Android 5 消息机制源码分析

消息模型

基本要素：
消息队列、消息发送、消息读取、消息分发、消息循环线程。
操作系统原理中的生产者线程和消费者线程有着类似的过程：

Android中的消息机制跟这个很类似，关键的几个名词如下：

• Handler
• Message
• Message Queue
• Looper

---

总览

这里是从网上找的一张图，在此感谢原作者。这里使用的是Android 5.1源码。
从这张图上我们可以大致看出Android中消息的执行过程。
Handler是依附于当前线程的，它在创建的时候，会使用当前线程的Looper来构造内部的消息循环系统。在Handler的运行过程中，由Handler发送一个Message给Handler所在线程的MessageQueue消息队列，Looper负责对消息进行转发处理。一个线程对应的Handler可以有多个，但是MessageQueue和Looper则只有一个。

Message

Message实现了Parcelable接口，是一个可序列化的类。
构造方法是一个空方法，注释中可以看到，建议使用obtian()去获取一个Message实例而不是通过new：

// sometimes we store linked lists of these things
/*package*/ Message next;
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;

/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
public Message() {
}

/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
   synchronized (sPoolSync) {
       if (sPool != null) {
           Message m = sPool;
           sPool = m.next;
           m.next = null;
           m.flags = 0; // clear in-use flag
           sPoolSize--;
           return m;
        }
   }
   return new Message();
}

这里的实现代码比较简单，第一次的时候sPool值为null，也就是会执行new操作，创建一个Message对象，之后会把这个对象进行复用，通过Message的结构可以看到，维持了一个类似链表的复用关系。sPool代表接下来要被重用的Message对象，sPoolSize表示被重用的对象数目；MAX_POOL_SIZE是pool的最大容量，默认为50个。

public void recycle() {
   if (isInUse()) {
      if (gCheckRecycle) {
           throw new IllegalStateException("This message cannot be recycled because it "
                 + "is still in use.");
      }
      return;
   }
   recycleUnchecked();
}

/**
* Recycles a Message that may be in-use.
* Used internally by the MessageQueue and Looper when disposing of queued Messages.
*/
void recycleUnchecked() {
    // Mark the message as in use while it remains in the recycled object pool.
    // Clear out all other details.
    flags = FLAG_IN_USE;
    what = 0;
    arg1 = 0;
    arg2 = 0;
    obj = null;
    replyTo = null;
    sendingUid = -1;
    when = 0;
    target = null;
    callback = null;
    data = null;

    synchronized (sPoolSync) {
        if (sPoolSize < MAX_POOL_SIZE) {
           next = sPool;
           sPool = this;
           sPoolSize++;
        }
     }
 }

回收操作会导致每次被使用完毕的Message进入复用链。

MessageQueue

消息队列，这里就是一个单链表。学过数据结构大家都知道，对于链表的基本操作主要就是访问，插入和删除。这里主要是从消息队列中获取消息，以及往队列中插入一个消息。

    private final boolean mQuitAllowed;
    @SuppressWarnings("unused")
    private long mPtr; // used by native code
    Message mMessages;//消息队列
    private final ArrayList mIdleHandlers = new ArrayList();
    private IdleHandler[] mPendingIdleHandlers;
    private boolean mQuitting;

    // Indicates whether next() is blocked waiting in pollOnce() with a non-zero timeout.
    private boolean mBlocked;

    // The next barrier token.
    // Barriers are indicated by messages with a null target whose arg1 field carries the token.
    private int mNextBarrierToken;

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

mMessages指向消息队列。这列声明了几个Native方法，意味着Java层的实现依赖于C层。

首先看一下进队的操作：

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.
            // 新消息会插入到链表的表头，意味着队列需要调整唤醒时间
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            //插入到队列中间通常不用唤醒事件队列，除非在队头部有一个同步分隔栏，并且这个消息是队列中最早进来的异步消息
            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;
}

首先判断了消息的target（其实就是发送消息的Handler）是否存在，又检查了当前这个消息是否正在使用。校验过了，会设置msg.markInUse();表明当前消息在使用中。接着就是插入单链表的操作了，判断链表当前是否为空，为空则插入的消息成为队首元素，采用头插法进行插入，不过需要注意同步分隔栏。最后，如果needWake为true，调用native方法nativeWake()唤醒。

看看这个函数做了啥：
【frameworks/base/core/jni/android_os_MessageQueue.cpp】

static JNINativeMethod gMessageQueueMethods[] = {
    /* name, signature, funcPtr */
    { "nativeInit", "()J", (void*)android_os_MessageQueue_nativeInit },
    { "nativeDestroy", "(J)V", (void*)android_os_MessageQueue_nativeDestroy },
    { "nativePollOnce", "(JI)V", (void*)android_os_MessageQueue_nativePollOnce },
    { "nativeWake", "(J)V", (void*)android_os_MessageQueue_nativeWake },
    { "nativeIsIdling", "(J)Z", (void*)android_os_MessageQueue_nativeIsIdling }
};

