Android Thread 介绍与实例

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]

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]

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