surfaceview在按下home键，再返回时出错

public class MLTestActivity extends Activity {
    @Override
    public void onCreate(final Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(new MyView(this));
    }
    
    @Override
    public void onResume(){
        super.onResume();
       
    }

   //视图内部类
    class MyView extends SurfaceView implements SurfaceHolder.Callback
    {
       private final SurfaceHolder holder;
       private final MyThread myThread;
       public MyView(final Context context) {
           super(context);
           // TODO Auto-generated constructor stub
           holder = this.getHolder();
           holder.addCallback(this);
           myThread = new MyThread(holder);//创建一个绘图线程
       }

       @Override
       public void surfaceChanged(final SurfaceHolder holder, final int format, final int width,
                final int height) {
           // TODO Auto-generated method stub
             
       }

       @Override
       public void surfaceCreated(final SurfaceHolder holder) {
            // TODO Auto-generated method stub
           myThread.isRun = true;
           myThread.start();
      }

      @Override
       public void surfaceDestroyed(final SurfaceHolder holder) {
          // TODO Auto-generated method stub
          myThread.isRun = false;
      }
    }
    
    
   class MyThread extends Thread{
       private SurfaceHolder holder;
       public boolean isRun ;
       float radius = 10f;
       Paint p;
       public MyThread(SurfaceHolder holder)
       {
           this.holder =holder;
           isRun = true;
           p = new Paint();
       }
       
       public void run(){
           int count = 0;
           while(isRun)
           {
               Canvas c = null;
               try{
                   synchronized (holder){
                       c = holder.lockCanvas();
                       c.drawColor(Color.BLACK);
                       //doDraw(c);
                       if(count % 2 == 0)
                           p.setColor(Color.RED);
                       else
                           p.setColor(Color.YELLOW);
                       Rect r = new Rect(100, 50, 300, 250);
                       c.drawRect(r, p);
                       c.drawText("这是第"+(count++)+"秒", 100, 310, p);
                       Thread.sleep(1000);
                       Thread.sleep(5);
                   }
               }catch(Exception e){
                   e.printStackTrace();
               }finally{
                   if(c!= null)
                       holder.unlockCanvasAndPost(c);
               }
           }
       }
       
       public void doDraw(Canvas c){
           
           //这个很重要，清屏操作，清楚掉上次绘制的残留图像
           c.drawColor(Color.BLUE);
           p.setColor(Color.YELLOW);
           c.translate(200, 200);
           c.drawCircle(0,0, radius++, p );

           if(radius > 100){
               radius = 10f;
           }

       }
       
   }

}

即使在代码里设置了isRun flag，在按下home键再回到surfaceview后，还是会报同样的错误

W/dalvikvm( 2802): threadid=1: thread exiting with uncaught exception (group=0x40a3b1f8)
E/AndroidRuntime( 2802): FATAL EXCEPTION: main
E/AndroidRuntime( 2802): java.lang.IllegalThreadStateException: Thread already started.
E/AndroidRuntime( 2802):     at java.lang.Thread.start(Thread.java:1045)
E/AndroidRuntime( 2802):     at andr.google.test.MLTestActivity$MyView.surfaceCreated(MLTestActivity.java:44)
E/AndroidRuntime( 2802):     at android.view.SurfaceView.updateWindow(SurfaceView.java:533)
E/AndroidRuntime( 2802):     at android.view.SurfaceView.onWindowVisibilityChanged(SurfaceView.java:226)
E/AndroidRuntime( 2802):     at android.view.View.dispatchWindowVisibilityChanged(View.java:5844)
E/AndroidRuntime( 2802):     at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:945)
E/AndroidRuntime( 2802):     at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:945)
E/AndroidRuntime( 2802):     at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:945)
E/AndroidRuntime( 2802):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:965)
E/AndroidRuntime( 2802):     at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2442)
E/AndroidRuntime( 2802):     at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 2802):     at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime( 2802):     at android.app.ActivityThread.main(ActivityThread.java:4424)
E/AndroidRuntime( 2802):     at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 2802):     at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime( 2802):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
E/AndroidRuntime( 2802):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:552)
E/AndroidRuntime( 2802):     at dalvik.system.NativeStart.main(Native Method)
W/ActivityManager(  190):   Force finishing activity andr.google.test/.MLTestActivity

解决办法
在onResume中调用setContentView( view )，SurfaceView会调用surfaceChanged->surfaceCreated方法

具体见http://blog.youkuaiyun.com/pengjianbosoft/article/details/7171276