java synchronized没有起作用

package com.wxb.syn;

class Semaphore{}

public class BalanceTest extends Thread{
public static int balance = 100;
public Semaphore semp = new Semaphore();
public void run() {
synchronized(this){
// System.out.println("balance=" + balance);
if(balance == 100) {
// try {
// sleep(1000);
// } catch (InterruptedException e) {
// // TODO Auto-generated catch block
// e.printStackTrace();
// }
balance = balance - 100;
System.out.println(balance);
System.out.println("withdraw 100 successful!");
}else{
System.out.println("failed");
}
}

}
public static void main(String[] args) {
    BalanceTest bt = new BalanceTest();
    bt.start();
    BalanceTest bt2 = new BalanceTest();
    bt2.start();
    BalanceTest bt3 = new BalanceTest();
    bt3.start();
    BalanceTest bt4 = new BalanceTest();
    bt4.start();
    BalanceTest bt5 = new BalanceTest();
    bt5.start();
    BalanceTest bt6 = new BalanceTest();
    bt6.start();
}

}

不加sleep只有一个successful，加了之后都成功synchronized好像不起作用

问题应该是出在这一句：synchronized(this)
搞个synchronized方法，run的时候调用它