单例模式

定义

  一个类至多有一个对象实例。这个实例被称为单例。在使用时才初始化被称为懒汉式单例，类一加载就初始化被叫做饿汉式单例。有8种常见实现方式。

实现

1. 静态常量法：可用

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

2. 静态代码块：可用

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

3. 静态方法：不可用

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

  线程A进入第7句但还未来得及初始化时，线程B在第5句判断为null，所以也可以进入第6句，有并发问题。

4. 同步方法：可用但不推荐

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

5. 同步代码块：不可用

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

  线程A进入第7句但还未来得及初始化时，线程B在第5句判断为null，虽然得等到A初始化完成，但也可以进入第7句，有并发问题。

6. 双重检查：可用且推荐用

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

7. 静态内部类：可用且推荐用

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

8. 枚举：可用且推荐用

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

原文来至：https://www.cnblogs.com/zhaoyan001/p/6365064.html