public class SingleDemo { 
  private static SingleDemo s = null; 
  private SingleDemo(){} 
  public static SingleDemo getInstance(){ 
    if(s == null){ 
      s = new SingleDemo(); 
    } 
    return s; 
  } 
} 

public class ThreadDemo3 { 
   
  public static void main(String[] args) { 
    SingleDemo s1 = SingleDemo.getInstance(); 
    SingleDemo s2 = SingleDemo.getInstance(); 
    System.out.println(s2 == s2); 
  } 
} 

public class ThreadTest implements Runnable { 
  //存放单例对象，使用Set是为了不存放重复元素 
  public Set<SingleDemo> singles = new HashSet<SingleDemo>(); 
  @Override 
  public void run() { 
    //获取单例 
    SingleDemo s = SingleDemo.getInstance(); 
    //添加单例 
    singles.add(s); 
  } 
} 

public class ThreadDemo3 { 
   
  public static void main(String[] args) { 
//   SingleDemo s1 = SingleDemo.getInstance(); 
//   SingleDemo s2 = SingleDemo.getInstance(); 
//   System.out.println(s2 == s2); 
    ThreadTest t = new ThreadTest(); 
    new Thread(t).start(); 
    new Thread(t).start(); 
    new Thread(t).start(); 
    new Thread(t).start(); 
    new Thread(t).start(); 
    new Thread(t).start(); 
    new Thread(t).start(); 
    new Thread(t).start(); 
    System.out.println(t.singles); 
  } 
} 

public class SingleDemo {
 private static SingleDemo s = null;
 private SingleDemo(){}
 public static synchronized SingleDemo getInstance(){
  if(s == null){
   s = new SingleDemo();
  }
  return s;
 }
}

public class SingleDemo {
 private static SingleDemo s = null;
 private SingleDemo(){}
 public static SingleDemo getInstance(){
  /*如果第一个线程获取到了单例的实例对象，
   * 后面的线程再获取实例的时候不需要进入同步代码块中了*/
  if(s == null){
   //同步代码块用的锁是单例的字节码文件对象，且只能用这个锁
   synchronized(SingleDemo.class){
    if(s == null){
     s = new SingleDemo();
    }
   }
  }
  return s;
 }
}