java.lang.IllegalThreadStateException: Thread already started.

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);
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);
}
}catch(Exception e){
e.printStackTrace();
}finally{
if(c!= null)
holder.unlockCanvasAndPost(c);
}
}
}
}