正确理解 AsyncTask，Looper,Handler三者之间的关系(基于android 4.0)

Looper 和Handler 是理解好AsyncTask的一个基础，我们可以先从这里开始，先给出一个主线程和子线程互相通信的例子。

  1 package com.example.loopertest;
  2 
  3 import android.app.Activity;
  4 import android.os.Bundle;
  5 import android.os.Handler;
  6 import android.os.Looper;
  7 import android.os.Message;
  8 import android.util.Log;
  9 
 10 public class MainActivity extends Activity {
 11 
 12     public static final int SIGNAL_1 = 0x1;
 13     public static final int SIGNAL_2 = 0x2;
 14 
 15     public static int flagValue = 0;
 16     private LooperThread thread;
 17     private Handler uiHandler = new Handler() {
 18         @Override
 19         public void handleMessage(Message msg) {
 20             // TODO Auto-generated method stub
 21             switch (msg.what) {
 22             case SIGNAL_1:
 23                 Log.v("MainActivity", "主线程收到子线程发来的消息");
 24                 flagValue++;
 25                 if (flagValue == 5) {
 26                     Log.v("MainActivity", "now flagvalue is over 5");
 27                     Log.v("MainActivity",
 28                             "quit 前 thread.isAlive?" + thread.isAlive());
 29                     thread.cHandler.getLooper().quit();
 30                     Log.v("MainActivity",
 31                             "quit 后 thread.isAlive?" + thread.isAlive());
 32                 } else {
 33                     Log.v("MainActivity", "thread.isAlive?" + thread.isAlive());
 34                     thread.cHandler.sendEmptyMessageDelayed(SIGNAL_1, 3000);
 35                 }
 36                 break;
 37             case SIGNAL_2:
 38                 thread.cHandler.sendEmptyMessage(SIGNAL_1);
 39                 break;
 40             default:
 41                 break;
 42             }
 43         }
 44     };
 45 
 46     @Override
 47     protected void onCreate(Bundle savedInstanceState) {
 48         super.onCreate(savedInstanceState);
 49         setContentView(R.layout.activity_main);
 50 
 51         thread = new LooperThread();
 52         thread.start();
 53         uiHandler.sendEmptyMessage(SIGNAL_2);
 54 
 55         new Thread() {
 56             public void run() {
 57                 while (true) {
 58                     try {
 59                         Thread.sleep(10000);
 60                     } catch (InterruptedException e) {
 61                         // TODO Auto-generated catch block
 62                         e.printStackTrace();
 63                     }
 64                     Log.v("MainActivity",
 65                             "第三个子线程里面每隔10s判断 thread.isAlive?"
 66                                     + thread.isAlive());
 67                 }
 68             }
 69         }.start();
 70 
 71     }
 72 
 73     class LooperThread extends Thread {
 74         public Handler cHandler;
 75 
 76         @Override
 77         public void run() {
 78 
 79             // 实例化messagequeue
 80             Looper.prepare();
 81 
 82             cHandler = new Handler() {
 83                 @Override
 84                 public void handleMessage(Message msg) {
 85                     // TODO Auto-generated method stub
 86                     switch (msg.what) {
 87                     case SIGNAL_1:
 88                         Log.v("MainActivity", "子线程收到主线程发来的消息");
 89                         uiHandler.sendEmptyMessageDelayed(SIGNAL_1, 3000);
 90                         break;
 91                     default:
 92                         break;
 93                     }
 94                 }
 95             };
 96             Log.v("MainActivity", "loop以前的语句");
 97             Looper.loop();
 98 
 99             Log.v("MainActivity", "loop以后的语句");
100 
101         }
102 
103     }
104 
105 }

看一下执行结果

04-30 07:17:58.754: V/MainActivity(597): loop以前的语句
04-30 07:17:58.784: V/MainActivity(597): 子线程收到主线程发来的消息
04-30 07:18:01.794: V/MainActivity(597): 主线程收到子线程发来的消息
04-30 07:18:01.794: V/MainActivity(597): thread.isAlive?true
04-30 07:18:04.804: V/MainActivity(597): 子线程收到主线程发来的消息
04-30 07:18:07.814: V/MainActivity(597): 主线程收到子线程发来的消息
04-30 07:18:07.814: V/MainActivity(597): thread.isAlive?true
04-30 07:18:08.780: V/MainActivity(597): 第三个子线程里面每隔10s判断 thread.isAlive?true
04-30 07:18:10.824: V/MainActivity(597): 子线程收到主线程发来的消息
04-30 07:18:13.834: V/MainActivity(597): 主线程收到子线程发来的消息
04-30 07:18:13.834: V/MainActivity(597): thread.isAlive?true
04-30 07:18:16.844: V/MainActivity(597): 子线程收到主线程发来的消息
04-30 07:18:18.782: V/MainActivity(597): 第三个子线程里面每隔10s判断 thread.isAlive?true
04-30 07:18:19.844: V/MainActivity(597): 主线程收到子线程发来的消息
04-30 07:18:19.844: V/MainActivity(597): thread.isAlive?true
04-30 07:18:22.854: V/MainActivity(597): 子线程收到主线程发来的消息
04-30 07:18:25.864: V/MainActivity(597): 主线程收到子线程发来的消息
04-30 07:18:25.864: V/MainActivity(597): now flagvalue is over 5
04-30 07:18:25.864: V/MainActivity(597): quit 前 thread.isAlive?true
04-30 07:18:25.874: V/MainActivity(597): loop以后的语句
04-30 07:18:25.874: V/MainActivity(597): quit 后 thread.isAlive?false
04-30 07:18:28.785: V/MainActivity(597): 第三个子线程里面每隔10s判断 thread.isAlive?false

这个例子就是用来在android里面主线程和子线程进行通信的，大家可以看一下代码，另外还有第三个线程在不断侦测子线程如何结束。

实际上就是子线程和主线程每隔3s 通信一次，然后通信的时候那个参数值就每次加1，一直加到5的时候子线程就结束了。

例子里面也可以看出来 looper的消息队列在没有quit的时候子线程是会一直执行的，也就是谁looper.loop()后面的代码是不会执行的，

只有当quit以后 loop()的代码才会执行，这点大家要注意了。

然后我们可以借着这个清晰得例子，来理一下 looper和handler之间是如何通信的。

首先我们调用的是Looper.prepare(); 这句话，我们来看一下源码是怎么写的，

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

一下就能看出来如果get()的值不是空那么就要抛出这个异常，这样就能解释我们一个线程肯定只能有一个looper了，并且Looper.prepare() 在一个线程里面只能调用一次，否则也要抛异常。

当然了我们可以点到这个set方法里面看一下大概做了什么操作。

1  // sThreadLocal.get() will return null unless you've called prepare().
2     static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

然后我们进入threadlocal这个类里面的set方法看看。

 1 /**
 2      * Sets the value of this variable for the current thread. If set to
 3      * {@code null}, the value will be set to null and the underlying entry will
 4      * still be present.
 5      *
 6      * @param value the new value of the variable for the caller thread.
 7      */
 8     public void set(T value) {
 9         Thread currentThread = Thread.currentThread();
10         Values values = values(currentThread);
11         if (values == null) {
12             values = initializeValues(currentThread);
13         }
14         values.put(this, value);
15     }

到这个地方就很明显了看第9行，

 Thread currentThread = Thread.currentThread();

这也就是为什么说一个线程只能有一个looper。也能说明Looper和线程的绑定就是在这个方法里面完成的。有兴趣的同学还可以继续看values这个内部类，我们在这里先不去挖的太深。

说完prepare 方法我们再说说loop方法，因为有很多人都不明白为什么loop方法以后的语句都不执行。我们还是直接上源代码。

 1  /**
 2      * Run the message queue in this thread. Be sure to call
 3      * {@link #quit()} to end the loop.
 4      */
 5     public static void loop() {
 6         Looper me = myLooper();
 7         if (me == null) {
 8             throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
 9         }
10         MessageQueue queue = me.mQueue;
11         
12         // Make sure the identity of this thread is that of the local process,
13         // and keep track of what that identity token actually is.
14         Binder.clearCallingIdentity();
15         final long ident = Binder.clearCallingIdentity();
16         
17         while (true) {
18             Message msg = queue.next(); // might block
19             if (msg != null) {
20                 if (msg.target == null) {
21                     // No target is a magic identifier for the quit message.
22                     return;
23                 }
24 
25                 long wallStart = 0;
26                 long threadStart = 0;
27 
28                 // This must be in a local variable, in case a UI event sets the logger
29                 Printer logging = me.mLogging;
30                 if (logging != null) {
31                     logging.println(">>>>> Dispatching to " + msg.target + " " +
32                             msg.callback + ": " + msg.what);
33                     wallStart = SystemClock.currentTimeMicro();
34                     threadStart = SystemClock.currentThreadTimeMicro();
35                 }
36 
37                 msg.target.dispatchMessage(msg);
38 
39                 if (logging != null) {
40                     long wallTime = SystemClock.currentTimeMicro() - wallStart;
41                     long threadTime = SystemClock.currentThreadTimeMicro() - threadStart;
42 
43                     logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
44                     if (logging instanceof Profiler) {
45                         ((Profiler) logging).profile(msg, wallStart, wallTime,
46                                 threadStart, threadTime);
47                     }
48                 }
49 
50                 // Make sure that during the course of dispatching the
51                 // identity of the thread wasn't corrupted.
52                 final long newIdent = Binder.clearCallingIdentity();
53                 if (ident != newIdent) {
54                     Log.wtf(TAG, "Thread identity changed from 0x"
55                             + Long.toHexString(ident) + " to 0x"
56                             + Long.toHexString(newIdent) + " while dispatching to "
57                             + msg.target.getClass().getName() + " "
58                             + msg.callback + " what=" + msg.what);
59                 }
60                 
61                 msg.recycle();
62             }
63         }
64     }

6-10行我们能看出来是取得了 looper的消息队列，然后17行开始就是一个while true循环了！这也就能解释为啥loop方法一调用我们loop后面的代码就不会执行，

18行也能看出来，代码告诉我们是不断的在消息队列里面取消息，并且有可能会阻塞~

我们再来看一下这个37行的代码

msg.target.dispatchMessage(msg);

到这个地方可能很多人就能猜到了，这个地方就是我们处理消息的地方。那么这个msg.target我们应该怎么理解？（很多人无法理解looper handler 之间的关系其实是因为这个地方始终理解不了）

我们可以先放一放，先去看我们handler的代码，在我们文中开头的第一个例子中，我们在prepare()以后就创建了一个handler，

我们去看看handler的构造函数。

 1  /**
 2      * Default constructor associates this handler with the queue for the
 3      * current thread.
 4      *
 5      * If there isn't one, this handler won't be able to receive messages.
 6      */
 7     public Handler() {
 8         if (FIND_POTENTIAL_LEAKS) {
 9             final Class<? extends Handler> klass = getClass();
10             if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
11                     (klass.getModifiers() & Modifier.STATIC) == 0) {
12                 Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
13                     klass.getCanonicalName());
14             }
15         }
16 
17         mLooper = Looper.myLooper();
18         if (mLooper == null) {
19             throw new RuntimeException(
20                 "Can't create handler inside thread that has not called Looper.prepare()");
21         }
22         mQueue = mLooper.mQueue;
23         mCallback = null;
24     }

17-21行可以看出来，我们在调用handler的构造函数的时候，会先取当前线程的looper 如果取不到就会报异常了~~

然后我们发消息的时候是调用的send函数，

 1 /**
 2      * Sends a Message containing only the what value, to be delivered
 3      * after the specified amount of time elapses.
 4      * @see #sendMessageDelayed(android.os.Message, long) 
 5      * 
 6      * @return Returns true if the message was successfully placed in to the 
 7      *         message queue.  Returns false on failure, usually because the
 8      *         looper processing the message queue is exiting.
 9      */
10     public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
11         Message msg = Message.obtain();
12         msg.what = what;
13         return sendMessageDelayed(msg, delayMillis);
14     }

