Android中handler的作用与线程

Android handler 的主要用途根据官网上的解释，可以概括为两点：

There are two main uses for a Handler: (1) to schedule messages and runnables to be executed as some point in the future; and (2) to enqueue an action to be performed on a different thread than your own.

也就是说handler保存了两个队列，一个消息队列，一个runnable队列，handler负责顺序一个一个执行两个队列里需要处理的元素。首先需要强调一下，handler并不是开辟了一个新的线程去执行，而是在其调用线程中，这个可以用以下代码去测试：

package com.relin.handler;import android.app.Activity;import android.os.Bundle;import android.os.Handler;public class AndroidHandlerActivity extends Activity {    /** Called when the activity is first created. */private Handler handler = new Handler();    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        handler.post(r);System.out.println("Activity-------->"+Thread.currentThread().getId());System.out.println("Activityname----->"+Thread.currentThread().getName());    }        Runnable r = new Runnable(){    @Override    public void run(){    System.out.println("handler-------->"+Thread.currentThread().getId());    System.out.println("hanldername----->"+Thread.currentThread().getName());    try{    Thread.sleep(5000);    }catch(Exception e){        }    }    };}

上述代码首先定义了一个Runable对象，但是handler.post(r)并没有开辟新的线程，很容易发现，handler实际上使用的线程就是主线程。当然我们也可以用自定义一个线程，让handler在另一个线程中执行，这样主线程不会因为handler中有大量工作需要处理而进入阻塞状态：

package com.relin.bundle;import android.app.Activity;import android.os.Bundle;import android.os.Handler;import android.os.HandlerThread;import android.os.Looper;import android.os.Message;public class AndroidBundleActivity extends Activity {    /** Called when the activity is first created. */    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        System.out.println("activity----->" + Thread.currentThread().getId());        //由android系统内核提供，其中有一个looper专门用于处理消息队列        HandlerThread handlerThread = new HandlerThread("handler_thread");        //在使用getLooper之前，必须先调用start，不然looper是null        handlerThread.start();        MyHandler myHandler = new MyHandler(handlerThread.getLooper());        Message msg = myHandler.obtainMessage();        Bundle b =  new Bundle();        b.putInt("age", 20);        b.putString("name", "relin");        msg.setData(b);        msg.sendToTarget();    }        class MyHandler extends Handler{    public MyHandler(){    }        public MyHandler(Looper looper){    super(looper);    }    @Overridepublic void handleMessage(Message msg) {// TODO Auto-generated method stubsuper.handleMessage(msg);Bundle b = msg.getData();int age = b.getInt("age");String name = b.getString("name");System.out.println("age is " + age + ", name is " + name);System.out.println("handler----->" + Thread.currentThread().getId());}        }}

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