
}
}

到此looper的初始化结束，我们来看handler中另外的初始化

mQueue = mLooper.mQueue;

这里通过上面初始化的looper的一个成员变量mQueue得到MessageQueue对象，点击进入looper查看mQueue初始化的地方

发现还是在looper.prepare()

sThreadLocal.set(new Looper(quitAllowed));

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

我们发现这个构造函数被私有化了，在这里进行初始化MessageQueue，顾名思义，MessageQueue是一个消息通道

当handler发送消息时，有以下几种发送消息方式

/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}

/**
* Sends a Message containing only the what value.
*
* @return Returns true if the message was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}

/**
* Sends a Message containing only the what value, to be delivered
* after the specified amount of time elapses.
* @see #sendMessageDelayed(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}

/**
* Sends a Message containing only the what value, to be delivered
* at a specific time.
* @see #sendMessageAtTime(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.
*/

public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}

/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.  Note that a
*         result of true does not mean the message will be processed -- if
*         the looper is quit before the delivery time of the message
*         occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
*         delivered, using the
*         {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.  Note that a
*         result of true does not mean the message will be processed -- if
*         the looper is quit before the delivery time of the message
*         occurs then the message will be dropped.
*/
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);
}

/**
* Enqueue a message at the front of the message queue, to be processed on
* the next iteration of the message loop.  You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
* <b>This method is only for use in very special circumstances -- it
* can easily starve the message queue, cause ordering problems, or have
* other unexpected side-effects.</b>
*
* @return Returns true if the message was successfully placed in to the
*         message queue.  Returns false on failure, usually because the
*         looper processing the message queue is exiting.
*/
public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}

最终会调用 MessageQueue的 enqueueMessage(Message msg, long when)来进行消息插入，这里的

msg.target就是一个handler，在这里发送消息时，必须先初始化handler，

if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}

既然消息进来了，应该会有个类来负责不断的取消息并处理，我们想到了应该是Looper来处理，看下looper的loop()，方法

/**
* 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(); // 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();
}
}

在这里，首先通过mylooper()得到当前线程的Looper对象me，再通过me.mQueue变量得到当前looper的MessageQueue对象queue,然后for(;;)循环阻塞，不停的调用queue的next方法，我们点击去看下next方法的实现

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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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;
}
}

其实这里就是从MessageQueue中不断取出message并从MessageQueue中删除取出过的message，再回到我们的loop()方法，我们发现调用完next()方法取出message以后会调用handler的

msg.target.dispatchMessage(msg);

/**
* 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);
}
}

最终会调用handleMessage（msg）,我们点进去看下handleMessage()的实现

/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}

发现这里什么也没有实现，从官方注释可以看出这里是处理消息回来以后的实现，

自此，我们可以整理出handler是android消息机制的顶层接口，是用之前，先调用looper.prepare()初始化Messagequeue，并将当前线程当前looper保存在ThreadLocal中，然后通过handler发送消息，最终调用MessageQueue的equeueMessage将消息插入到MessageQueue中，然后再调用Looper.loop(),Looper.loop()会调用MessageQueue的next ()方法不断的从MessageQueue中获取消息并删除掉，再通过handler的dispatchMessage()分发消息到handler所在线程中，handler再通过handleMessage进行消息处理