简析Handler、MessageQueue、Looper

1.前言

在android中，网络、文件等耗时操作在执行时，不可避免的造成线程操作阻塞，这就很尴尬了，毕竟点击，UI绘制都是再主线程中执行的，若是主线程阻塞掉，不可避免的造成用户体验下降，甚至会出现anr。所以异步操作在app中是不可少的，然而，android中做出了UI操作必须放置在UI线程也就是主线程中的限制，导致子线程并不能对视图界面进行及时的更新，怎么办呢，这就需要一个可以进行线程间通信的方法出现，Handler
Looper MessageQueue应运而生。

2.从Looper开始

Hander-Looper机制的起始都是从一句Looper.prepare()开始的，哪怕是mainThread。所以，可以从prepare()入手:

public static void prepare() {
prepare(true);
}

</pre>继续往下看：<p></p><p></p><pre name="code" class="java" style="font-size:14px;">    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));
}

ThreadLocal用作对对应线程提供线程变量副本，可以发现，prepare()中创建了Looper对象！而if中的异常判断则会避免一个线程中多个Looper的出现。沿着方法往下看，就是new Looper(quiallowed);

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

可以看出，quiAllowed与正在讨论的Looper关系并不大，所以暂且不管，new Looper完成了Looper对应的MessageQueue构建，以及当前Thread引用的获取。preper()就此打住，接下来就是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();
}
}

按照老方法，第一个先看的就是myLooper(),虽然看之前就从字面明白，获取的必然是当前线程对应的Looper副本......

public static Looper myLooper() {
return sThreadLocal.get();
}

嗯，果不其然......

往下走就是判断是否存在Looper对象，这也是先调用Looper.loop()必然报错的原因

之后就是MessageQueue对象的获取，至于Binder.clearCallingIdentity()以及loop()方法中的相关内容暂且不用管，知道和进程间通信有关就行......

之后就是for(;;)循环，通过MessageQueue的next方法不停的获取下一个Message,在通过Message.target.dispatchMessage分发出去，提前预告，Message.target是一个Handler对象。Message.recycleUnchecked会在之后讲解。

到此为止，讲解了Looper在Handler-Looper机制中的主要功用，总结就是，线程对应的Looper对象拥有一个自己的MessageQueue，并且不断的通过自己的MessageQueue获取新的Message, 注释中有一句很醒目的话

// No message indicates that the message queue is quitting.

这就很有意思了，若是取到的msg为空就代表messagequeue退出执行了，并且中断执行当前方法，然而，在我们mainThread中也没见Looper退出的情况啊（退出后，触摸事件都会废掉，你怕不怕）。下节讲述机智的MessageQueue。

3.MessageQueue分析

代码中第一次提到MessageQueue还是在Looper初创时，从MessageQueue构造方法看起：

MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}

quitAllowed这参数看起来有点意思，从名字就可以知道，作用是判断是否可退，这就不奇怪了，若是mainThread中的MessageQueue在程序还在执行时退出了，后果不堪想象，所以肯定在mainThread中是不可退出的，Looper中的prepareMainLooper方法也默默地支持了这种说法：

public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}

记得之前prepare中说暂不讨论的参数么，就是这个判断是否可以推出MessageQueue的boolean。

本地方法nativeInit（）暂不影响分析，等需要的时候在讲解

下一个讲解的方法就是在Looper中提到的那个next():

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;
}
}

首先到的是 方法中的long类型 ptr ,先做搁置，不予理会

剩下的两个int型参数 pendingIdleHandlerCount与nextPollTimeoutMillis，一个是指IdleHandler调用的数量（后边会讲），一个则是最近下次Message事件发生的时间。然后，就又是熟悉的for(;;)循环，Binder。flushPendingCommands()暂且不管，后续会有关于Binder的博文。之后就是一个nativePollonce(ptr,nextPollTimeoutMillis)方法，名字听起来有些古怪：轮询一次......输入值有ptr,以及最短下次事件发生时间，这就有意思了，观察来看可能与事件的发生有关。由于是native方法，所以就直接讲解功用吧，具体代码，后续会进行分析。就是这个方法会只能的阻塞线程，之前说过，app惧怕线程阻塞，然而这里的阻塞更接近于等待而不是持续占用系统资源，一旦有相关的时间发生，线程就会在等待的状态恢复，执行事件内容,而这样机制的实现则基于Linux的poll,后续会在native代码中讲解。