可以看到jni注册到了函数android_os_MessageQueue_nativeWake ：

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast(ptr);
    return nativeMessageQueue->wake();
}

void NativeMessageQueue::wake() {
    mLooper->wake();
}

这里一看，又转到了Looper中去了。
【system/core/libutils/Looper.cpp】

void Looper::wake() {
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ wake", this);
#endif

    ssize_t nWrite;
    do {
        nWrite = write(mWakeWritePipeFd, "W", 1);
    } while (nWrite == -1 && errno == EINTR);

    if (nWrite != 1) {
        if (errno != EAGAIN) {
            ALOGW("Could not write wake signal, errno=%d", errno);
        }
    }
}

这里向mWakeWritePipeFd管道里中写了个”W”。

接下来看看出队操作：

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;// 取消息队列里当前第一个消息
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                // 如果从队列里拿到的msg是个“同步分割栏”，那么就寻找其后第一个“异步消息”
                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;// 重置消息队列的头部
                    }
                    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;
    }
}

这个方法有点儿长，主要就是从消息队列中取出一个消息，并从队列中移除。从for(;;)可以看出，这个是一个无线循环。nativePollOnce(ptr, nextPollTimeoutMillis);可能会阻塞。

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());
}

这里从注释上，我们可以看到，当前队列如果被Barrier卡住，也就是队列中插了一个同步分隔栏，那么就去找队列中的下一个异步消息。方法最后有个IdleHandler的循环，当消息队列中没有消息需要马上处理时，会判断用户是否设置了Idle Handler，如果有的话，则会尝试处理mIdleHandlers中所记录的所有Idle Handler，此时会逐个调用这些Idle Handler的queueIdle()方法。

Looper

【frameworks/base/core/java/android/os/Looper.java】

public final class Looper {
    private static final String TAG = "Looper";

    // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal sThreadLocal = new ThreadLocal();
    private static Looper sMainLooper;  // guarded by Looper.class

    final MessageQueue mQueue;
    final Thread mThread;

    private Printer mLogging;
    ...
}

可以看到，Looper中维护了一个ThreadLocal变量，用于线程隔离存储。

 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");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

这两个方法是平时用的最多的，当我们在非UI线程中使用Handler时，一般都需要先创建一个Looper，然后才能发送消息。那么创建的操作就是这里的prepare方法。从代码的实现来看，当实例化Looper时，会把当前线程对应的Looper存储到ThreadLocal中，从而保证每个线程的Looper是唯一的，与其他线程之间隔离的。
此外还有如下的方法：

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

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

prepareMainLooper是给主线程使用的，我们可以在ActivityThread的main方法中看到它。而getMainLooper为在应用中获取主线程的Looper提供了便捷。

接下来看看Looper的主要方法loop()：

/**
 * 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();
    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();
    }
}

方法开始处获取Looper对象实例，并做了校验，下面这个异常相信大家都遇到过：”No Looper; Looper.prepare() wasn’t called on this thread.”
在获取到消息队列的实例之后，开始了无限循环。
关键的是下面这两句：

Message msg = queue.next();
msg.target.dispatchMessage(msg);

第一个就是前面说过的MessageQueue的next()方法，阻塞式获取Message对象。
第二句，前面说过，这个target就是发送消息到MessageQueue的handler对象。把消息又转发到Handler的的handleMessage方法中去具体处理。这样看来，Looper其实就是一个中转的作用。
我们再进到MessageQueue的next()中看一下：

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);
            ...
}

刚才看过这个方法，不过这个地方有个nativePollOnce没有分析。nativePollOnce()起到了阻塞作用，保证消息循环不会在无消息处理时一直在那里空跑。看看这个Native方法是怎么实现的？
从前面的注册器上看：
实现在jni函数：android_os_MessageQueue_nativePollOnce

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

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

这个地方又出现了一个Looper。这个是Looper在C++层的实现，那这个有什么不一样的地方么？
【system/core/include/utils/Looper.h】
【system/core/libutils/Looper.cpp】
看下构造函数：

Looper::Looper(bool allowNonCallbacks) :
        mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
        mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
    int wakeFds[2];
    // 创建管道
    int result = pipe(wakeFds);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe.  errno=%d", errno);
    // 管道的“读取端”
    mWakeReadPipeFd = wakeFds[0];
    // 管道的“写入端”
    mWakeWritePipeFd = wakeFds[1];
    //读
    result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);
    LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking.  errno=%d",
            errno);
    //写
    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;

    // 创建epoll实例，并注册唤醒管道
    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;
    // 监听管道读取端
    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);
}

