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