转载:android笔记--android中的多线程--Handler, Looper, MessageQueue, Message类

什么时候使用多线程:

1. 耗时操作使用多线程, 耗时操作放在UI线程中会导致用户的操作无法得到响应.

2. 阻塞操作使用多线程, 理由同上.

3. 多核CUP的设备使用多线程, 可以有效提高CPU的利用率.

4. 并行操作使用多线程.

 

android中的多线程模型主要涉及的类有:Looper, Handler, MessageQueue, Message等. 

 一:Looper类:

 1 static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
 2     private static Looper sMainLooper;  // guarded by Looper.class
 3 
 4     final MessageQueue mQueue;
 5     final Thread mThread;
 6 public static void loop() {
 7         final Looper me = myLooper();
 8         if (me == null) {
 9             throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread.");
10         }
11         final MessageQueue queue = me.mQueue;
12 
13         // Make sure the identity of this thread is that of the local process,
14         // and keep track of what that identity token actually is.
15         Binder.clearCallingIdentity();
16         final long ident = Binder.clearCallingIdentity();
17 
18         for (;;) {
19             Message msg = queue.next(); // might block
20             if (msg == null) {
21                 // No message indicates that the message queue is quitting.
22                 return;
23             }
24 
25             // This must be in a local variable, in case a UI event sets the logger
26             Printer logging = me.mLogging;
27             if (logging != null) {
28                 logging.println(">>>>> Dispatching to " + msg.target + " " +
29                         msg.callback + ": " + msg.what);
30             }
31 
32             msg.target.dispatchMessage(msg);//超级重要
33 
34             if (logging != null) {
35                 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
36             }
37 
38             // Make sure that during the course of dispatching the
39             // identity of the thread wasn‘t corrupted.
40             final long newIdent = Binder.clearCallingIdentity();
41             if (ident != newIdent) {
42                 Log.wtf(TAG, "Thread identity changed from 0x"
43                         + Long.toHexString(ident) + " to 0x"
44                         + Long.toHexString(newIdent) + " while dispatching to "
45                         + msg.target.getClass().getName() + " "
46                         + msg.callback + " what=" + msg.what);
47             }
48 
49             msg.recycle();
50         }
51     }
52 public void quit() {
53         mQueue.quit(false);
54     }
55 public void quitSafely() {//最好用此方法
56         mQueue.quit(true);
57     }

 

Looper类用来创建消息队列. 每个线程最多只能有一个消息队列:Looper唯一的构造方法如下

1 private Looper(boolean quitAllowed) {
2         mQueue = new MessageQueue(quitAllowed);
3         mThread = Thread.currentThread();
4     }

 

android中UI线程默认具有消息队列, 但非UI线程在默认情况下是不具备消息队列的. 如果需要在非UI线程中开启消息队列, 需要调用Looper.prepare()方法, 在该方法的执行过程中会创建一个Looper对象

 1  public static void prepare() {
 2         prepare(true);
 3     }
 4 
 5     private static void prepare(boolean quitAllowed) {
 6         if (sThreadLocal.get() != null) {
 7             throw new RuntimeException("Only one Looper may be created per thread");
 8         }
 9         sThreadLocal.set(new Looper(quitAllowed));
10     }

 

而Looper的构造函数中会创建一个MessageQueue instance(Looper的构造函数是私有的, 在Looper类之外无法创建其对象).  

此后再为该线程绑定一个Handler instance, 然后调用Looper.loop()方法, 就可以不断的从消息队列中取出消息和处理消息了.

