java单例的几种实现方式

来源：互联网发布：java开源考试系统编辑：程序博客网时间：2024/06/05 07:37

1.懒汉式

线程不安全，如果两个线程同时执行到了

 if (instance == null) {//这一句判断都为真，紧接着就会创建两个实例

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

线程安全的，每次调用方法都会对对象枷锁，非常影响性能

public class Singleton {      private static Singleton instance;      private Singleton (){}      public static synchronized Singleton getInstance() {  //对方法枷锁性能不高    if (instance == null) {          instance = new Singleton();      }      return instance;      }  }

线程安全的，double-check

public class Singleton {      private volatile static Singleton singleton;      private Singleton (){}      public static Singleton getSingleton() {      if (singleton == null) {          synchronized (Singleton.class) {          if (singleton == null) {              singleton = new Singleton();          }          }      }      return singleton;      }  }

2.饿汉式的，这种是借助jvm的特性来实现的，因为静态变量在类加载的时候就初始化完成了，所以不需要线程同步，缺点就是累加载时就完成了

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

3.静态内部类，jvm在调用getInstance()时才会进行类的初始化

public class Singleton {      private static class SingletonHolder {      private static final Singleton INSTANCE = new Singleton();      }      private Singleton (){}      public static final Singleton getInstance() {      return SingletonHolder.INSTANCE;      }  }

4.通过枚举实现，枚举实例的创建时线程安全的，并且对序列化有保证

public enum Singleton {      INSTANCE;      public void whateverMethod() {      }  }