之后就简单了，代码上同步锁，做事件类型的判断，要注意的是

if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run.  Loop and wait some more.
mBlocked = true;
continue;
}

当IdleHandler没有要发生的事件时，则会返回到for(;;)最端，另外要说的就是，当所有的Message都执行完时，nativePollonce中的时间传入是-1，也就是说进入线程等待状态，直至有事件发生。

顺便提下MessageQueue中的IdleHandler接口，实现这个接口，则会在MessageQueue执行完所有的或者当前空闲时，执行IdleHander中的内容，具体用法请百度或谷歌，不再讲述。

这里可能有点跳跃，讲下Message是如何加入Message链的

MessageQueue enquequeMessage 上代码：

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;
}

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;
}

可以看到前两个if的本质就是在判断加入的msg是否有目标handler以及是否处于在使用状态，当msg有目标handler且自身并没有处于使用状态，则会上同步锁，手心啊判断的就是Looper是否推出，之后对msg对应状态进行修改，并根据msg属性判断是否需要唤醒线程，这就和上边讲的 在nativepollonce方法中传入-1进行无限制等待对应起来了，当需要时，会通过nativeWake方法进行线程激活。

4.Handler

Looper,MessageQueue，Handler相交会简单不少，闲话不说，从new Handler开始：

public Handler() {
this(null, 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;
}

其中async是指消息是否同步，不影响分析不予理会

之后的if中则是判断当前Handler以及其继承类在被调用时存在的状态，三个isXXX对应的是，匿名类，成员变量以及局部变量（比如在方法中进行声明并new出来），改举动是为了方式出现内存泄漏

之后的mLooper,如果不是进行指定，则对应的是当前线程的Looper,也就是说若是当前线程Looper还没有被建立，则会抛出异常，new完之后则是开发最为关心的post了：

public final boolean post(Runnable r)
{
return  sendMessageDelayed(getPostMessage(r), 0);
}

public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
}

可见，无论是post还是postDelayed，其本质都是对应的sendMessageDelayed，只不过对延迟时间设置的不同而已，继续往下看：

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);
}

嗯 ，这次找到正主了，无论如何花式的post或者send，最后都会在enqueueMessage方法中通过调用Looper对应的queue添加到Message链中，然后，进行数据处理，以及判断是否需要激活线程。

至于message分发就不再讲述了：

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

很简单的源码，这就是为什么我们使用可以通过父类的handleMessage方法获取Message的原因。

进阶部分

这里只简单的分析下MessageQueue中重要的native方法（此处将使用5.1的源码）。

nativeInit

nativeInit 位于android_os_MessageQueue.cpp（android5.1\frameworks\base\core\jni）

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}

nativeMessageQueue->incStrong(env);
return reinterpret_cast<jlong>(nativeMessageQueue);
}

其中incStrong作用强引用计数，可忽略。可以看出在nativeInit中只是进行了底层 MessageQueue与 应用层MessageQueue的对应生命，两者之间并没有什么关系，但是，这个一jlong形式返回的对象则就是前面提到的mPtr，在唤醒或pollonce的时候，传入的mPtr，也会被转换为NativeMessageQueue.

nativePollOnce

nativePollonce 同样位于android_os_MessageQueue.cpp（android5.1\frameworks\base\core\jni）

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jclass clazz,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, timeoutMillis);
}

在nativePollOnce中，首先将前边提到mPtr转换为对应的NativeMessageQueue,然后调用方法pollOnce:

void NativeMessageQueue::pollOnce(JNIEnv* env, int timeoutMillis) {
mInCallback = true;
mLooper->pollOnce(timeoutMillis);
mInCallback = false;
if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}

查看头文件，可发现mLooper,其实就是Looper的指针

然后，继续跟踪pollOnce（）（/ frameworks / native / jb-dev / . / libs / utils / Looper.cpp）

