1 最佳实践

Spring的Bean默认是单例的，方便Spring容器管理Bean的生命周期。如果采用非单例，则Bean初始化后交由JavaEE容器管理，Spring不再跟踪管理Bean的生命周期

2 代码

2.1 饿汉

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

2.2 懒汉

线程安全问题：在getSingleton2()上或方法内部增加synchronized关键字

class Singleton2{
    private static Singleton2 single;
    private Singleton2(){}
    public static Singleton2 getSingleton2(){
        if(single == null){
            single = new Singleton2();
        }
        return single;
    }
}

2.3 单例模式扩展：产生两个实例

class Singleton3{
    private static int maxNum = 2;
    private static ArrayList<Singleton3> singletonList = new ArrayList<Singleton3>();
    private static int currentNum = 0;
    static{
        for(int i=0; i<maxNum; i++){
            singletonList.add(new Singleton3());
        }
    }
    
    private Singleton3(){}
    public static Singleton3 getSingleton3(){
        Random ran = new Random();
        currentNum = ran.nextInt(maxNum);
        return singletonList.get(currentNum);
    }
}