Question

受comment to an given answer的启发我尝试创建多线程模式的线程安全实现，它依赖于唯一键并对它们执行锁定（我从 JB Nizet 获得了这个想法这是question）的回答。

问题

我提供的实施是否可行？

我对Multiton（或Singleton）是否总体良好的模式不感兴趣，它会导致讨论。我只想要一个干净而有效的实施方案。

反政府：

您必须知道要在编译时创建多少个实例。

赞成

整个班级或整个地图都没有锁定。可以同时拨打getInstance。
通过密钥对象获取实例，而不仅仅是无界int或String，因此您可以确保在方法调用后获得非null实例。
线程安全（至少这是我的印象）。

public class Multiton
{
  private static final Map<Enum<?>, Multiton> instances = new HashMap<Enum<?>, Multiton>();

  private Multiton() {System.out.println("Created instance."); }

  /* Can be called concurrently, since it only synchronizes on id */
  public static <KEY extends Enum<?> & MultitionKey> Multiton getInstance(KEY id)
  {
    synchronized (id)
    {
      if (instances.get(id) == null)
        instances.put(id, new Multiton());
    }
    System.out.println("Retrieved instance.");
    return instances.get(id);
  }

  public interface MultitionKey { /* */ }

  public static void main(String[] args) throws InterruptedException
  {
    //getInstance(Keys.KEY_1);
    getInstance(OtherKeys.KEY_A);

    Runnable r = new Runnable() {
      @Override
      public void run() { getInstance(Keys.KEY_1); }
    };

    int size = 100;
    List<Thread> threads = new ArrayList<Thread>();
    for (int i = 0; i < size; i++)
      threads.add(new Thread(r));

    for (Thread t : threads)
      t.start();

    for (Thread t : threads)
      t.join();
  }

  enum Keys implements MultitionKey
  {
    KEY_1;

    /* define more keys */
  }

  enum OtherKeys implements MultitionKey
  {
    KEY_A;

    /* define more keys */
  }
}

我试图阻止调整地图大小和滥用我同步的枚举。在我完成它之前，它更像是一个概念证明！：）

public class Multiton
{
  private static final Map<MultitionKey, Multiton> instances = new HashMap<MultitionKey, Multiton>((int) (Key.values().length/0.75f) + 1);
  private static final Map<Key, MultitionKey> keyMap;

  static
  {
    Map<Key, MultitionKey> map = new HashMap<Key, MultitionKey>();
    map.put(Key.KEY_1, Keys.KEY_1);
    map.put(Key.KEY_2, OtherKeys.KEY_A);
    keyMap = Collections.unmodifiableMap(map);
  }

  public enum Key {
    KEY_1, KEY_2;
  }

  private Multiton() {System.out.println("Created instance."); }

  /* Can be called concurrently, since it only synchronizes on KEY */
  public static <KEY extends Enum<?> & MultitionKey> Multiton getInstance(Key id)
  {
    @SuppressWarnings ("unchecked")
    KEY key = (KEY) keyMap.get(id);
    synchronized (keyMap.get(id))
    {
      if (instances.get(key) == null)
        instances.put(key, new Multiton());
    }
    System.out.println("Retrieved instance.");
    return instances.get(key);
  }

  private interface MultitionKey { /* */ }

  private enum Keys implements MultitionKey
  {
    KEY_1;

    /* define more keys */
  }

  private enum OtherKeys implements MultitionKey
  {
    KEY_A;

    /* define more keys */
  }
}

Answer 1

绝对不是线程安全的。这是许多可能出错的事情的简单例子。

线程A正试图放入密钥id1。由于放置在id2，线程B正在调整存储桶表的大小。因为它们具有不同的同步监视器，所以它们可以并行进行比赛。

Thread A                      Thread B
--------                      --------
b = key.hash % map.buckets.size   

                             copy map.buckets reference to local var
                             set map.buckets = new Bucket[newSize]
                             insert keys from old buckets into new buckets

insert into map.buckets[b]

在此示例中，假设Thread A看到了map.buckets = new Bucket[newSize]修改。它不能保证（因为没有发生 - 在边缘之前），但它可能。在这种情况下，它会将（key，value）对插入错误的桶中。没有人会找到它。

作为一个轻微的变体，如果Thread A将map.buckets引用复制到本地var并完成所有工作，那么它将插入到正确的存储桶中，但错误的存储桶表;它不会插入Thread B即将安装的新表中，供大家查看。如果key 1上的下一个操作恰好看到新表（再次，不保证但可能会这样），那么它将不会看到Thread A's个动作，因为它们是在长期被遗忘的存储桶阵列上完成的