经过一番调查，我们发现最终都是调用的这个方法

 1 /**
 2      * Enqueue a message into the message queue after all pending messages
 3      * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
 4      * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
 5      * You will receive it in {@link #handleMessage}, in the thread attached
 6      * to this handler.
 7      * 
 8      * @param uptimeMillis The absolute time at which the message should be
 9      *         delivered, using the
10      *         {@link android.os.SystemClock#uptimeMillis} time-base.
11      *         
12      * @return Returns true if the message was successfully placed in to the 
13      *         message queue.  Returns false on failure, usually because the
14      *         looper processing the message queue is exiting.  Note that a
15      *         result of true does not mean the message will be processed -- if
16      *         the looper is quit before the delivery time of the message
17      *         occurs then the message will be dropped.
18      */
19     public boolean sendMessageAtTime(Message msg, long uptimeMillis)
20     {
21         boolean sent = false;
22         MessageQueue queue = mQueue;
23         if (queue != null) {
24             msg.target = this;
25             sent = queue.enqueueMessage(msg, uptimeMillis);
26         }
27         else {
28             RuntimeException e = new RuntimeException(
29                 this + " sendMessageAtTime() called with no mQueue");
30             Log.w("Looper", e.getMessage(), e);
31         }
32         return sent;
33     }

注意看 24行，

msg.target = this;

然后我们回到loop方法里面

msg.target.dispatchMessage(msg);

一下就能看出来，其实target就是handler。

所以loop方法里面处理消息实际上就是调用的handler的dispatchMessage 这个方法！

我们进去看这个方法

 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     }

看14行，发现其实调用的是handler的这个方法

1  /**
2      * Subclasses must implement this to receive messages.
3      */
4     public void handleMessage(Message msg) {
5     }

而这个方法也是我们每次都去重写的。到这里我们就算是理清楚了 handler 和looper 之间消息传递的一个过程。

其实就是先调用looper.prepare() 然后才能创建handler. 一个线程只能有一个looper 一个线程队列 messagequeue.

handler发消息的时候发message的时候实际上是把自己（handler本身）放在了message的 target变量里面，这样在loop

方法里面无线循环的时候我们才能回调到 handler的handleMessage方法~~~~

同样的我们也可以分析一下为什么quit方法可以让loop循环结束？(我就不分析了留给大家自己分析其实也是不难的)

搞清楚looper和handler的关系以后我们就可以看看 AsyncTask这个东西。

 1 package com.example.asynctest;
 2 
 3 import android.app.Activity;
 4 import android.app.ProgressDialog;
 5 import android.os.AsyncTask;
 6 import android.os.Bundle;
 7 import android.os.Handler;
 8 import android.os.Looper;
 9 import android.util.Log;
10 import android.widget.TextView;
11 
12 public class MainActivity extends Activity {
13 
14     private TextView tv1;
15 
16     @Override
17     protected void onCreate(Bundle savedInstanceState) {
18         super.onCreate(savedInstanceState);
19         setContentView(R.layout.activity_main);
20         tv1 = (TextView) this.findViewById(R.id.tv1);
21         /**
22          * Task的实例必须在UI thread中创建； execute方法必须在UI thread中调用；
23          * 不要手动的调用onPreExecute(),
24          * onPostExecute(Result)，doInBackground(Params...),
25          * onProgressUpdate(Progress...)这几个方法； 该task只能被执行一次，否则多次调用时将会出现异常；
26          */
27         new MyAsyncTask().execute();
28 
29     }
30 
31     private class MyAsyncTask extends AsyncTask<Void, Integer, Void> {
32 
33         @Override
34         protected void onPreExecute() {
35             // TODO Auto-generated method stub
36             Log.v("mainactivity", Thread.currentThread().getName()
37                     + " onPreExecute ");
38         }
39 
40         /**
41          * 这里不能直接操作ui 因为不是在主线程里操作的
42          */
43         @Override
44         protected Void doInBackground(Void... params) {
45             // TODO Auto-generated method stub
46             // 模拟数据的加载,耗时的任务
47             for (int i = 0; i < 100; i++) {
48                 try {
49                     Thread.sleep(80);
50                 } catch (InterruptedException e) {
51                     e.printStackTrace();
52                 }
53                 publishProgress(i);
54             }
55 
56             Log.v("mainactivity", Thread.currentThread().getName()
57                     + " doInBackground ");
58             return null;
59         }
60 
61         /**
62          * 在主线程执行
63          */
64         @Override
65         protected void onProgressUpdate(Integer... values) {
66             tv1.setText(values[0] + "");
67             Log.v("mainactivity", Thread.currentThread().getName()
68                     + " onProgressUpdate ");
69         }
70 
71         /**
72          * 可以操作ui
73          */
74         @Override
75         protected void onPostExecute(Void result) {
76             // 进行数据加载完成后的UI操作
77             tv1.setText("LOAD DATA SUCCESS ");
78             Log.e("mainactivity", Thread.currentThread().getName()
79                     + " onPostExecute ");
80         }
81     }
82 
83 }

