MultiResourceLock.java

/**
 * Licensed to The Apereo Foundation under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for additional
 * information regarding copyright ownership.
 *
 *
 * The Apereo Foundation licenses this file to you under the Educational
 * Community License, Version 2.0 (the "License"); you may not use this file
 * except in compliance with the License. You may obtain a copy of the License
 * at:
 *
 *   http://opensource.org/licenses/ecl2.txt
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
 * License for the specific language governing permissions and limitations under
 * the License.
 *
 */

package org.opencastproject.util;

import com.entwinemedia.fn.Fn;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicInteger;

import javax.annotation.ParametersAreNonnullByDefault;

/**
 * Synchronization utility to concurrently access a variable set of resources.
 */
@ParametersAreNonnullByDefault
public class MultiResourceLock {
  private final ConcurrentHashMap<Object, AtomicInteger> lockMap = new ConcurrentHashMap<>();

  public MultiResourceLock() {
  }

  /**
   * Synchronize access to a given resource.
   * Execute function <code>function</code> only, if currently now other function accesses resource <code>resource</code>.
   * <p>
   * Implementation note: The given resource is not used in any synchronization primitives, i.e. no monitor of
   * that object are being held.
   */
  public <A, K> A synchronize(final K resource, final Fn<K, A> function) {
    final AtomicInteger counter;
    synchronized (lockMap) {
      AtomicInteger newCounter = new AtomicInteger();
      AtomicInteger currentCounter = lockMap.putIfAbsent(resource, newCounter);
      counter = currentCounter != null ? currentCounter : newCounter;
      counter.incrementAndGet();
    }

    final A ap;
    synchronized (counter) {
      ap = function.apply(resource);
    }

    synchronized (lockMap) {
      if (counter.decrementAndGet() == 0)
        lockMap.remove(resource);
    }
    return ap;
  }

  /** For testing purposes only. */
  int getLockMapSize() {
    return lockMap.size();
  }
}