Handler的作用

异步通信，消息传递

Handler的基本用法

Handler的用法，示例1、（子线程向主线程发送消息）

public class HandlerActivity extends AppCompatActivity {Handler handler = new Handler() {    @Override    public void handleMessage(Message msg) {        Toast.makeText(HandlerActivity.this, "接收到了消息", Toast.LENGTH_SHORT).show();    }};@Overrideprotected void onCreate(@Nullable Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_handler);}public void onClickView(View v) {    switch (v.getId()) {        case R.id.button: {            Thread thread = new Thread() {                @Override                public void run() {                    Message msg = Message.obtain();                    handler.sendMessage(msg);                }            };            thread.start();            break;        }    }}}

这个界面只有一个Button, 点击事件是：启动一个线程，在这个线程里使用handler，发送一个消息；这个消息不带任何数据。

Message: 这个可以使用new来生成，但是最好还是使用Message.obtain()来获取一个实例。这样便于消息池的管理。 handler初始化的时候，我们复写了handleMessage方法，这个方法用来接收，子线程中发过来的消息。

示例2、主线程向子线程发送消息

public class HandlerActivity extends AppCompatActivity {Handler handler = new Handler() {    @Override    public void handleMessage(Message msg) {        Log.d("child", "----" + Thread.currentThread().getName());    }};Handler childHandler = null;@Overrideprotected void onCreate(@Nullable Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_handler);    Thread child = new Thread("child thread") {        @Override        public void run() {            Looper.prepare();            childHandler = new Handler(Looper.myLooper()) {                @Override                public void handleMessage(Message msg) {                    Log.d("child", "----" + Thread.currentThread().getName() + ",  " + msg.obj);                }            };            Looper.loop();        }    };    child.start();}public void onClickView(View v) {    switch (v.getId()) {        case R.id.button: {            Thread thread = new Thread() {                @Override                public void run() {                    Message msg = Message.obtain();                    handler.sendMessage(msg);                }            };            thread.start();            break;        }        case R.id.button1: {            if (childHandler != null) {                Message msg = Message.obtain(childHandler);                msg.obj = "123---" + SystemClock.uptimeMillis();                childHandler.sendMessage(msg);            } else {                Log.d("-----", "onClickView: child Handler is null");            }            break;        }    }}}

主线程向子线程发送消息时，我们需要使用的是handler,但是这个handler是需要在子线程中实例化，否则子线程无法接收到消息。

在child thread中准备的工作：

• 1. Looper.prepare();
• 1. 实例化childHandler的时候出入了一个Looper实例。
• 1. Looper.loop()调用

     假如我们直接在子线程中直接实例化一个handler，而不传入Looper实例。程序会直接抛出异常：
     java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
     这个异常是在hanlder源码：

     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());
     }
     }
     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;
     }
     出现的，从这里可知，在线程中实例化一个handler需要一个Looper实例。

     如果其他线程的实例出来的Looper会怎么样？

     首先看Looper源码中是怎么写的。Looper的构造方法是私有的，这样我们只能通过Looper的静态方法来实例化一个Looper.在Looper类中还有一个ThreadLocal<Looper> sThreadLocal 变量，而在上面的代码中使用了Looper.myLooper()方法来给handler一个Looper实例。

     * Looper.myLooper()

     public static @Nullable Looper myLooper() {return sThreadLocal.get();}

     这个方法返回了一个存放在ThreadLocal<Looper>中的Looper实例。

     为什么在调用Looper.myLooper()方法之前需要先调用 Looper.prepare()呢？

     Looper.prepare源码：

  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));
  }
  方法很简单，就是检查sThreadLocal里面是否有有值，然后将Looper的实例存放到ThreadLocal，如果不为空，直接就是 RuntimeException 异常。这也保证了一个线程中只能有一个Looper实例。因此，Looper.prepare方法只能调用一次。

  那么ThreadLocal的作用是什么呢？

  在这个程序中，可以大概理解成线程间数据的隔离。意思就是我存放在ThreadLocal中的数据，只能在我本线程中可以获得到值，在其他线程中获取不到（其他线程中获取的是null）。

