Android异步处理三：Handler+Looper+MessageQueue深入详解

Android异步处理系列文章索引

Android异步处理一：使用Thread+Handler实现非UI线程更新UI界面

Android异步处理二：使用AsyncTask异步更新UI界面

Android异步处理三：Handler+Looper+MessageQueue深入详解

Android异步处理四：AsyncTask的实现原理

在《Android异步处理一：使用Thread+Handler实现非UI线程更新UI界面》中，我们讲到使用Thread+Handler的方式来实现界面的更新，其实是在非UI线程发送消息到UI线程，通知UI线程进行界面更新，这一篇我们将深入学习Android线程间通讯的实现原理。

概述：Android使用消息机制实现线程间的通信，线程通过Looper建立自己的消息循环，MessageQueue是FIFO的消息队列，Looper负责从MessageQueue中取出消息，并且分发到消息指定目标Handler对象。Handler对象绑定到线程的局部变量Looper，封装了发送消息和处理消息的接口。

例子：在介绍原理之前，我们先介绍Android线程通讯的一个例子，这个例子实现点击按钮之后从主线程发送消息"hello"到另外一个名为” CustomThread”的线程。

代码下载

LooperThreadActivity.java

[java] view plaincopyprint?
  1. package com.zhuozhuo; 
  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. import android.view.View; 
  10. import android.view.View.OnClickListener; 
  11.  
  12. publicclass LooperThreadActivity extends Activity{ 
  13.     /** Called when the activity is first created. */ 
  14.      
  15.     privatefinalint MSG_HELLO = 0; 
  16.     private Handler mHandler; 
  17.      
  18.     @Override 
  19.     publicvoid onCreate(Bundle savedInstanceState) { 
  20.         super.onCreate(savedInstanceState); 
  21.         setContentView(R.layout.main); 
  22.         new CustomThread().start();//新建并启动CustomThread实例  
  23.          
  24.         findViewById(R.id.send_btn).setOnClickListener(new OnClickListener() { 
  25.              
  26.             @Override 
  27.             publicvoid onClick(View v) {//点击界面时发送消息  
  28.                 String str = "hello"; 
  29.                 Log.d("Test", "MainThread is ready to send msg:" + str); 
  30.                 mHandler.obtainMessage(MSG_HELLO, str).sendToTarget();//发送消息到CustomThread实例  
  31.                  
  32.             } 
  33.         }); 
  34.          
  35.     } 
  36.      
  37.      
  38.      
  39.      
  40.      
  41.     class CustomThread extends Thread { 
  42.         @Override 
  43.         publicvoid run() { 
  44.             //建立消息循环的步骤  
  45.             Looper.prepare();//1、初始化Looper  
  46.             mHandler = new Handler(){//2、绑定handler到CustomThread实例的Looper对象  
  47.                 publicvoid handleMessage (Message msg) {//3、定义处理消息的方法  
  48.                     switch(msg.what) { 
  49.                     case MSG_HELLO: 
  50.                         Log.d("Test", "CustomThread receive msg:" + (String) msg.obj); 
  51.                     } 
  52.                 } 
  53.             }; 
  54.             Looper.loop();//4、启动消息循环  
  55.         } 
  56.     } 
  57. }  
package com.zhuozhuo;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;

public class LooperThreadActivity extends Activity{
    /** Called when the activity is first created. */
	
	private final int MSG_HELLO = 0;
    private Handler mHandler;
	
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new CustomThread().start();//新建并启动CustomThread实例
        
        findViewById(R.id.send_btn).setOnClickListener(new OnClickListener() {
			
			@Override
			public void onClick(View v) {//点击界面时发送消息
				String str = "hello";
		        Log.d("Test", "MainThread is ready to send msg:" + str);
				mHandler.obtainMessage(MSG_HELLO, str).sendToTarget();//发送消息到CustomThread实例
				
			}
		});
        
    }
    
    
    
    
    
    class CustomThread extends Thread {
    	@Override
    	public void run() {
    		//建立消息循环的步骤
    		Looper.prepare();//1、初始化Looper
    		mHandler = new Handler(){//2、绑定handler到CustomThread实例的Looper对象
    			public void handleMessage (Message msg) {//3、定义处理消息的方法
    				switch(msg.what) {
    				case MSG_HELLO:
    					Log.d("Test", "CustomThread receive msg:" + (String) msg.obj);
    				}
    			}
    		};
    		Looper.loop();//4、启动消息循环
    	}
    }
}

main.xml

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  1. <?xmlversion="1.0"encoding="utf-8"?> 
  2. <LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android" 
  3.     android:orientation="vertical" 
  4.     android:layout_width="fill_parent" 
  5.     android:layout_height="fill_parent" 
  6.     > 
  7. <TextView   
  8.     android:layout_width="fill_parent"  
  9.     android:layout_height="wrap_content"  
  10.     android:text="@string/hello" 
  11.     /> 
  12. <Buttonandroid:text="发送消息"android:id="@+id/send_btn"android:layout_width="wrap_content"android:layout_height="wrap_content"></Button> 
  13. </LinearLayout>  
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello"
    />
<Button android:text="发送消息" android:id="@+id/send_btn" android:layout_width="wrap_content" android:layout_height="wrap_content"></Button>
</LinearLayout>

Log打印结果：

原理：

我们看到，为一个线程建立消息循环有四个步骤：

1、  初始化Looper

2、  绑定handler到CustomThread实例的Looper对象

3、  定义处理消息的方法

4、  启动消息循环

下面我们以这个例子为线索，深入Android源代码，说明Android Framework是如何建立消息循环，并对消息进行分发的。

1、  初始化Looper : Looper.prepare()

Looper.java

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  1. privatestaticfinal ThreadLocal sThreadLocal = new ThreadLocal(); 
  2. publicstaticfinalvoid prepare() { 
  3.         if (sThreadLocal.get() != null) { 
  4.             thrownew RuntimeException("Only one Looper may be created per thread"); 
  5.         } 
  6.         sThreadLocal.set(new Looper()); 
  7. }  
private static final ThreadLocal sThreadLocal = new ThreadLocal();
public static final void prepare() {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper());
}

一个线程在调用Looper的静态方法prepare()时，这个线程会新建一个Looper对象，并放入到线程的局部变量中，而这个变量是不和其他线程共享的（关于ThreadLocal的介绍）。下面我们看看Looper()这个构造函数：

Looper.java

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  1. final MessageQueue mQueue; 
  2. private Looper() { 
  3.         mQueue = new MessageQueue(); 
  4.         mRun = true; 
  5.         mThread = Thread.currentThread(); 
  6.     }  
final MessageQueue mQueue;
private Looper() {
        mQueue = new MessageQueue();
        mRun = true;
        mThread = Thread.currentThread();
    }

可以看到在Looper的构造函数中，创建了一个消息队列对象mQueue，此时，调用Looper. prepare()的线程就建立起一个消息循环的对象（此时还没开始进行消息循环）。

2、  绑定handler到CustomThread实例的Looper对象 : mHandler= new Handler()

Handler.java

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  1. final MessageQueue mQueue; 
  2. final Looper mLooper; 
  3. public Handler() { 
  4.         if (FIND_POTENTIAL_LEAKS) { 
  5.             final Class<? extends Handler> klass = getClass(); 
  6.             if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) && 
  7.                     (klass.getModifiers() & Modifier.STATIC) == 0) { 
  8.                 Log.w(TAG, "The following Handler class should be static or leaks might occur: " + 
  9.                     klass.getCanonicalName()); 
  10.             } 
  11.         } 
  12.  
  13.         mLooper = Looper.myLooper(); 
  14.         if (mLooper == null) { 
  15.             thrownew RuntimeException( 
  16.                 "Can't create handler inside thread that has not called Looper.prepare()"); 
  17.         } 
  18.         mQueue = mLooper.mQueue; 
  19.         mCallback = null; 
  20. }  
final MessageQueue mQueue;
 final Looper mLooper;
public Handler() {
        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 = null;
}

Handler通过mLooper = Looper.myLooper();绑定到线程的局部变量Looper上去，同时Handler通过mQueue =mLooper.mQueue;获得线程的消息队列。此时，Handler就绑定到创建此Handler对象的线程的消息队列上了。

3、定义处理消息的方法：Override public void handleMessage (Message msg){}

     子类需要覆盖这个方法，实现接受到消息后的处理方法。

4、启动消息循环 ： Looper.loop()