这段代码来看，是在Looper中创建了一个管道，然后通过epoll监听管道的读取端，当向消息队列发送消息时，最终是向管道的写入端写入数据。前面写入”W”只是一个通知有消息来了。

接着前面看下pollOnce函数：
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    int result = 0;
    for (;;) {
        ...
        if (result != 0) {
        ...
            if (outFd != NULL) *outFd = 0;
            if (outEvents != NULL) *outEvents = 0;
            if (outData != NULL) *outData = NULL;
            return result;
        }
        result = pollInner(timeoutMillis);
    }
}

调到了pollInner函数：

int Looper::pollInner(int timeoutMillis) {
    ...
    // Poll.
    int result = POLL_WAKE;
    mResponses.clear();
    mResponseIndex = 0;

    // We are about to idle.
    mIdling = true;

    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
    ....

    for (int i = 0; i < eventCount; i++) {
        int fd = eventItems[i].data.fd;
        uint32_t epollEvents = eventItems[i].events;
        if (fd == mWakeReadPipeFd) {//这个是唤醒的读管道描述符
            if (epollEvents & EPOLLIN) {
                awoken();//从管道中感知到EPOLLIN,调用awoken()
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
            }
        } else {//其他情况的消息事件
            ssize_t requestIndex = mRequests.indexOfKey(fd);
            if (requestIndex >= 0) {
                int events = 0;
                if (epollEvents & EPOLLIN) events |= EVENT_INPUT;
                if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;
                if (epollEvents & EPOLLERR) events |= EVENT_ERROR;
                if (epollEvents & EPOLLHUP) events |= 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: ;
    //调用等待处理的消息回调
    // 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) {
           ....
            { // obtain handler
                sp handler = messageEnvelope.handler;
                Message message = messageEnvelope.message;
                mMessageEnvelopes.removeAt(0);
                mSendingMessage = true;
                mLock.unlock();
                handler->handleMessage(message);
            } // release handler

            mLock.lock();
            mSendingMessage = false;
            result = POLL_CALLBACK;
        } else {
            // The last message left at the head of the queue determines the next wakeup time.
            mNextMessageUptime = messageEnvelope.uptime;
            break;
        }
    }
    .......
    //调用所有response记录的回调
    for (size_t i = 0; i < mResponses.size(); i++) {
        Response& response = mResponses.editItemAt(i);
        if (response.request.ident == 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);
            }

            response.request.callback.clear();
            result = POLL_CALLBACK;
        }
    }
    return result;
}

从上面的代码中，我们可以看到，当有消息事件发过来时，首先Looper.loop()会通过queue.next()从MessageQueue中取消息，此时可能会在next()方法中阻塞，也就是nativePollOnce(ptr, nextPollTimeoutMillis);方法做的事。这里会继续调到C++层，通过android_os_MessageQueue_nativePollOnce(xxx)将时间值传递给nativeMessageQueue->pollOnce(xxx);接着处理逻辑就转移到了C++层的Looper中处理。由mLooper->pollOnce(timeoutMillis);调到pollOnce函数，然后调用pollInner(timeoutMillis)，在这个函数中对请求的消息和响应做了处理：

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

这里将时间值设置给epoll_wait函数，也就是获取消息事件等待的超时时间了。
当eventCount大于0时，意味着有消息事件来了，我们比较关心的是fd == mWakeReadPipeFd的情况，我们看到这里调用了awoken()唤醒函数;

void Looper::awoken() {
#if DEBUG_POLL_AND_WAKE
    ALOGD("%p ~ awoken", this);
#endif

    char buffer[16];
    ssize_t nRead;
    do {
        nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
    } while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
}

这里只是从管道的读取端进行读数据操作。

Handler

构造函数：

public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class 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;
}

这里又是经常看到的异常信息了。
下面这个也是一个常用的方法，实际上调用的也是Message的obtain方法。

public final Message obtainMessage()
{
    return Message.obtain(this);
}

我们通常使用的发送消息方法：

public final boolean sendMessage(Message msg)
{
    return sendMessageDelayed(msg, 0);
}

public final boolean sendEmptyMessage(int what)
{
    return sendEmptyMessageDelayed(what, 0);
}

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

最后都会调用到同一个方法：

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);
}

其实也就做了一件事情，就是在uptimeMillis时间计时完成前把消息插入消息队列中。
在Looper.java的loop()方法中，调用到msg.target.dispatchMessage()时，流程再次转到handler中：

/**
  * Handle system messages here.
  */
 public void dispatchMessage(Message msg) {
     if (msg.callback != null) {
         handleCallback(msg);
     } else {
         if (mCallback != null) {
             if (mCallback.handleMessage(msg)) {
                 return;
             }
         }
         handleMessage(msg);
     }
 }

最后会走到handler的handleMessage方法对消息进行处理。