线程Java中安全multitons线程Java中安全multitons(Thread safe m

鉴于以下多例：

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];
    }
}

我们怎样才能保持它的线程安全和懒惰而不getInstance（）方法的昂贵同步和双重检查锁定的争议？ 对于单身的有效方法是提到这里 ，但似乎并没有延伸到multitons。

Answer 1:

这会给你提供一个线程安全的存储机制为您Multitons。 唯一的缺点是，它可以创建不会在的putIfAbsent（）调用中使用的多例。 可能性很小，但它存在不存在。 当然遥控器上的机会，它确实发生了，但它仍然不会造成伤害。

从有利的一面，也没有预分配或所需的初始化并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) 
{
    Multiton result = instances.get(which);

    if (result == null) 
    {
        Multiton m = new Multiton(...);
        result = instances.putIfAbsent(which, m);

        if (result == null)
            result = m;
    }

    return result;
}

Answer 2:

更新：与Java 8，也可以是更简单：

public class Multiton {
    private static final ConcurrentMap<String, Multiton> multitons = new ConcurrentHashMap<>();

    private final String key;
    private Multiton(String key) { this.key = key; }

    public static Multiton getInstance(final String key) {
        return multitons.computeIfAbsent(key, Multiton::new);
    }
}

嗯，这很好！

---

原来的答案

这是建立在一溶液中Memoizer图案作为在JCiP描述 。 它采用像其他答案之一ConcurrentHashMap的，但不是直接存储多例的情况下，这可能会导致产生未使用的情况下，它存储导致创建多例的计算。 这额外的层解决了未使用的情况下的问题。

public class Multiton {

    private static final ConcurrentMap<Integer, Future<Multiton>> multitons = new ConcurrentHashMap<>();
    private static final Callable<Multiton> creator = new Callable<Multiton>() {
        public Multiton call() { return new Multiton(); }
    };

    private Multiton(Strnig key) {}

    public static Multiton getInstance(final Integer key) throws InterruptedException, ExecutionException {
        Future<Multiton> f = multitons.get(key);
        if (f == null) {
            FutureTask<Multiton> ft = new FutureTask<>(creator);
            f = multitons.putIfAbsent(key, ft);
            if (f == null) {
                f = ft;
                ft.run();
            }
        }
        return f.get();
    }
}

Answer 3:

你可以使用锁定阵列，至少能够得到不同的情况下同时来：

private static final Multiton[] instances = new Multiton[...];
private static final Object[] locks = new Object[instances.length];

static {
    for (int i = 0; i < locks.length; i++) {
        locks[i] = new Object();
    }
}

private Multiton(...) {
    //...
}

public static Multiton getInstance(int which) {
    synchronized(locks[which]) {
        if(instances[which] == null) {
            instances[which] = new Multiton(...);
        }
        return instances[which];
    }
}

Answer 4:

在Java 8的出现和一些改进ConcurrentMap和lambda表达式，现在可以实现Multiton （也许甚至Singleton在更为整洁时尚）：

public class Multiton {
  // Map from the index to the item.
  private static final ConcurrentMap<Integer, Multiton> multitons = new ConcurrentHashMap<>();

  private Multiton() {
    // Possibly heavy construction.
  }

  // Get the instance associated with the specified key.
  public static Multiton getInstance(final Integer key) throws InterruptedException, ExecutionException {
    // Already made?
    Multiton m = multitons.get(key);
    if (m == null) {
      // Put it in - only create if still necessary.
      m = multitons.computeIfAbsent(key, k -> new Multiton());
    }
    return m;
  }
}

我怀疑-尽管这会让我感到不舒服-这getInstance可以进一步减少到：

// Get the instance associated with the specified key.
public static Multiton getInstance(final Integer key) throws InterruptedException, ExecutionException {
  // Put it in - only create if still necessary.
  return multitons.computeIfAbsent(key, k -> new Multiton());
}

Answer 5:

您正在寻找一个AtomicReferenceArray 。

public class Multiton {
  private static final AtomicReferenceArray<Multiton> instances = new AtomicReferenceArray<Multiton>(1000);

  private Multiton() {
  }

  public static Multiton getInstance(int which) {
    // One there already?
    Multiton it = instances.get(which);
    if (it == null) {
      // Lazy make.
      Multiton newIt = new Multiton();
      // Successful put?
      if ( instances.compareAndSet(which, null, newIt) ) {
        // Yes!
        it = newIt;
      } else {
        // One appeared as if by magic (another thread got there first).
        it = instances.get(which);
      }
    }

    return it;
  }
}

文章来源: Thread safe multitons in Java