转载:android笔记--android中的多线程--Handler, Looper, MessageQueue, Message类
 1 public Handler() {
 2         this(null, false);
 3     }
 4 public Handler(Callback callback) {
 5         this(callback, false);
 6     }
 7 public Handler(Looper looper) {
 8         this(looper, null, false);
 9     }
10 public Handler(Looper looper, Callback callback) {
11         this(looper, callback, false);
12     }
13 public Handler(Callback callback, boolean async) {
14         if (FIND_POTENTIAL_LEAKS) {
15             final Class<? extends Handler> klass = getClass();
16             if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
17                     (klass.getModifiers() & Modifier.STATIC) == 0) {
18                 Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
19                     klass.getCanonicalName());
20             }
21         }
22 
23         mLooper = Looper.myLooper();
24         if (mLooper == null) {
25             throw new RuntimeException(
26                 "Can‘t create handler inside thread that has not called Looper.prepare()");
27         }
28         mQueue = mLooper.mQueue;
29         mCallback = callback;
30         mAsynchronous = async;
31     }
32 public Handler(Looper looper, Callback callback, boolean async) {
33         mLooper = looper;
34         mQueue = looper.mQueue;
35         mCallback = callback;
36         mAsynchronous = async;
37     }
View Code

(创建handler的时候必须关联一个looper)

Looper.myLoop()方法可以得到线程的Looper对象, 如果为null, 说明此时该线程尚未开启消息队列.

1 public static Looper myLooper() {
2         return sThreadLocal.get();
3     }

二:Handler: 

Handler类用于处理消息. 该类具有四个构造函数:(如上)

1. public Handler(). 创建好的Handler instance将绑定在代码所在的线程的消息队列上, 因此一定要确定该线程开启了消息队列, 否则程序将发生错误. 使用这个构造函数创建Handler instance, 一般来说, 我们需要重写Hanler类的handleMessage()方法, 以便在之后的消息处理时调用.

2. public Handler(Callback callback). Callback是Handler内部定义的一个接口, 因此想要使用这个构造函数创建Handler对象, 需要自定义一个类实现Callback接口, 并重写接口中定义的handleMessage()方法. 这个构造函数其实与无参的构造函数类似, 也要确保代码所在的线程开启了消息队列. 不同的是在之后处理消息时, 将优先调用callback的handleMessage()方法,返回true,则消息处理结束;返回false。则会调用Handler对象的handleMssage()方法.

1  final MessageQueue mQueue;
2  final Looper mLooper;
3  final Callback mCallback;
4  public interface Callback {
5         public boolean handleMessage(Message msg);
6     }
7  private static void handleCallback(Message message) {
8         message.callback.run();
9     }
 1 /**
 2      * Handle system messages here.
 3      */
 4     public void dispatchMessage(Message msg) {
 5         if (msg.callback != null) {
 6             handleCallback(msg);//如上的方法
 7         } else {
 8             if (mCallback != null) {
 9                 if (mCallback.handleMessage(msg)) {
10                     return;
11                 }
12             }
13             handleMessage(msg);
14         }
15     }

 

3. public Handler(Looper looper). 这个构造函数表示创建一个Handler instance, 并将其绑定在looper所在的线程上. 此时looper不能为null. 此时一般也需要重写Hanler类的handleMessage()方法

4. public Handler(Looper looper, Callback callback). 可以结合2和3理解.

