首页 > 代码库 > 【内核研究】消息队列_MessageQueue
【内核研究】消息队列_MessageQueue
消息队列采用排队方式对消息进行处理,即先到的消息会先得到处理,但如果消息本身指定了被处理的时刻,则必须等到该时刻才能处理该消息。消息在MessageQueue中使用Message类表示,队列中的消息以链表的结构进行保存,Message对象内部包含一个next变量,该变量指向下一个消息对象。
MessageQueue中的两个主要函数是“取出消息”和“添加消息”,分别是next()和enquenceMessage()。
next()函数
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) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); final Message msg = mMessages; if (msg != null) { final long when = msg.when; if (now >= when) { mBlocked = false; mMessages = msg.next; msg.next = null; if (Config.LOGV) Log.v("MessageQueue", "Returning message: " + msg); return msg; } else { nextPollTimeoutMillis = (int) Math.min(when - now, Integer.MAX_VALUE); } } else { nextPollTimeoutMillis = -1; } // If first time, then get the number of idlers to run. if (pendingIdleHandlerCount < 0) { 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; } }
该函数的内部流程分三步:
1. 调用nativePollOnce(mPtr, int time)。这是一个JNI函数,作用是从消息队列中取出一个消息。MessageQueue类内部本身并没有保存消息队列,真正的消息队列数据保存在JNI中的C代码中,在C环境中创建了一个NativeMessageQueue数据对象,这就是nativePollOnce()第一个参数的意义。它是一个int型变量,在C环境中,该变量被强制转换为一个NativeMessageQueue对象。在C环境中,如果消息队列中没有消息,将导致当前线程被挂起(wait),如果消息队列中有消息,则C代码中将把该消息赋值给Java环境中的mMessages变量。
2. 接下来的代码被包含在synchronized(this)中,this被用作取消息和写消息的锁。仅仅是判断消息所指定的执行时间是否到了。如果到了,就返回该消息,并将mMessages变量置空;如果时间还没有到,则尝试读取下一个消息。
3. 如果mMessages为空,则说明C环境中的消息队列没有可执行的消息了,因此,执行mPendingIdleHanlder列表中的“空闲回调函数”。可以向MessageQueue中注册一些“空闲回调函数”,从而当线程中没有消息可处理的时候去执行这些“空闲代码”。
enquenceMessage()函数
final boolean enqueueMessage(Message msg, long when) { if (msg.when != 0) { throw new AndroidRuntimeException(msg + " This message is already in use."); } if (msg.target == null && !mQuitAllowed) { throw new RuntimeException("Main thread not allowed to quit"); } final boolean needWake; synchronized (this) { if (mQuiting) { RuntimeException e = new RuntimeException( msg.target + " sending message to a Handler on a dead thread"); Log.w("MessageQueue", e.getMessage(), e); return false; } else if (msg.target == null) { mQuiting = true; } msg.when = when; //Log.d("MessageQueue", "Enqueing: " + msg); Message p = mMessages; if (p == null || when == 0 || when < p.when) { msg.next = p; mMessages = msg; needWake = mBlocked; // new head, might need to wake up } else { Message prev = null; while (p != null && p.when <= when) { prev = p; p = p.next; } msg.next = prev.next; prev.next = msg; needWake = false; // still waiting on head, no need to wake up } } if (needWake) { nativeWake(mPtr); } return true; }该函数内部分为两步:
1. 将参数msg赋值给mMessages。
2. 调用nativeWake(mPtr)。这是一个JNI函数,其内部会将mMessages消息添加到C环境中的消息队列中,并且如果消息线程正处于挂起(wait)状态,则唤醒该线程。
【内核研究】消息队列_MessageQueue