java单例问题

之前看资料,有人根据对象的创建时间将单例的实现分为懒汉式和饿汉式:

懒汉式:

 public class Singleton {
     private volatile static Singleton instance;

     private Singleton(){
             System.out.println("Singleton has loaded");
         }

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

1 通过延迟加载,提高内存使用效率;

2 双重为空判断:第一个判断可以减少锁判断;第二个判断可以减少对象重复重复创建;

饿汉式:

 public class Singleton {
     // 通过静态实现单例,但是存在创建时间过早问题
     privatestatic Singleton instance = new Singleton();

     private Singleton(){
             System.out.println("Singleton has loaded");
         }

     public static Singleton getInstance() {
         return instance;
     }
 }

1 代码精简;

2 通过静态实现在类加载时就创建对象,避免了同步问题;但是也造成实例化过早,存在内存浪费;

按需饿汉式:

public class Singleton {
    private Singleton(){
        System.out.println("Singleton has loaded");
    }

    public static Singleton getInstance() {
        return Quote.instance;
    }

    private static class Quote {
        private static final Singleton instance = new Singleton();
    }
}

1 使用内部类来创建对象,实现延迟加载;

参考资料:

　　https://baijiahao.baidu.com/s?id=1604691509810750218&wfr=spider&for=pc