对象创建型

定义

保证一个类仅有一个实例，并提供一个访问它的全局访问点

使用场景

通常使用在创建需要花费较大资源的对象上，如Hibernate中SessionFactory这样的重量级对象

入门案例：

代码：

饿汉式：

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

懒汉式：

public class Singleton2 {
    private static Singleton2 singleton = null;
    private Singleton2(){}
    public static Singleton2 getInstance(){
        if(singleton == null){
            singleton = new Singleton2();
        }
        return singleton;
    }
}

懒汉式-多线程：

public class Singleton3 {
    private static Singleton3 singleton = null;
    private Singleton3(){}

    //注意这是synchronized用得是类锁不是this
    public static synchronized Singleton3 getInstance(){
        if(singleton == null){
            singleton = new Singleton3();
        }
        return singleton;
    }
}

懒汉式-多线程-双重判断（高效）：

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