Java中的线程安全多态

Question

鉴于以下多态：

public class Multiton 

{

    private static final Multiton[] instances = new Multiton[...];

    private Multiton(...) 

    {

        //...

    }

    public static Multiton getInstance(int which) 

    {

        if(instances[which] == null) 

        {

            instances[which] = new Multiton(...);

        }

        return instances[which];

    }

}

我们如何在没有昂贵的getInstance（）方法同步和双重检查锁定争议的情况下使它保持线程安全和懒惰？这里提到了单例的有效方法，但似乎并没有扩展到多例。

Answer 1

这将为您提供Multitons的线程安全存储机制。唯一的缺点是可以创建在putIfAbsent（）调用中不使用的Multiton。可能性很小，但确实存在。当然，尽管确实有发生，但仍然不会造成任何伤害。从正面看，不需要预分配或初始化，也没有预定义的大小限制。private static ConcurrentHashMap<Integer, Multiton> instances = new ConcurrentHashMap<Integer, Multiton>();public static Multiton getInstance(int which)&nbsp;{&nbsp; &nbsp; Multiton result = instances.get(which);&nbsp; &nbsp; if (result == null)&nbsp;&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; Multiton m = new Multiton(...);&nbsp; &nbsp; &nbsp; &nbsp; result = instances.putIfAbsent(which, m);&nbsp; &nbsp; &nbsp; &nbsp; if (result == null)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; result = m;&nbsp; &nbsp; }&nbsp; &nbsp; return result;}

Answer 2

您可以使用一组锁，至少能够同时获取不同的实例：private static final Multiton[] instances = new Multiton[...];private static final Object[] locks = new Object[instances.length];static {&nbsp; &nbsp; for (int i = 0; i < locks.length; i++) {&nbsp; &nbsp; &nbsp; &nbsp; locks[i] = new Object();&nbsp; &nbsp; }}private Multiton(...) {&nbsp; &nbsp; //...}public static Multiton getInstance(int which) {&nbsp; &nbsp; synchronized(locks[which]) {&nbsp; &nbsp; &nbsp; &nbsp; if(instances[which] == null) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; instances[which] = new Multiton(...);&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; return instances[which];&nbsp; &nbsp; }}