Android学习心得（24） --- Android Handler消息机制源码分析

Android Handler消息机制源码分析

相关类

1. Looper
2. MessageQueue
3. Message
4. Handler

基础知识

1. ThreadLocal

2. epoll

3. pipe

4. idleHandler

使用过程

使用方法：
1、Looper.prepare()
2、创建Handler实例，重写handleMessage处理函数
3、Looper.loop()
4、调用相关的发送Message函数，将消息发送出去
5、调用相关处理函数处理

Looper类

Androidxref.com - Looper.java源码链接

1. prepare()

/** 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"); 
        // 每个线程拥有一个Lopper，不能多次创建
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

2. Looper构造函数：

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed); // 创建了对应的MessageQueue
    mRun = true;
    mThread = Thread.currentThread();
}

Looper中会创建MessageQueue，并设置当前运行状态和当前线程

3. Looper对象获取

/**
 * 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();
}

4. 获取主线程Looper

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

5. loop()

负责分发消息，主要通过在for循环中实现
Message msg = queue.next();
msg.target.dispatchMessage(msg);

/**
 * 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();  // 获取Looper
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;  // 获取其MessageQueue

    // 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 (;;) {
        // 取出一条消息
        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);
        }

        // 分发消息
        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.recycle();
    }
}

MessageQueue

Androidxref - MessageQueue源码链接
Androidxref - android_os_MessageQueue.cpp源码链接
Androidxref - Looper.cpp源码链接
为了能够很多的理解该类的作用，主要从三方面入手：创建过程、消息处理、发送消息
源码中定义了5个native函数，分别实现了MessageQueue的主体功能

private native static int nativeInit();
private native static void nativeDestroy(int ptr);
private native static void nativePollOnce(int ptr, int timeoutMillis);
private native static void nativeWake(int ptr);
private native static boolean nativeIsIdling(int ptr);

1. 创建过程

Looper在创建过程中会创建一个MessageQueue，并将其存放到成员变量mQueue中

// MessageQueue构造函数
MessageQueue(boolean quitAllowed) {
    mQuitAllowed = quitAllowed;
    mPtr = nativeInit();
}

主要调用了nativeInit()函数来实现创建功能

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

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

NativeMessageQueue函数，该函数功能主要创建本地Looper类对象，从功能函数中可以看到，MessageQueue最后操作的对象还是Native层Looper类对象

NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {
    mLooper = Looper::getForThread();
    if (mLooper == NULL) {
        mLooper = new Looper(false);
        Looper::setForThread(mLooper);
    }
}

Looper.cpp构造函数
Looper创建过程主要是创建管道、使用epoll来监听管道上的数据

Looper::Looper(bool allowNonCallbacks) :
        mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
        mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
    int wakeFds[2];
    // 创建匿名管道
    int result = pipe(wakeFds);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe.  errno=%d", errno);

    mWakeReadPipeFd = wakeFds[0];
    mWakeWritePipeFd = wakeFds[1];

    // 读写管道设置为非阻塞
    result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking.  errno=%d",
            errno);

    result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake write pipe non-blocking.  errno=%d",
            errno);

    mIdling = false;

    // Allocate the epoll instance and register the wake pipe.
    // 创建epoll对象并进行注册
    mEpollFd = epoll_create(EPOLL_SIZE_HINT);
    LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance.  errno=%d", errno);

    struct epoll_event eventItem;
    memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
    eventItem.events = EPOLLIN;
    eventItem.data.fd = mWakeReadPipeFd;
    result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake read pipe to epoll instance.  errno=%d",
            errno);
}

2. 消息处理

MessageQueue.next()用于取出一条消息

Message next() {
    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }

        // 调用本地方法等待nextPollTimeoutMillis，具体等待时候会在下方msg中不断更新
        nativePollOnce(mPtr, nextPollTimeoutMillis);

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            // mMessages指向队列头
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // 当msg.target为null，即该消息为SyncBarrier，则忽略该消息，并向后找到第一个异步消息
                do {
                    prevMsg = msg;
                    msg = msg.next;
                    // 找到第一个isAsynchronous标志msg
                } 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.   
                    // 该消息执行时间未到，等待nextPollTimeoutMillis
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    // 类似于链表删除，将要处理的消息从中断开
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;  
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (false) Log.v("MessageQueue", "Returning message: " + msg);
                    msg.markInUse();
                    // 返回要处理的消息
                    return msg;  
                }
            } else {
                // No more messages.
                // 当等待时间为-1时阻塞
                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.
        // 当没有消息要处理，检查是否安装了idle回调函数，调用并获取其状态
        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();  // 返回该处理的函数的状态
            } catch (Throwable t) {
                Log.wtf("MessageQueue", "IdleHandler threw exception", t);
            }

            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);  // 移除
                }
            }
        }

        // 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;  // 重置等待时间
    }
}

nativePollOnce函数，等待功能通过epoll_wait实现

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

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

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    ...
    ...
   result = pollInner(timeoutMillis);
}

int Looper::pollInner(int timeoutMillis) {
    ...
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
    ...
}

3. 发送消息

向MessageQueue中发送消息使用enqueueMessage
主要是根据时间插入消息，要考虑有SybBarrier和异步消息的情况

boolean enqueueMessage(Message msg, long when) {
    if (msg.isInUse()) {  // 如果消息正在处理，则报错
        throw new AndroidRuntimeException(msg + " This message is already in use.");
    }
    if (msg.target == null) {  // 如果消息target为空，则报错
        throw new AndroidRuntimeException("Message must have a target.");
    }

    synchronized (this) {   // 使用this来同步，表示一次只能有一个进入enqueueMessage
        if (mQuitting) {  
            RuntimeException e = new RuntimeException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w("MessageQueue", e.getMessage(), e);
            return false;
        }

        msg.when = when;
        Message p = mMessages;  // 获取头消息
        boolean needWake;
        // 插入消息操作
        // 1. 当前没有消息，则插入头部
        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.
            // 判断是否有SynBarrier && 是否有异步消息 
            needWake = mBlocked && p.target == null && msg.isAsynchronous();  
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                // 根据消息的时间来确定存放的位置
                if (p == null || when < p.when) {
                    break;
                }
                // 如果有异步消息，则不用唤醒
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            // 插入线程
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }

        // We can assume mPtr != 0 because mQuitting is false.
        // 唤醒线程
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

Message类

这个就不再多说了，直接给出源码:
Androidxref - Message.java源码链接

Handler类

Handler主要实现的是消息的发送和处理
两种方式： send && post

1. Send

该方法是根据Message自身的ID来判断，从而执行不同的处理操作

public final boolean sendMessage(Message msg);

public final boolean sendEmptyMessage(int what);

public final boolean sendEmptyMessageDelayed(int what, long delayMillis);

public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis);

public final boolean sendMessageDelayed(Message msg, long delayMillis);

public boolean sendMessageAtTime(Message msg, long uptimeMillis);

public final boolean sendMessageAtFrontOfQueue(Message msg);

2. Post

而Post方法则是需要有消息的响应代码，不再局限于预定于内容，而是可以定制相关处理方法

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

public final boolean postAtTime(Runnable r, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}

public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)
{
    return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);
}

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

public final boolean postAtFrontOfQueue(Runnable r)
{
    return sendMessageAtFrontOfQueue(getPostMessage(r));
}

最终上述两个方法都会调用enqueueMessage方法添加消息

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);
}