Android多线程分析之四：MessageQueue的实现
在前面两篇文章《Android多线程分析之二：Thread的实现》，《Android多线程分析之三：Handler，Looper的实现》中分别介绍了 Thread 的创建，运行，销毁的过程以及 Thread与 Handler，Looper 之间的关联：Thread 在其 run() 方法中创建和运行消息处理循环 Looper，而 Looper::loop() 方法不断地从 MessageQueue 中获取消息，并由 Handler 分发处理该消息。接下来就来介绍 MessageQueue 的运作机制，MessageQueue。
参考源码：
android/framework/base/core/java/android/os/MessageQueue.java android/framework/base/core/java/android/os/Message.java android/frameworks/base/core/jni/android_os_MessageQueue.h android/frameworks/base/core/jni/android_os_MessageQueue.cpp
// True if the message queue can be quit.
private final boolean mQuitAllowed;
private int mPtr; // used by native code
Message mMessages;
private boolean mQuiting;
// Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
private boolean mBlocked;MessageQueue 的构造函数很简单：
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
nativeInit();
}static void android_os_MessageQueue_nativeInit(JNIEnv* env, jobject obj) {
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return;
}
nativeMessageQueue->incStrong(env);
android_os_MessageQueue_setNativeMessageQueue(env, obj, nativeMessageQueue);
}
static void android_os_MessageQueue_setNativeMessageQueue(JNIEnv* env, jobject messageQueueObj,
NativeMessageQueue* nativeMessageQueue) {
env->SetIntField(messageQueueObj, gMessageQueueClassInfo.mPtr,
reinterpret_cast<jint>(nativeMessageQueue));
}class MessageQueue : public RefBase {
public:
/* Gets the message queue‘s looper. */
inline sp<Looper> getLooper() const {
return mLooper;
}
bool raiseAndClearException(JNIEnv* env, const char* msg);
virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj) = 0;
protected:
MessageQueue();
virtual ~MessageQueue();
protected:
sp<Looper> mLooper;
};
class NativeMessageQueue : public MessageQueue {
public:
NativeMessageQueue();
virtual ~NativeMessageQueue();
virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj);
void pollOnce(JNIEnv* env, int timeoutMillis);
void wake();
private:
bool mInCallback;
jthrowable mExceptionObj;
};NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
void NativeMessageQueue::pollOnce(JNIEnv* env, int timeoutMillis) {
mInCallback = true;
mLooper->pollOnce(timeoutMillis);
mInCallback = false;
}
void NativeMessageQueue::wake() {
mLooper->wake();
}对于Android MessageQueue 来说，其主要的工作就是：接收投递进来的消息，获取下一个需要处理的消息。这两个功能是通过 enqueueMessage() 和 next() 方法实现的。next() 在前一篇文章介绍 Looper.loop() 时提到过。
在分析这两个函数之前，先来介绍一下 Message：前面说过 Message 是完备的，即它同时带有消息内容和处理消息的 Handler 或 callback。下面列出它的主要成员变量：
public int what; // 消息 id public int arg1; // 消息参数 public int arg2; // 消息参数 public Object obj; // 消息参数 long when; // 处理延迟时间，由 Handler 的 sendMessageDelayed/postDelayed 设置 Handler target; // 处理消息的 Handler Runnable callback; // 处理消息的回调 Message next; // 链表结构，指向下一个消息
接下来分析 enqueueMessage：
final boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}
boolean needWake;
synchronized (this) {
if (mQuiting) {
return false;
}
msg.when = when;
Message p = mMessages;
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;
}
}
if (needWake) {
nativeWake(mPtr);
}
return true;
} 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);
}下面来分析如何从 MessageQueue 中获取合适的消息, 这是 next() 要做的最主要的事情，next() 方法还做了其他一些事情，这些其它事情是为了提高系统效果，利用消息队列在空闲时通过 idle handler 做一些事情，比如 gc 等等。但它们和获取消息关系不大，所以这部分将从略介绍。
final Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(mPtr, nextPollTimeoutMillis);
synchronized (this) {
if (mQuiting) {
return null;
}
// 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;
}
// 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;
}
} 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;
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
msg.target.dispatchMessage(msg);
msg.recycle();
}
}如果队列中没有消息或者第一个待处理的消息时机未到，且也没有其他利用队列空闲要处理的事务，则将队列设置为设置 blocked 状态，进入等待状态；否则就利用队列空闲处理其它事务。至此，已经对 Android 多线程相关的主要概念 Thread, HandlerThread, Handler, Looper, Message, MessageQueue 作了一番介绍，下一篇就要讲讲 AsyncTask，这是为了简化 UI 多线程编程为提供的一个便利工具类。
Android多线程分析之四：MessageQueue的实现,布布扣,bubuko.com
Android多线程分析之四：MessageQueue的实现
原文地址：http://blog.csdn.net/kesalin/article/details/37765707