我们看一下日志和运行效果。

看一下日志

04-30 09:04:16.345: V/mainactivity(935): main onPreExecute 
04-30 09:04:16.494: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:16.884: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:16.914: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:16.914: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:16.914: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.264: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.264: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.284: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.284: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.365: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.414: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.414: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.484: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.534: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.614: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.684: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.754: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.834: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.914: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:17.994: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.074: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.154: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.244: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.324: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.404: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.484: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.565: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.644: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.724: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.804: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.884: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:18.964: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.053: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.134: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.214: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.294: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.375: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.454: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.534: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.614: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.694: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.784: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.864: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:19.944: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.024: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.104: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.184: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.264: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.344: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.425: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.504: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.593: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.674: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.754: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.834: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.914: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:20.994: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.073: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.154: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.233: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.313: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.404: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.485: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.563: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.643: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.723: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.803: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.883: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:21.963: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.043: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.123: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.204: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.293: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.373: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.454: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.533: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.613: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.693: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.773: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.854: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:22.934: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.024: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.103: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.184: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.264: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.344: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.424: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.504: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.584: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.664: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.744: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.834: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.914: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:23.994: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:24.074: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:24.154: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:24.234: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:24.314: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:24.394: V/mainactivity(935): main onProgressUpdate 
04-30 09:04:24.474: V/mainactivity(935): AsyncTask #5 doInBackground 
04-30 09:04:24.474: V/mainactivity(935): main onProgressUpdate

　其实还是蛮好理解的，可以清楚的看到

AsyncTask

的执行周期 以及那些方法都是在哪个线程执行的（ui还是子?）

然后 我们就来分析一下 这个AsyncTask类到底是怎么做的？

这是我们调用的excute方法，

1 public final AsyncTask<Params, Progress, Result> execute(Params... params) {
2         return executeOnExecutor(sDefaultExecutor, params);
3     }

跟进去看看。

 1 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
 2             Params... params) {
 3         if (mStatus != Status.PENDING) {
 4             switch (mStatus) {
 5                 case RUNNING:
 6                     throw new IllegalStateException("Cannot execute task:"
 7                             + " the task is already running.");
 8                 case FINISHED:
 9                     throw new IllegalStateException("Cannot execute task:"
10                             + " the task has already been executed "
11                             + "(a task can be executed only once)");
12             }
13         }
14 
15         mStatus = Status.RUNNING;
16 
17         onPreExecute();
18 
19         mWorker.mParams = params;
20         exec.execute(mFuture);
21 
22         return this;
23     }

3-13行我们可以看出来，这个excute方法只能执行一次，不然就要抛异常。同时也能看出来

onPreExecute(); 这个方法是在主线程执行的。

然后我们着重看一下19行，这个地方，mWorker 是什么？

找了一下 发现

1     private final WorkerRunnable<Params, Result> mWorker;

实际上是Workerrunnable这个类的对象，我们进去看看这个类。

 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        Params[] mParams;
    }

