单例模式

单例模式避免了资源的多次读写导致的同步、性能问题。

饿汉

先生成，用的时候直接引用

//A
public class SingletonInstance {
    private static final SingletonInstance instance= new SingletonInstance();

    public SingletonInstance getInstance(){
        return instance;
    }
}
//B 
public class SingletonInstance {
    private static final SingletonInstance instance;

    static {
        instance = new SingletonInstance();
    }

    public SingletonInstance getInstance() {
        return instance;
    }
}
//C
public class SingletonInstance {
    private static class SingletonHolder {
        private static final SingletonInstance instance = new SingletonInstance();
    }

    public SingletonInstance getInstance() {
        return SingletonHolder.instance;
    }
}

懒汉 - 双重校验锁

用的时候再生成

public class SingletonInstance {
    private static volatile SingletonInstance instance;

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

枚举

枚举类的成员默认是Static Final的

public enum SingletonInstance {
    getInstance()
}