设计模式

1、单例模式

饿汉模式

优点：执行效率高，性能高，没有任何的锁

缺点：某些情况下，可能会造成内存的浪费

public class MyStaticHungrySingleton {

    private static final MyStaticHungrySingleton myStaticHungrySingleton;

    static{
        myStaticHungrySingleton = new MyStaticHungrySingleton();
    }

    private MyStaticHungrySingleton(){}

    public MyStaticHungrySingleton getInstance(){
        return myStaticHungrySingleton;
    }
}

public class MyHungrySingleton {
    private static final MyHungrySingleton myHungrySingleton = new MyHungrySingleton();

    private MyHungrySingleton(){}

    private MyHungrySingleton getInstance(){
        return myHungrySingleton;
    }
}

懒汉式单例

优点：节省内存

缺点：性能下降

public class LazySimpleSingletion {

    private static LazySimpleSingletion instance;

    private LazySimpleSingletion(){}

    public synchronized static LazySimpleSingletion getInstance(){
        if(instance == null){
            instance = new LazySimpleSingletion();
        }
        return instance;
    }
}
//性能下降

public class LazyDoubleCheckSingletion {
    private volatile static LazyDoubleCheckSingletion instance;

    private LazyDoubleCheckSingletion(){}

    public static LazyDoubleCheckSingletion getInstance(){
        //检查是否要阻塞
        if(instance==null){
            synchronized (LazyDoubleCheckSingletion.class){
                //检查是否要创建实例
                if(instance==null){
                    instance = new LazyDoubleCheckSingletion();
                }
            }
        }
        return instance;
    }
}
//指令重排序