单例模式

1. 饿汉模式

这种设计存在的问题是无法懒加载

public class Singleton1 {
    private Singleton1(){
        
    }
    public static final Singleton1 INSTANCE=new Singleton1();
    public static Singleton1 getInstance(){
        return INSTANCE;
    }
}

2. 非线程安全懒汉模式

这种设计虽然是懒加载但是在多线程环境下会有线程安全的问题

public class Singleton2 {
    private Singleton2() {

    }

    private static Singleton2 INSTANCE;

    public static Singleton2 getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Singleton2();
        }
        return INSTANCE;
    }

}

3. double check synchronized

最小化锁粒度同步，由于jmv 编译器等优化原因会导致空指针异常

public class Singleton3 {
    private Singleton3() {

    }

    private static Singleton3 INSTANCE;

    public static Singleton3 getInstance() {
        if (null == INSTANCE) {
            synchronized (Singleton3.class) {
                if (null == INSTANCE) {
                    INSTANCE = new Singleton3();
                }
            }
        }
        return INSTANCE;
    }
}

4. volatile + double check

使用 volatile + double check保证了可见行，解决了3中有可能出现等空指针异常

public class Singleton4 {
    private Singleton4() {

    }

    private static volatile  Singleton4 INSTANCE;

    public static Singleton4 getInstance() {
        if (null == INSTANCE) {
            synchronized (Singleton4.class) {
                if (null == INSTANCE) {
                    INSTANCE = new Singleton4();
                }
            }
        }
        return INSTANCE;
    }
}

5. 使用静态内部类

巧妙的使用jvm类加载特性，完成使用时一次加载，并且线程安全

public class Singleton5 {
    private Singleton5() {

    }

    private static class Singleton5Holder {
        private static final Singleton5 INSTANCE = new Singleton5();
    }

    public static Singleton5 getInstance() {
        return Singleton5Holder.INSTANCE;
    }
}

6. 使用枚举

利用enum在使用时只初始化一次的特性

public class Singleton6 {
    private Singleton6() {

    }

    private enum SingletonEnum {
        INSTANCE;

        private Singleton6 instance;

        SingletonEnum() {
            instance = new Singleton6();
        }

        private Singleton6 getInstance() {
            return instance;
        }
    }

    public static Singleton6 getInstance() {
        return SingletonEnum.INSTANCE.getInstance();

    }
}