synchronized关键字实现信号量同步功能

下面的代码，不加mys.acquire();和mys.release();的时候，各线程会交替输出。
加上的话，各线程会串行输出，说明信号量机制生效了。

MySemaphore.java

interface Semaphore { 
  void acquire();
  void release();
} 

public class MySemaphore implements Semaphore{
	private StringBuffer runFlag = new StringBuffer("true");

  public void acquire(){
  	synchronized(runFlag){
      if(runFlag.toString().equals("true")){
      	runFlag.setLength(0);
      	runFlag.append("false");
      }else{
        try{
            runFlag.wait();
        	runFlag.setLength(0);
      	    runFlag.append("false");
        }catch(Exception e){
      	  e.printStackTrace();
        }
      }
    }
  }

  public void release(){
  	synchronized(runFlag){
        runFlag.setLength(0);
      	runFlag.append("true");
      	try{
      	  runFlag.notify();
      	}catch(Exception e){
      		e.printStackTrace();
      	}
    }
  }

	public static void main(String[] args){
		
		
		Runnable run = new Runnable(){
			    MySemaphore mys = new MySemaphore();
			
				public void run(){
					mys.acquire();
					System.out.println(Thread.currentThread().getName() + " start");
					for(int j=0;j<10;j++){
					  System.out.println(Thread.currentThread().getName() + " start " + j);
					}
					System.out.println(Thread.currentThread().getName() + " end");
					mys.release();
				}
		};
		
		Thread t;
		for(int i=0;i<10;i++){
		  t = new Thread(run);
		  t.start();
		}
	}
}