发现这是一个静态的抽象类~并且继承了

Callable 这个接口

当然了 因为这是抽象类，所以我们无需实现这个接口。

既然

mWorker  是 WorkerRunnable 他的对象，哪我们去看看 是如何new出来的呢？看构造方法

 1 /**
 2      * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
 3      */
 4     public AsyncTask() {
 5         mWorker = new WorkerRunnable<Params, Result>() {
 6             public Result call() throws Exception {
 7                 mTaskInvoked.set(true);
 8 
 9                 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
10                 return postResult(doInBackground(mParams));
11             }
12         };
13 
14         mFuture = new FutureTask<Result>(mWorker) {
15             @Override
16             protected void done() {
17                 try {
18                     final Result result = get();
19 
20                     postResultIfNotInvoked(result);
21                 } catch (InterruptedException e) {
22                     android.util.Log.w(LOG_TAG, e);
23                 } catch (ExecutionException e) {
24                     throw new RuntimeException("An error occured while executing doInBackground()",
25                             e.getCause());
26                 } catch (CancellationException e) {
27                     postResultIfNotInvoked(null);
28                 } catch (Throwable t) {
29                     throw new RuntimeException("An error occured while executing "
30                             + "doInBackground()", t);
31                 }
32             }
33         };
34     }

到这 我们相信 mWorker大家已经知道是怎么回事了  5-12行表明了mWorker 是一个对象，并且他的回调方法里面 也调用了

doInBackground 这个方法，至与

postResult 这个方法 我们可以等会再看，先往下面继续看，我们再拿到

mWorker 这个对象以后 又利用他 去初始化了

mFuture  这个对象，且这个对象是由

FutureTask 这个类生成的，哪我们就去看看这个类。

public class FutureTask<V> implements RunnableFuture<V> {

发现这类继承自 RunnableFuture 这个接口，哪我们去看看这个接口，

public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

我们发现原来看似复杂的FutureTask 也不过就是一个Runnable 对象吗~~

回到excute方法

我们发现了这句话

exec.execute(mFuture);

实际上就是这么调用的

 public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

那么我们赶紧去看一下 sDefaultExecutor 这是什么东西？

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

再看看

SERIAL_EXECUTOR 这是什么？

/**
     * An {@link Executor} that executes tasks one at a time in serial
     * order.  This serialization is global to a particular process.
     */
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

原来是一个静态的 final 对象~~且这个对象是由SerialExecutor 这个类生成的，哪我们赶紧去看一下这个类，

 1 private static class SerialExecutor implements Executor {
 2         final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
 3         Runnable mActive;
 4 
 5         public synchronized void execute(final Runnable r) {
 6             mTasks.offer(new Runnable() {
 7                 public void run() {
 8                     try {
 9                         r.run();
10                     } finally {
11                         scheduleNext();
12                     }
13                 }
14             });
15             if (mActive == null) {
16                 scheduleNext();
17             }
18         }
19 
20         protected synchronized void scheduleNext() {
21             if ((mActive = mTasks.poll()) != null) {
22                 THREAD_POOL_EXECUTOR.execute(mActive);
23             }
24         }
25     }

第二行明显的是创建了一个线程队列

当执行excute方法的时候

实际上就是把这个runnable 先放到线程队列里面，然后再去执行线程队列里的第一个线程，

20-24行就是从这个线程队列里面取值，如果能取到就执行这句话

THREAD_POOL_EXECUTOR.execute(mActive);

而THREAD_POOL_EXECUTOR就是一个线程池。

1 /**
2      * An {@link Executor} that can be used to execute tasks in parallel.
3      */
4     public static final Executor THREAD_POOL_EXECUTOR
5             = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
6                     TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

也可以看一下这个常量

1  private static final int CORE_POOL_SIZE = 5;
2     private static final int MAXIMUM_POOL_SIZE = 128;
3     private static final int KEEP_ALIVE = 1;

private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(10);

