安卓17版本上google引入了无锁消息队?老版本MessageQueue的锁解密剖析

/**     * 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.     */publicfinal boolean sendMessage(@NonNull Message msg){return sendMessageDelayed(msg, 0);    }

注意：post消息其实也是最后调用到了sendMessage相关方法

/**     * Causes the Runnable r to be added to the message queue.     * The runnable will be run on the thread to which this handler is      * attached.      *       * @param r The Runnable that will be executed.     *      * @return Returns true if the Runnable was successfully placed in to the      *         message queue.  Returns false on failure, usually because the     *         looper processing the message queue is exiting.     */publicfinal boolean post(@NonNull Runnable r){return  sendMessageDelayed(getPostMessage(r), 0);    }

这里会调用到sendMessageDelayed

publicfinal boolean sendMessageDelayed(@NonNull Message msg, long delayMillis){if (delayMillis < 0) {            delayMillis = 0;        }return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }

这里又会调用到sendMessageAtTime方法

public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis){        MessageQueue queue = mQueue;return enqueueMessage(queue, msg, uptimeMillis);//调用到enqueueMessage    }

再看看enqueueMessage方法

private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,long uptimeMillis){        msg.target = this;        msg.workSourceUid = ThreadLocalWorkSource.getUid();if (mAsynchronous) {            msg.setAsynchronous(true);        }returnqueue.enqueueMessage(msg, uptimeMillis);//这里最后调用到MessageQueue的enqueueMessage方法    }

再看看MessageQueue的enqueueMessage方法：

frameworks/base/core/java/android/os/MessageQueue.java

boolean enqueueMessage(Message msg, long when){if (msg.target == null) {thrownew IllegalArgumentException("Message must have a target.");        }        synchronized (this) {//这里是锁            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);            }        }returntrue;    }

MessageQueue的native部分锁lock展示：

看看native层面如何发送Message可以看到主要调用是sendMessage方法 system/core/libutils/Looper.cpp

voidLooper::sendMessage(const sp<MessageHandler>& handler, const Message& message){nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);    sendMessageAtTime(now, handler, message);}

在看看sendMessageAtTime方法

voidLooper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,const Message& message){size_t i = 0;    { // acquire lock        AutoMutex _l(mLock);//这里也有一个锁保护size_t messageCount = mMessageEnvelopes.size();while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {            i += 1;        }MessageEnvelope messageEnvelope(uptime, handler, message);        mMessageEnvelopes.insertAt(messageEnvelope, i, 1);// Optimization: If the Looper is currently sending a message, then we can skip// the call to wake() because the next thing the Looper will do after processing// messages is to decide when the next wakeup time should be.  In fact, it does// not even matter whether this code is running on the Looper thread.if (mSendingMessage) {return;        }    } // release lock// Wake the poll loop only when we enqueue a new message at the head.if (i == 0) {        wake();    }}

可以看到在 Looper::sendMessageAtTime方法中对mMessageEnvelopes进行操作时候都是有lock进行保护的。

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