Android Thread 介绍与实例

时间:2022-03-28 06:21:02

Android中很重要的一个机制就是线程+消息,当然线程并不是android独有的,下面,简单的说说使用线程的时候应该注意的地方

我们采用最简单的方法来建立一个android的线程+消息的例子

1.Thread + Handler

[java]

 

复制代码 代码如下:


package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

    TextView mTextView = null;
//  static TextView mTextView = null; 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView)findViewById(R.id.textview);
        Thread th = new Thread(new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub 
                for(int i = 0;i<1000;i++)
                {
                    try {
                        Thread.sleep(500);
                        System.out.println("Thread running :"+i+"!");
                        Message msg = new Message();
                        msg.what = i;
                        mHandler.sendMessage(msg);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block 
                        e.printStackTrace();
                    }
                }
            }
        });
        th.start();

    }
    public Handler mHandler = new Handler(){
//    public static Handler mHandler = new Handler(){ 

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

            mTextView.setText(String.valueOf(msg.what));
        }

    };

}

 

package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

 TextView mTextView = null;
// static TextView mTextView = null;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView)findViewById(R.id.textview);
        Thread th = new Thread(new Runnable() {

   @Override
   public void run() {
    // TODO Auto-generated method stub
    for(int i = 0;i<1000;i++)
    {
     try {
      Thread.sleep(500);
      System.out.println("Thread running :"+i+"!");
      Message msg = new Message();
      msg.what = i;
      mHandler.sendMessage(msg);
     } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
     }
    }
   }
  });
        th.start();

    }
    public Handler mHandler = new Handler(){
//    public static Handler mHandler = new Handler(){

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

   mTextView.setText(String.valueOf(msg.what));
  }

    };

}

 

 

当我们用以上方式建立线程时,进入应用之后,线程开始运行,Handler接收消息改变UI中的TextView,此时一切正常

当按下退出时,程序退出,但是程序进程还在stack中,因此主线程之子线程,也就是我们定义的th(th_1)不会退出,此时,在log信息中可以看到,system.out还在print数字

当再次进入程序的时候,可以看到,log中打印的信息double,但是UI会按照新线程(th_2)的次序改变

此时th_1仍在运行,th_1使用的 handler_1也在运行,只不过上一个Activity的状态已经是finish,因此不会改变UI  this ->mFinished= true

其实只要th_1中有关于上一个Activity的引用,那么Activity就不会销毁,java的机制就是这样,这是我们推荐的线程机制,下面着重说一下可能遇到的问题

2.同样是刚刚的例子,我们将Handler定义成static

[java]
public static Handler mHandler = new Handler(){

public static Handler mHandler = new Handler(){此时,在退出应用再重新进入时,由于Handler并不会有新的实例,因此,th_1与th_2同时发消息给一个static Handler 或者说是指向了同一块内存区域,这时就会出现TextView上的数字来回跳的现象

3.这样也可以

使用static定义Handler也不是不可以,只要在Activity的onCreate()中重新实例一个Handler,这样,JVM分配另一块内存给新的Handler,这样运行就正常了

[java]

复制代码 代码如下:


package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

    public Handler mHandler = null;
    TextView mTextView = null;
//  static TextView mTextView = null; 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView)findViewById(R.id.textview);
        mHandler = new TestHandler();
        Thread th = new Thread(new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub 
                for(int i = 0;i<1000;i++)
                {
                    try {
                        Thread.sleep(500);
                        System.out.println("Thread running :"+i+"!");
                        Message msg = new Message();
                        msg.what = i;
                        mHandler.sendMessage(msg);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block 
                        e.printStackTrace();
                    }
                }
            }
        });
        th.start();

    }
    class TestHandler extends Handler
    {
        @Override
        public void handleMessage(Message msg) {
            // TODO Auto-generated method stub 
            super.handleMessage(msg);
            System.out.println("Handler running :"+msg.what+"!");
            mTextView.setText(String.valueOf(msg.what));
        }

    }
}

 

package com.example.test_thread;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.widget.TextView;

public class MainActivity extends Activity {

    public Handler mHandler = null;
 TextView mTextView = null;
// static TextView mTextView = null;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView)findViewById(R.id.textview);
        mHandler = new TestHandler();
        Thread th = new Thread(new Runnable() {

   @Override
   public void run() {
    // TODO Auto-generated method stub
    for(int i = 0;i<1000;i++)
    {
     try {
      Thread.sleep(500);
      System.out.println("Thread running :"+i+"!");
      Message msg = new Message();
      msg.what = i;
      mHandler.sendMessage(msg);
     } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
     }
    }
   }
  });
        th.start();

    }
    class TestHandler extends Handler
    {
  @Override
  public void handleMessage(Message msg) {
   // TODO Auto-generated method stub
   super.handleMessage(msg);
   System.out.println("Handler running :"+msg.what+"!");
   mTextView.setText(String.valueOf(msg.what));
  }

    }
}

 


当然,总的来说Java还是不推荐使用static变量的,这本身也不符合面向对象的变成思想,所以,建议除了一些final值,尽量还是多使用消息机制来解决问题,维护也轻松些