Java单例模式实现的几种方式

public class SingleTon1 {     private SingleTon1(){   }   private static SingleTon1 instance = null;      public static SingleTon1 getInstance(){    if(instance == null){      instance = new SingleTon1();    }    return instance;  }}

package singleton; public class SingleTon1 {        private SingleTon1(){       }   private static SingleTon1 instance = null;     //多线程问题解法一，但是效率不高！因为每次调用都会加锁！  public static synchronized SingleTon1 getInstance(){    if(instance == null){      instance = new SingleTon1();    }    return instance;  }  public void print(){    System.out.println("thread_id:"+Thread.currentThread().getId());  }     private static Object object = new Object();  //很巧妙的方法，只有在null的时候加锁，之后就不加啦  public static SingleTon1 getInstance2(){         if(instance == null){      synchronized (object){        instance = new SingleTon1();      }    }    return instance;  } }

package singleton; import java.util.concurrent.locks.ReentrantLock; public class SingleTon2 {     private SingleTon2(){       }  private static ReentrantLock lock = new ReentrantLock();  private static SingleTon2 instance = null;        public void print(){    System.out.println("thread_id:"+Thread.currentThread().getId());  }     public static SingleTon2 getInstance2(){         if(instance == null){      lock.lock();      if(instance == null){ //注意这里还要判断下！！        instance = new SingleTon2();      }      lock.unlock();    }    return instance;  }}

package singleton;  public class SingleTon3 {     public static void print(){    System.out.println("thread_id:"+Thread.currentThread().getId());  }     public static Nested getNested(){       return Nested.instance;  }  //这个是单例创建的类  static class Nested{   private Nested(){    }  static Nested instance = new Nested();  }}

package singleton; import singleton.SingleTon3.Nested; public class Test2 {   public static void main(String[] args) {    // TODO Auto-generated method stub    Nested singleton;    Myrunnable mm = new Myrunnable();    Myrunnable m1 = new Myrunnable();         Myrunnable2 m2 = new Myrunnable2();    new Thread(m1).start();    new Thread(m2).start();    if(m1.singleton == m2.singleton){ //是同一个      System.out.println("是同一个");    }else{      System.out.println("不是同一个");    }   }}  class Myrunnable implements Runnable{    Nested singleton;      @Override      public void run() {        // TODO Auto-generated method stub        singleton = SingleTon3.getNested();        SingleTon3.print();      }  }     class Myrunnable2 implements Runnable{    Nested singleton;    @Override    public void run() {      // TODO Auto-generated method stub      singleton = SingleTon3.getNested();      SingleTon3.print();    }  }