多线程环境下的单例实现

import java.util.concurrent.locks.ReentrantLock;



public class Singleton {

  private static Singleton instance;
  
  private static ReentrantLock lock = new ReentrantLock();

  private Singleton() {}

  public static Singleton getInstance() {
    if (instance == null) {
      try{
        lock.lock();
        if (instance != null){
          return instance;
        } 
        instance = new Singleton();
      } finally{
        lock.unlock();
      }
      return instance;
    } else {
      return instance;
    }
  }

  public static void main(String[] args) {
    Thread[] threads = new Thread[100];
    for (Thread t : threads) {
      t = new Thread(new Runnable() {

        @Override
        public void run() {
          System.out.println(getInstance());
        }
      });
      t.start();

    }
  }

}