 1  public final Message obtainMessage()
 2     {
 3         return Message.obtain(this);
 4     }
 5 private static Message getPostMessage(Runnable r) {
 6         Message m = Message.obtain();
 7         m.callback = r;
 8         return m;
 9     }
10 public final boolean post(Runnable r)
11     {
12        return  sendMessageDelayed(getPostMessage(r), 0);
13     }
14 public final boolean postAtTime(Runnable r, long uptimeMillis)
15     {
16         return sendMessageAtTime(getPostMessage(r), uptimeMillis);
17     }
18 public final boolean postDelayed(Runnable r, long delayMillis)
19     {
20         return sendMessageDelayed(getPostMessage(r), delayMillis);
21     }
22  public final boolean postAtFrontOfQueue(Runnable r)
23     {
24         return sendMessageAtFrontOfQueue(getPostMessage(r));
25     }
26  public final boolean sendMessage(Message msg)
27     {
28         return sendMessageDelayed(msg, 0);
29     }
30  public final boolean sendEmptyMessage(int what)
31     {
32         return sendEmptyMessageDelayed(what, 0);
33     }
34 public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
35         Message msg = Message.obtain();
36         msg.what = what;
37         return sendMessageDelayed(msg, delayMillis);
38     }
39 public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
40         Message msg = Message.obtain();
41         msg.what = what;
42         return sendMessageAtTime(msg, uptimeMillis);
43     }
44 //Handler所有post、send方法,最后调的都是此方法...
45 public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
46         MessageQueue queue = mQueue;
47         if (queue == null) {
48             RuntimeException e = new RuntimeException(
49                     this + " sendMessageAtTime() called with no mQueue");
50             Log.w("Looper", e.getMessage(), e);
51             return false;
52         }
53         return enqueueMessage(queue, msg, uptimeMillis);
54     }
55 //入队,其实调的是MessageQueue的入队方法
56 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
57         msg.target = this;
58         if (mAsynchronous) {
59             msg.setAsynchronous(true);
60         }
61         return queue.enqueueMessage(msg, uptimeMillis);
62     }
63 //Handler的hasMessage、removeCallback是MessageQueue的方法
64  public final void removeMessages(int what) {
65         mQueue.removeMessages(this, what, null);
66     }
67  public final void removeCallbacksAndMessages(Object token) {
68         mQueue.removeCallbacksAndMessages(this, token);
69     }
70 public final boolean hasMessages(int what) {
71         return mQueue.hasMessages(this, what, null);
72     }
73 public final boolean hasCallbacks(Runnable r) {
74         return mQueue.hasMessages(this, r, null);
75     }

 

 三,MessageQueue:

MessageQueue类用于表示消息队列. 队列中的每一个Message都有一个when字段, 这个字段用来决定Message应该何时出对处理. 消息队列中的每一个Message根据when字段的大小由小到大排列, 排在最前面的消息会首先得到处理, 因此可以说消息队列并不是一个严格的先进先出的队列.

 方法主要有:

Message next() {...}
void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new RuntimeException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;

            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
    }
//入队
 boolean enqueueMessage(Message msg, long when) {...}
//判断消息队列中是否包含某个消息
boolean hasMessages(Handler h, int what, Object object) {...}
//将某个消息从队列中移出去,该消息被回收到消息池中了
void removeMessages(Handler h, int what, Object object) {...}

四:Message:

Message类用于表示消息. Message对象可以通过arg1, arg2, obj字段和setData()携带数据, 此外还具有很多字段. when字段决定Message应该何时出对处理, target字段用来表示将由哪个Handler对象处理这个消息, next字段表示在消息队列中排在这个Message之后的下一个Message, callback字段如果不为null表示这个Message包装了一个runnable对象, what字段表示code, 即这个消息具体是什么类型的消息. 每个what都在其handler的namespace中, 我们只需要确保将由同一个handler处理的消息的what属性不重复就可以.

 