      所有准备工作都准备好了，是时候启动消息循环了！Looper的静态方法loop()实现了消息循环。

Looper.java

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  1. publicstaticfinalvoid loop() { 
  2.        Looper me = myLooper(); 
  3.        MessageQueue queue = me.mQueue; 
  4.         
  5.        // Make sure the identity of this thread is that of the local process,  
  6.        // and keep track of what that identity token actually is.  
  7.        Binder.clearCallingIdentity(); 
  8.        finallong ident = Binder.clearCallingIdentity(); 
  9.         
  10.        while (true) { 
  11.            Message msg = queue.next(); // might block  
  12.            //if (!me.mRun) {  
  13.            //    break;  
  14.            //}  
  15.            if (msg != null) { 
  16.                if (msg.target == null) { 
  17.                    // No target is a magic identifier for the quit message.  
  18.                    return; 
  19.                } 
  20.                if (me.mLogging!= null) me.mLogging.println( 
  21.                        ">>>>> Dispatching to " + msg.target + " " 
  22.                        + msg.callback + ": " + msg.what 
  23.                        ); 
  24.                msg.target.dispatchMessage(msg); 
  25.                if (me.mLogging!= null) me.mLogging.println( 
  26.                        "<<<<< Finished to    " + msg.target + " " 
  27.                        + msg.callback); 
  28.                 
  29.                // Make sure that during the course of dispatching the  
  30.                // identity of the thread wasn't corrupted.  
  31.                finallong newIdent = Binder.clearCallingIdentity(); 
  32.                if (ident != newIdent) { 
  33.                    Log.wtf("Looper", "Thread identity changed from 0x" 
  34.                            + Long.toHexString(ident) + " to 0x" 
  35.                            + Long.toHexString(newIdent) + " while dispatching to " 
  36.                            + msg.target.getClass().getName() + " " 
  37.                            + msg.callback + " what=" + msg.what); 
  38.                } 
  39.                 
  40.                msg.recycle(); 
  41.            } 
  42.        } 
  43.    }  
 public static final void loop() {
        Looper me = myLooper();
        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();
        
        while (true) {
            Message msg = queue.next(); // might block
            //if (!me.mRun) {
            //    break;
            //}
            if (msg != null) {
                if (msg.target == null) {
                    // No target is a magic identifier for the quit message.
                    return;
                }
                if (me.mLogging!= null) me.mLogging.println(
                        ">>>>> Dispatching to " + msg.target + " "
                        + msg.callback + ": " + msg.what
                        );
                msg.target.dispatchMessage(msg);
                if (me.mLogging!= null) me.mLogging.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("Looper", "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.recycle();
            }
        }
    }

while(true)体现了消息循环中的“循环“，Looper会在循环体中调用queue.next()获取消息队列中需要处理的下一条消息。当msg != null且msg.target != null时，调用msg.target.dispatchMessage(msg);分发消息，当分发完成后，调用msg.recycle();回收消息。

msg.target是一个handler对象，表示需要处理这个消息的handler对象。Handler的void dispatchMessage(Message msg)方法如下：

Handler.java

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  1. publicvoid dispatchMessage(Message msg) { 
  2.         if (msg.callback != null) { 
  3.             handleCallback(msg); 
  4.         } else { 
  5.             if (mCallback != null) { 
  6.                 if (mCallback.handleMessage(msg)) { 
  7.                     return; 
  8.                 } 
  9.             } 
  10.             handleMessage(msg); 
  11.         } 
  12. }  
public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
}

可见，当msg.callback== null 并且mCallback == null时，这个例子是由handleMessage(msg);处理消息，上面我们说到子类覆盖这个方法可以实现消息的具体处理过程。

 

总结：从上面的分析过程可知，消息循环的核心是Looper，Looper持有消息队列MessageQueue对象，一个线程可以把Looper设为该线程的局部变量，这就相当于这个线程建立了一个对应的消息队列。Handler的作用就是封装发送消息和处理消息的过程，让其他线程只需要操作Handler就可以发消息给创建Handler的线程。由此可以知道，在上一篇《Android异步处理一：使用Thread+Handler实现非UI线程更新UI界面》中，UI线程在创建的时候就建立了消息循环(在ActivityThread的public static final void main(String[] args)方法中实现)，因此我们可以在其他线程给UI线程的handler发送消息，达到更新UI的目的。

本博文地址：http://blog.csdn.net/mylzc/article/details/6771331 转载请注明出处

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原文地址：https://www.cnblogs.com/LiaoHao/p/3270008.html