单例模式五种写法：懒汉、饿汉、双重检验锁、静态内部类、枚举。

本文链接：https://blog.youkuaiyun.com/Frame_M/article/details/78949995

本文深入探讨了单例模式的不同实现方式，包括懒汉式、饿汉式、双重检查锁定及静态内部类等，并分析了它们在多线程环境下的安全性和效率。

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public class singletonTest {

/* 懒汉式多线程下不安全 */

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

/* 懒汉式多线程下安全性较高 */

private singletonTest(){}
  private static singletonTest instance;
  public synchronized static singletonTest getInstance(){
    if(instance==null){
      instance=new singletonTest();
    }
    return instance;
  }

/* 恶汉式*/

private singletonTest(){}
  private static singletonTest instance=new singletonTest();
  public static singletonTest getInstance(){
    return instance;
  }

/*双重检验锁*/

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

/* 双重检验锁优化因为编译器中存在指令重排序的问题注意java5之前无效*/

  private singletonTest(){}
  private volatile static singletonTest instance;
  public static singletonTest getInstance(){
    if(instance==null){
      synchronized (singletonTest.class) {
        if(instance==null){
          instance=new singletonTest();
        }
      }
    }
    return instance;
  }

/*静态类部类*/

  private singletonTest(){}
  private static class getSingletonTestHolder{
    private static singletonTest instatce=new singletonTest();
  }
  public static final singletonTest getInstance(){
    return getSingletonTestHolder.instatce;
  }

/*枚举*/

  public enum EasySingleton{
    INSTANCE;
  }

}

这么多，你都看到这里了，果然是真爱啊！！！！！