 1     public int what;
 2     public int arg1; 
 3     public int arg2;
 4     public Object obj;
 5     public Messenger replyTo;
 6     /** If set message is in use */
 7     static final int FLAG_IN_USE = 1 << 0;
 8     /** If set message is asynchronous */
 9    static final int FLAG_ASYNCHRONOUS = 1 << 1;
10     /** Flags to clear in the copyFrom method */
11    static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;
12     int flags;
13     long when;    
14     Bundle data;    
15      Handler target;        
16      Runnable callback;       
17     // sometimes we store linked lists of these things
18     Message next;
19 
20     private static final Object sPoolSync = new Object();
21     private static Message sPool;
22     private static int sPoolSize = 0;
23 
24     private static final int MAX_POOL_SIZE = 50;
 1 public static Message obtain() {
 2         synchronized (sPoolSync) {
 3             if (sPool != null) {
 4                 Message m = sPool;
 5                 sPool = m.next;
 6                 m.next = null;
 7                 sPoolSize--;
 8                 return m;
 9             }
10         }
11         return new Message();
12     }
13 public static Message obtain(Message orig) {
14         Message m = obtain();
15         m.what = orig.what;
16         m.arg1 = orig.arg1;
17         m.arg2 = orig.arg2;
18         m.obj = orig.obj;
19         m.replyTo = orig.replyTo;
20         if (orig.data != null) {
21             m.data = new Bundle(orig.data);
22         }
23         m.target = orig.target;
24         m.callback = orig.callback;
25 
26         return m;
27     }
28 public static Message obtain(Handler h) {
29         Message m = obtain();
30         m.target = h;
31 
32         return m;
33     }
34 ......
35 void clearForRecycle() {
36         flags = 0;
37         what = 0;
38         arg1 = 0;
39         arg2 = 0;
40         obj = null;
41         replyTo = null;
42         when = 0;
43         target = null;
44         callback = null;
45         data = null;
46     }
47 public void recycle() {
48         clearForRecycle();
49 
50         synchronized (sPoolSync) {
51             if (sPoolSize < MAX_POOL_SIZE) {
52                 next = sPool;
53                 sPool = this;
54                 sPoolSize++;
55             }
56         }
57     }
58 public void copyFrom(Message o)
59 public long getWhen()
60 public void setTarget(Handler target)
61 public Handler getTarget()
62 public Runnable getCallback()
63 public Bundle getData() {
64         if (data == null) {
65             data = new Bundle();
66         }
68         return data;
69     }
70 public Bundle peekData() {
71         return data;
72     }
73 public void setData(Bundle data)
74  public void sendToTarget()

 

将消息压入消息队列: Message对象的target字段关联了哪个线程的消息队列, 这个消息就会被压入哪个线程的消息队列中.

1. 调用Handler类中以send开头的方法可以将Message对象压入消息队列中, 调用Handler类中以post开头的方法可以将一个runnable对象包装在一个Message对象中, 然后再压入消息队列, 此时入队的Message其callback字段不为null, 值就是这个runnable对象. 调用Handler对象的这些方法入队的Message, 其target属性会被赋值为这个handler对象.

2. 调用Message对象的sendToTarget()方法可以将其本身压入与其target字段(即handler对象)所关联的消息队列中. 

 

将未来得及处理的消息从消息队列中删除:

调用Handler对象中以remove开头的方法就可以.

 

从消息队列中取出消息并处理消息: 所有在消息队列中的消息, 都具有target字段. 消息是在target所关联的线程上被取出和处理的.

 1 /**
 2      * Handle system messages here.
 3      */
 4     public void dispatchMessage(Message msg) {
 5         if (msg.callback != null) {
 6             handleCallback(msg);
 7         } else {
 8             if (mCallback != null) {
 9                 if (mCallback.handleMessage(msg)) {
10                     return;
11                 }
12             }
13             handleMessage(msg);
14         }
15     }

 

1. 如果取出的Message对象的callback字段不为null, 那么就调用callback字段的run()方法(callback字段的类型是runnable). 注意此时并不开启一个新的线程运行run()方法, 而是直接在handler对象(即Message的target字段)所关联的线程上运行.

2. 如果取出的Message对象的callback字段为null, 且Handler对象中的callback字段也为null, 那么这个消息将由Handler对象的handleMessage(msg)方法处理. 注意Message对象的callback字段是Runnable类型的而Handler对象的callback字段是Callback类型的, Handler对象的callback字段是在创建Handler instance的时候指定的, 如果没有指定则这个字段为null, 详见Handler类的四个构造方法.

3. 如果取出的Message对象的callback字段为null, 且Handler对象中的callback字段不为null, 那么这个消息将由Handler对象中的callback字段的handleMessage方法优先处理.

 

线程间通信: 有了以上的叙述, 线程间的通信也就好理解了. 假如一个handler关联了A线程上的消息队列, 那么我们可以在B线程上调用handler的相关方法向A线程上的消息队列压入一个Message, 这个Message将在A线程上得到处理.

转载:android笔记--android中的多线程--Handler, Looper, MessageQueue, Message类,布布扣,bubuko.com

转载:android笔记--android中的多线程--Handler, Looper, MessageQueue, Message类

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