有兴趣的可以看下这个线程池的参数设置。

但实际上我们回到

SerialExecutor 

这个类的5=15行 

我们发现 实际上我们还是从队列里面取任务出来做，任务做完了，才去取下一个任务。

所以 AsyncTask 他本质上还是一个单线程执行的 东西，（当然执行的时候是在子线程 而不是主线程执行的）


我们再回到一开始的地方

 1 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
 2             Params... params) {
 3         if (mStatus != Status.PENDING) {
 4             switch (mStatus) {
 5                 case RUNNING:
 6                     throw new IllegalStateException("Cannot execute task:"
 7                             + " the task is already running.");
 8                 case FINISHED:
 9                     throw new IllegalStateException("Cannot execute task:"
10                             + " the task has already been executed "
11                             + "(a task can be executed only once)");
12             }
13         }
14 
15         mStatus = Status.RUNNING;
16 
17         onPreExecute();
18 
19         mWorker.mParams = params;
20         exec.execute(mFuture);
21 
22         return this;
23     }

实际上我们执行的就是mFuture 这个任务，这个任务在子线程里面被执行。

所以我们去看一下FutureTask 他的run方法

1 /**
2      * Sets this Future to the result of its computation
3      * unless it has been cancelled.
4      */
5     public void run() {
6         sync.innerRun();
7     }

找到run方法

 1  void innerRun() {
 2             if (!compareAndSetState(READY, RUNNING))
 3                 return;
 4 
 5             runner = Thread.currentThread();
 6             if (getState() == RUNNING) { // recheck after setting thread
 7                 V result;
 8                 try {
 9                     result = callable.call();
10                 } catch (Throwable ex) {
11                     setException(ex);
12                     return;
13                 }
14                 set(result);
15             } else {
16                 releaseShared(0); // cancel
17             }
18         }

可以看出来 run方法实际执行的是

callable.call 这个方法，所谓这个方法 不过也是我们一开始构造函数里的mWorker对象罢了，

 1   public AsyncTask() {
 2         mWorker = new WorkerRunnable<Params, Result>() {
 3             public Result call() throws Exception {
 4                 mTaskInvoked.set(true);
 5 
 6                 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
 7                 return postResult(doInBackground(mParams));
 8             }
 9         };
10 
11         mFuture = new FutureTask<Result>(mWorker) {
12             @Override
13             protected void done() {
14                 try {
15                     final Result result = get();
16 
17                     postResultIfNotInvoked(result);
18                 } catch (InterruptedException e) {
19                     android.util.Log.w(LOG_TAG, e);
20                 } catch (ExecutionException e) {
21                     throw new RuntimeException("An error occured while executing doInBackground()",
22                             e.getCause());
23                 } catch (CancellationException e) {
24                     postResultIfNotInvoked(null);
25                 } catch (Throwable t) {
26                     throw new RuntimeException("An error occured while executing "
27                             + "doInBackground()", t);
28                 }
29             }
30         };
31     }

所以线程中执行的是3-8行里的代码！！！！！！！！！！！！！！！！！！！！！！！！！！

所以doInBackground 这方法是在子线程里面去执行的。

执行完毕以后才调用了下面的方法

1 private Result postResult(Result result) {
2         Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
3                 new AsyncTaskResult<Result>(this, result));
4         message.sendToTarget();
5         return result;
6     }

最终我们找到这个handler里面看一下看看这里面做了什么。

 1 private static class InternalHandler extends Handler {
 2         @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
 3         @Override
 4         public void handleMessage(Message msg) {
 5             AsyncTaskResult result = (AsyncTaskResult) msg.obj;
 6             switch (msg.what) {
 7                 case MESSAGE_POST_RESULT:
 8                     // There is only one result
 9                     result.mTask.finish(result.mData[0]);
10                     break;
11                 case MESSAGE_POST_PROGRESS:
12                     result.mTask.onProgressUpdate(result.mData);
13                     break;
14             }
15         }
16     }

再看看这个finish方法

1  private void finish(Result result) {
2         if (isCancelled()) {
3             onCancelled(result);
4         } else {
5             onPostExecute(result);
6         }
7         mStatus = Status.FINISHED;
8     }

于是 整个AsyncTask的生命周期 就全部得到印证，执行顺序 到这里也就差不多了。

至此 我们 AsyncTask，Looper,Handler 这三者的源码分析 也就告一段落。

---恢复内容结束---

posted @ 2015-04-30 18:32 希尔瓦娜斯女神阅读(1067) 评论(0) 编辑收藏举报

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正确理解 AsyncTask，Looper,Handler三者之间的关系(基于android 4.0)

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