Java单例模式

单例模式线程安全的四种创建方式

静态代码块,静态内部类,双重检锁,枚举模式

代码展示

静态代码块

private static final SingletonDemo SINGLETON_DEMO;
static {
    SINGLETON_DEMO = new SingletonDemo();
}

private SingletonDemo(){

}
public static SingletonDemo getInstance(){
    return  SINGLETON_DEMO ;
}

静态内部类

private singletonDemo(){
}

public static singletonDemo getInstance(){
    return Inner.singletonDemo;
}

public static class Inner {
    private static singletonDemo singletonDemo = new singletonDemo() ;
}

双重检锁

private static volatile SingletonDemo singletonDemo = null ;
private SingletonDemo(){
}

public static SingletonDemo getInstance(){
    try {
        Thread.sleep(5000);
    }catch(InterruptedException e) {
        throw new RuntimeException(e);
    }
    //双重校验锁  保证线程安全
    if(singletonDemo==null){
        synchronized(SingletonDemo.class){
            if(singletonDemo==null){
                singletonDemo = new SingletonDemo();
            }
        }
    }
    return singletonDemo ;
}

枚举模式

private SingletonDemo(){
}

public static SingletonDemo getInstance(){
    return InnerEnum.INNER_ENUM.getInstance();
}
private static enum InnerEnum {
    INNER_ENUM;

    private final SingletonDemo singletonDemo ;

    private InnerEnum(){
        singletonDemo = new SingletonDemo() ;
    }
    private SingletonDemo getInstance(){
         return singletonDemo;
    }
}