Android中handler的作用與線程

Android handler 的主要用途根據官網上的解釋，可以概括為兩點：

         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. 

         也就是說handler保存了兩個隊列，一個消息隊列，一個runnable隊列，handler負責順序一個一個執行兩個隊列里需要處理的元素。首先需要強調一下，handler并不是開辟了一個新的線程去執行，而是在其調用線程中，這個可以用以下代碼去測試：

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){

        }
    }
};

}</pre>

           上述代碼首先定義了一個Runable對象，但是handler.post(r)并沒有開辟新的線程，很容易發現，handler實際上使用的線程就是主線程。當然我們也可以用自定義一個線程，讓handler在另一個線程中執行，這樣主線程不會因為handler中有大量工作需要處理而進入阻塞狀態：

 


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系統內核提供，其中有一個looper專門用于處理消息隊列
        HandlerThread handlerThread = new HandlerThread("handler_thread");
        //在使用getLooper之前，必須先調用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);
    }

    @Override
    public void handleMessage(Message msg) {
        // TODO Auto-generated method stub
        super.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());
    }

}

}</pre>