  这样就能得出，其他线程中的Looper，在本线程中通过Looper.myLooper()获取不到数据。

  那么为什么在主线程中不需要传入Looper实例呢？

  通过查找Android源码，可以知道在ActivityThread中main方法里面，UI线程已经初始化了Looper.prepareMainLooper();这样就在UI线程中有Looper实例了。当然在main方法的下面，也调用了Looper.loop()方法。

  Looper.loop()方法是做什么的

  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    final Printer logging = me.mLogging;    if (logging != null) {        logging.println(">>>>> Dispatching to " + msg.target + " " +                msg.callback + ": " + msg.what);    }    final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;    final long traceTag = me.mTraceTag;    if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {        Trace.traceBegin(traceTag, msg.target.getTraceName(msg));    }    final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();    final long end;    try {        msg.target.dispatchMessage(msg);        end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();    } finally {        if (traceTag != 0) {            Trace.traceEnd(traceTag);        }    }    if (slowDispatchThresholdMs > 0) {        final long time = end - start;        if (time > slowDispatchThresholdMs) {            Slog.w(TAG, "Dispatch took " + time + "ms on "                    + Thread.currentThread().getName() + ", h=" +                    msg.target + " cb=" + msg.callback + " msg=" + msg.what);        }    }    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();**}**

  }

这个方法里面，我们需要注意到：

1. Looper me这个不允许为空
2. for (;;)循环
3. Message msg = queue.next()

4. msg.target.dispatchMessage(msg);

5. 在looper.loop方法里面，检查了本线程中的Looper实例，也对应了在调用looper.loop方法之前必须先调用looper.prepare方法。

6. for循环是一个死循环，这个需要一直取出MessageQueue队列中的数据，也就是刚才所列的第三个 queue.next方法。这个方法会阻塞，直到有消息从消息队列中取出来。

   next方法源码：

   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 iterationint 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;        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());        }        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;                }                msg.next = null;                if (DEBUG) Log.v(TAG, "Returning message: " + msg);                msg.markInUse();                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(TAG, "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;}

   }

7. queue.next方法，返回一个Message，而这个message会被传入到msg.target的dispatchMessage方法中。msg.target是一个handler，就是发送消息的实例。
   dispatchMessage(Message)源码：
   public void dispatchMessage(Message msg) {<br/>if (msg.callback != null) {<br/>handleCallback(msg);<br/>} else {<br/>if (mCallback != null) {<br/>if (mCallback.handleMessage(msg)) {<br/>return;<br/>}<br/>}<br/>handleMessage(msg);<br/>}<br/>}<br/>
   Handler初始化的时候，我们并没有传入msg.callback和mCallback这两个回调。所以这个方法最终执行的是handleMessage方法。
   上面我们分析了ThreadLocal,Looper,Message,MessageQueue,下面开始分析handler发送消息的方式。

   Handler发送消息

   Handler 通过sendMessage方法发送消息。这个方法最终调用的是

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {<br/>msg.target = this;<br/>if (mAsynchronous) {<br/>msg.setAsynchronous(true);<br/>}<br/>return queue.enqueueMessage(msg, uptimeMillis);<br/>}<br/>

参数：MessageQueue queue则是Handler中mQueue变量，这个变量在Handler(Callback callback, boolean async)初始化完成。它是Looper中一个final MessageQueue的变量，在初始化Looper的时候，就开始初始化MessageQueue。这也是一个线程中只有一个MessageQueue的原因. Message msg 是封装的消息。 long uptimeMillis 延迟发送时间。 之前分析looper.loop方法的时候，说了msg.target.dispatchMessage, 这个target就是在这个方法里面赋值的。 从这个方法里面，可以知道handler的sendMessage，只是把消息(Message实例)添加到了queue队列中。

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(TAG, 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.            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);        }    }    return true;}

Handler, Looper, Message, MessageQueue之间的关系

通过handler发送Message，将Message压入MessageQueue队列中；而Looper.loop方法又在不停的循环这个消息队列，取出压入MessageQueue的Message， 然后dispatchMessage分发，最终会调用handler.handleMessage方法来处理发送过来的Message.

本文转自 墨宇hz 51CTO博客，原文链接:http://blog.51cto.com/zzhhz/2045446