java - 使用Multiton模式的副作用

我需要关于我的multiton模式实现的建议，代码审查或改进。我想为mongodb服务器提供多连接支持。

public class MongoDatabaseFactory {
    private static volatile Map<String, MongoDatabase> connections = new ConcurrentHashMap<String, MongoDatabase>();

    public static MongoDatabase getDatabase(Databases database) throws MongoException {
        if (null == database) throw new MongoException("Database not found");
        if (null == database.name() || database.name().isEmpty()) throw new MongoException("Database not found");

        if (!connections.containsKey(database.name()) || null == connections.get(database.name())) {
            synchronized (database) {
                if (!connections.containsKey(database.name()) || null == connections.get(database.name())) {
                    connectDB(database);
                }
            }
        }

        if (!connections.get(database.name()).isAuthenticated()) {
            synchronized (database) {
                if (!connections.get(database.name()).isAuthenticated()) {
                    connectDB(database);
                }
            }
        }

        return connections.get(database.name());
    }
}

Multiton模式的最佳实践是什么？

最佳答案

正如Marko Topolnik所说，您当前的解决方案不是线程安全的。

我将其作为一个小练习，并编写了以下通用的线程安全Multition模式。它是否设计为在许多线程中都能很好地执行，并且适用于价值对象创建成本很高的情况。请注意，但是我不确定您的具体情况下没有更简单的解决方案。

import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;


public class ThreadSafeMultition <K, V> {
  private final ConcurrentHashMap<K, FutureTask<V>> map = new ConcurrentHashMap<K, FutureTask<V>>();
  private ValueFactory<K, V> factory;

  public ThreadSafeMultition(ValueFactory<K, V> factory) {
    this.factory = factory;
  }

  public V get(K key) throws InterruptedException, ExecutionException {
    FutureTask<V> f = map.get(key);
    if (f == null) {
      f = new FutureTask<V>(new FactoryCall(key));
      FutureTask<V> existing = map.putIfAbsent(key, f);
      if (existing != null)
        f = existing;
      else // Item added successfully. Now that exclusiveness is guaranteed, start value creation.
        f.run();
    }

    return f.get();
  }

  public static interface ValueFactory<K, V> {
    public V create(K key) throws Exception;
  }

  private class FactoryCall implements Callable<V> {
    private K key;

    public FactoryCall(K key) {
      this.key = key;
    }

    @Override
    public V call() throws Exception {
      return factory.create(key);
    }
  }
}

关于java - 使用Multiton模式的副作用，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/22374769/