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
int result = 0;
for (;;) {
while (mResponseIndex < mResponses.size()) {
const Response& response = mResponses.itemAt(mResponseIndex++);
int ident = response.request.ident;
if (ident >= 0) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - returning signalled identifier %d: "
"fd=%d, events=0x%x, data=%p",
this, ident, fd, events, data);
#endif
if (outFd != NULL) *outFd = fd;
if (outEvents != NULL) *outEvents = events;
if (outData != NULL) *outData = data;
return ident;
}
}
if (result != 0) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - returning result %d", this, result);
#endif
if (outFd != NULL) *outFd = 0;
if (outEvents != NULL) *outEvents = 0;
if (outData != NULL) *outData = NULL;
return result;
}
result = pollInner(timeoutMillis);
}
}

虽然找到了代码，发现参数并不对应，这时候就应该去头文件看看，会有惊喜：

int pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData);
inline int pollOnce(int timeoutMillis) {
return pollOnce(timeoutMillis, NULL, NULL, NULL);
}

这下就明白了，虽然调用的是pollOnce(int timeoutMillis)本质上则是还是之前的pollOnce(timeoutMillis,null,null,null);

再看下pollInner:

int Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);
#endif
// Adjust the timeout based on when the next message is due.
if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime);
if (messageTimeoutMillis >= 0
&& (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {
timeoutMillis = messageTimeoutMillis;
}
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - next message in %lldns, adjusted timeout: timeoutMillis=%d",
this, mNextMessageUptime - now, timeoutMillis);
#endif
}
// Poll.
int result = ALOOPER_POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
// Acquire lock.
mLock.lock();
// Check for poll error.
if (eventCount < 0) {
if (errno == EINTR) {
goto Done;
}
ALOGW("Poll failed with an unexpected error, errno=%d", errno);
result = ALOOPER_POLL_ERROR;
goto Done;
}
// Check for poll timeout.
if (eventCount == 0) {
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - timeout", this);
#endif
result = ALOOPER_POLL_TIMEOUT;
goto Done;
}
// Handle all events.
#if DEBUG_POLL_AND_WAKE
ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endif
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeReadPipeFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
}
} else {
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex >= 0) {
int events = 0;
if (epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
if (epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
if (epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
if (epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
pushResponse(events, mRequests.valueAt(requestIndex));
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
}
Done: ;
// Invoke pending message callbacks.
mNextMessageUptime = LLONG_MAX;
while (mMessageEnvelopes.size() != 0) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
if (messageEnvelope.uptime <= now) {
// Remove the envelope from the list.
// We keep a strong reference to the handler until the call to handleMessage
// finishes.  Then we drop it so that the handler can be deleted *before*
// we reacquire our lock.
{ // obtain handler
sp<MessageHandler> handler = messageEnvelope.handler;
Message message = messageEnvelope.message;
mMessageEnvelopes.removeAt(0);
mSendingMessage = true;
mLock.unlock();
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",
this, handler.get(), message.what);
#endif
handler->handleMessage(message);
} // release handler
mLock.lock();
mSendingMessage = false;
result = ALOOPER_POLL_CALLBACK;
} else {
// The last message left at the head of the queue determines the next wakeup time.
mNextMessageUptime = messageEnvelope.uptime;
break;
}
}
// Release lock.
mLock.unlock();
// Invoke all response callbacks.
for (size_t i = 0; i < mResponses.size(); i++) {
Response& response = mResponses.editItemAt(i);
if (response.request.ident == ALOOPER_POLL_CALLBACK) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
this, response.request.callback.get(), fd, events, data);
#endif
int callbackResult = response.request.callback->handleEvent(fd, events, data);
if (callbackResult == 0) {
removeFd(fd);
}
// Clear the callback reference in the response structure promptly because we
// will not clear the response vector itself until the next poll.
response.request.callback.clear();
result = ALOOPER_POLL_CALLBACK;
}
}
return result;
}

最为关键的一句代码就是：

eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

当timeoutMillis则会令线程处于等待状态，而且上可知，除了县城的等待与唤醒 Jlong对象之外，framework层关于MessageQueue looper的代码并没有与应用层中的数据有什么实质的交换，也就是说，Framework层的Looper机制与应用层对应相似，但是其主要任务就是对当前线程进行唤醒和等待。nativeWake就不予讲述了，有兴趣自行观看。 如果有时间，会讲解下epoll poll的使用与区别。

最后一点：android线程中，除了线程变量外，同一进程的变量在同进程不同的线程间是共享的，所以，所谓的android线程间通信，实质是线程的等待与唤醒