Answer 2

我说不可行。

同步id参数充满了危险 - 如果他们将此enum用于另一个同步机制会怎么样？当然，HashMap并不像评论所指出的那样是并发的。

要演示 - 试试这个：

Runnable r = new Runnable() {
  @Override
  public void run() { 
    // Added to demonstrate the problem.
    synchronized(Keys.KEY_1) {
      getInstance(Keys.KEY_1);
    } 
  }
};

这是一个使用原子而不是同步的实现，因此应该更有效。它比你的复杂得多，但处理Miltiton IS中的所有边缘情况都很复杂。

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

  // Ready for use - set to false while initialising.
  private final AtomicBoolean ready = new AtomicBoolean();
  // Everyone who is waiting for me to initialise.
  private final Queue<Thread> waiters = new ConcurrentLinkedQueue<>();
  // For logging (and a bit of linguistic fun).
  private final int forInstance;

  // We need a simple constructor.
  private Multiton(int forInstance) {
    this.forInstance = forInstance;
    log(forInstance, "New");
  }

  // The expensive initialiser.
  public void init() throws InterruptedException {
    log(forInstance, "Init");
    // ... presumably heavy stuff.
    Thread.sleep(1000);

    // We are now ready.
    ready();
  }

  private void ready() {
    log(forInstance, "Ready");
    // I am now ready.
    ready.getAndSet(true);
    // Unpark everyone waiting for me.
    for (Thread t : waiters) {
      LockSupport.unpark(t);
    }
  }

  // Get the instance for that one.
  public static Multiton getInstance(int which) throws InterruptedException {
    // One there already?
    Multiton it = instances.get(which);
    if (it == null) {
      // Lazy make.
      Multiton newIt = new Multiton(which);
      // Successful put?
      if (instances.compareAndSet(which, null, newIt)) {
        // Yes!
        it = newIt;
        // Initialise it.
        it.init();
      } else {
        // One appeared as if by magic (another thread got there first).
        it = instances.get(which);
        // Wait for it to finish initialisation.
        // Put me in its queue of waiters.
        it.waiters.add(Thread.currentThread());
        log(which, "Parking");
        while (!it.ready.get()) {
          // Park me.
          LockSupport.park();
        }
        // I'm not waiting any more.
        it.waiters.remove(Thread.currentThread());
        log(which, "Unparked");
      }
    }

    return it;
  }

  // Some simple logging.
  static void log(int which, String s) {
    log(new Date(), "Thread " + Thread.currentThread().getId() + " for Multiton " + which + " " + s);
  }
  static final DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
  // synchronized so I don't need to make the DateFormat ThreadLocal.

  static synchronized void log(Date d, String s) {
    System.out.println(dateFormat.format(d) + " " + s);
  }

  // The tester class.
  static class MultitonTester implements Runnable {
    int which;

    private MultitonTester(int which) {
      this.which = which;
    }

    @Override
    public void run() {
      try {
        Multiton.log(which, "Waiting");
        Multiton m = Multiton.getInstance(which);
        Multiton.log(which, "Got");
      } catch (InterruptedException ex) {
        Multiton.log(which, "Interrupted");
      }
    }
  }

  public static void main(String[] args) throws InterruptedException {
    int testers = 50;
    int multitons = 50;
    // Do a number of them. Makes n testers for each Multiton.
    for (int i = 1; i < testers * multitons; i++) {
      // Which one to create.
      int which = i / testers;
      //System.out.println("Requesting Multiton " + i);
      new Thread(new MultitonTester(which+1)).start();
    }

  }
}

Answer 3

我不是Java程序员，但是：HashMap对于并发访问是不安全的。我可能会推荐ConcurrentHashMap。

  private static final ConcurrentHashMap<Object, Multiton> instances = new ConcurrentHashMap<Object, Multiton>();

  public static <TYPE extends Object, KEY extends Enum<Keys> & MultitionKey<TYPE>> Multiton getInstance(KEY id)
  {
    Multiton result;
    synchronized (id)
    {
      result = instances.get(id);
      if(result == null)
      {
        result = new Multiton();
        instances.put(id, result);
      }
    }
    System.out.println("Retrieved instance.");
    return result;
  }

线程安全的多重模式

3 个答案: