android Handler 机制梳理

最近对android Handler实现原理比较感兴趣，抽空分析了一下Handler java层实现原理，梳理如下

main loop  在哪里创建的？SystemServer run   ActivityThread main

enqueueMessage逻辑？

类图：

1.loop.prepare  会检查当前线程是否存在loop  有则抛异常 否则创建loop实例 因此一个线程只能存在一个loop实例     该实例会创建一个messagequeue   故一个线程只有一个 messagequeue

2.loop.loop会让该实例所在线程进入死循环  不断从从messageQueue中获取消息调用msg.target.disparchMessage

3.创建handler时会获取所在线程的loop  并与loop的messageQueue关联

4.sendMssage 方法会将target设置handler自身并将message加入到messageQueue,同样post(Runnable)方法也一样

5.loop在接收到Message后获取其target然后调用dispatchMessage  派发消息调用handMessage

以下代码红色部分解释以上1、2、3、4、5步

1. Looper  prepare()

private static void prepare(boolean quitAllowed) {

    //检查所在线程是否存在loop 如存在抛出异常，所以prepare在同一个线程里只可以调用一次，即同一个线程只可有一个loop    if (sThreadLocal.get() != null) {        throw new RuntimeException("Only one Looper may be created per thread");    }

    //否则创建loop    sThreadLocal.set(new Looper(quitAllowed));}

private Looper(boolean quitAllowed) {

    //looper创建消息队列    mQueue = new MessageQueue(quitAllowed);    mThread = Thread.currentThread();}

2.  Looper.java 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();    //死循环    for (;;) {

        //读取消息队列消息        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);        }        //派发消息        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.recycleUnchecked();    }}

3.  Handler myLooper()

public Handler(Callback callback, boolean async) {    if (FIND_POTENTIAL_LEAKS) {        final Class<? extends Handler> 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());        }    }    //获取looper实例    mLooper = Looper.myLooper();    if (mLooper == null) {        throw new RuntimeException(            "Can't create handler inside thread that has not called Looper.prepare()");    }

    //获取looper消息队列    mQueue = mLooper.mQueue;    mCallback = callback;    mAsynchronous = async;}

4.Handler sendMessage  

/** * Return the Looper object associated with the current thread.  Returns * null if the calling thread is not associated with a Looper. */public static @Nullable Looper myLooper() {    return sThreadLocal.get();}

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

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

////////美丽的///////分隔线///////////////////////////////

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) {

    //将target设置为自己  即loop dispatchMessage方法中被调用

    //msg.target.dispatchMessage(msg);    msg.target = this;    if (mAsynchronous) {        msg.setAsynchronous(true);    }    return queue.enqueueMessage(msg, uptimeMillis);}

//post 方法是生成了一个带Callback的Message

//调用msg的target  target是发送Message的Handler

msg.target.dispatchMessage(msg);

/** * Handle system messages here. */public void dispatchMessage(Message msg) {

    //优先处理回调再处理handleMessage    if (msg.callback != null) {        handleCallback(msg);    } else {        if (mCallback != null) {            if (mCallback.handleMessage(msg)) {                return;            }        }        handleMessage(msg);    }}

private static Message getPostMessage(Runnable r) {    Message m = Message.obtain();    m.callback = r;    return m;}

5.Looper loop()

消息的回收利用机制，是维护了一个池，这个池以链表方式存储  当获取消息时从链头取出，若消息池中没有可复用则创建新消息，当回收（把消息池中添加）消息时将新回收消息添加至链头

回收消息代码片段：

/** * 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++;        }    }}

获取消息代码片段：

AsyncQueryHandler.WorkerHandler也继承自Handler 非UI线程操作处理类 在该类执行增删改操作

HandlerThread 独立线程的Loop管理类

工作流程如下：

1.创建AsyncQueryHandler实例会同时创建工作线程Handler

/** * 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();}

池最大为50条消息，即链表长度最多可回收50条消息

private static final int MAX_POOL_SIZE = 50;

////////////////////分隔线/////////////////////

后来我又发现了AsyncQueryHandler 什么鬼？

查看源码发现AsyncQueryHandler主要做了一个异步操作数据（ContextProvider）的架构.

AsyncQueryHandler继承自Handler  handMessage处理是否在自线程与Handler一样取决于构造该对象时所在线程

public AsyncQueryHandler(ContentResolver cr) {    super();

    //弱引用Resolver对象    mResolver = new WeakReference<ContentResolver>(cr);    synchronized (AsyncQueryHandler.class) {        if (sLooper == null) {

            //HandlerThread负责创建一个线程并生成loop            HandlerThread thread = new HandlerThread("AsyncQueryWorker");            thread.start();            sLooper = thread.getLooper();        }    }

    //获取HandlerThread生成的loop 因为在自线程所以mWorkeThreadHandler工作在自线程    mWorkerThreadHandler = createHandler(sLooper);}

2.HandlerThread 启动线程并创建loop  HandlerThread是继承Thread的

@Overridepublic void run() {    mTid = Process.myTid();    Looper.prepare();    synchronized (this) {        mLooper = Looper.myLooper();        notifyAll();    }    Process.setThreadPriority(mPriority);    onLooperPrepared();    Looper.loop();    mTid = -1;}

3.发生查询或者其他插入操作请求到工作线程

public final void startInsert(int token, Object cookie, Uri uri,        ContentValues initialValues) {    // Use the token as what so cancelOperations works properly    Message msg = mWorkerThreadHandler.obtainMessage(token);    msg.arg1 = EVENT_ARG_INSERT;    WorkerArgs args = new WorkerArgs();

    //处理者设置为自己    args.handler = this;    args.uri = uri;    args.cookie = cookie;    args.values = initialValues;    msg.obj = args;    //发消息到工作线程    mWorkerThreadHandler.sendMessage(msg);}

4.工作线程处理操作请求并将处理结果返回给AsyncQueryHandler(一般为UI线程中）

           if (resolver == null) return;           WorkerArgs args = (WorkerArgs) msg.obj;           int token = msg.what;           int event = msg.arg1;           switch (event) {                 case EVENT_ARG_QUERY:                       cursor = resolver.query(args.uri, args.projection,                    args.selection, args.selectionArgs,                    args.orderBy);                    args.result = cursor;               break;

            Message reply = args.handler.obtainMessage(token);            reply.obj = args;            reply.arg1 = msg.arg1;

            reply.sendToTarget();

            //将结果发消息回AsyncQueryHandler

protected class WorkerHandler extends Handler {    public WorkerHandler(Looper looper) {        super(looper);    }    @Override    public void handleMessage(Message msg) {

        //获取弱引用对象 这样AsyncQueryHandler被销毁后 不会一直引用着防止内存泄漏        final ContentResolver resolver = mResolver.get();