geode-core/src/main/java/org/apache/geode/distributed/internal/ProcessorKeeper21.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for additional information regarding
 * copyright ownership. The ASF licenses this file to You under the Apache 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://www.apache.org/licenses/LICENSE-2.0
 *
 * 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.apache.geode.distributed.internal;

import java.lang.ref.WeakReference;
import java.util.concurrent.atomic.AtomicInteger;

import org.apache.geode.internal.Assert;
import org.apache.geode.internal.ObjIdConcurrentMap;

/**
 * A message processor class typically creates an instance of ProcessorKeeper and holds it in a
 * static variable. In the constructor of the processor class, do this:
 *
 * <code>processorId = keeper.put()</code>
 *
 * When a message is processed, do this:
 *
 * <code>processor = <processorclass>.keeper().get(processorId);
 *
 * <p>
 * Processor ids are always greater than 0.
 * </p>
 */
public class ProcessorKeeper21 {

  /**
   * Key is a unique id, value is an instance of some processor class
   */
  private final ObjIdConcurrentMap<Object> map = new ObjIdConcurrentMap<Object>();

  /**
   * If true then use weak refs to reference the processors.
   */
  private final boolean useWeakRefs;

  private final AtomicInteger nextKey = new AtomicInteger(1);

  public ProcessorKeeper21() {
    this(true);
  }

  public ProcessorKeeper21(boolean useWeakRefs) {
    this.useWeakRefs = useWeakRefs;
  }

  private int getNextId() {
    int id = this.nextKey.getAndIncrement();
    if (id <= 0) {
      // id must be >= 0 since ObjIdMap does not supports keys < 0.
      // We don't use 0 just to keep it reserved as an illegal id.
      synchronized (this.nextKey) {
        id = this.nextKey.get();
        if (id <= 0) {
          this.nextKey.set(1);
        }
      }
      id = this.nextKey.getAndIncrement();
    }
    return id;
  }

  /**
   * Save the processor in this keeper, generate an id for the processor, and return that id so it
   * can be used to retrieve the processor later. This keeper keeps a weak reference to the
   * processor.
   *
   * @param processor the processor to keep
   * @return the unique id for processor
   */
  public int put(Object processor) {
    int id;
    final Object obj;
    if (this.useWeakRefs) {
      obj = new WeakReference<Object>(processor);
    } else {
      obj = processor;
    }
    do {
      id = getNextId();
    } while (this.map.putIfAbsent(id, obj) != null);
    Assert.assertTrue(id > 0);
    return id;
  }

  /**
   * Retrieve a processor that was previously put() in this keeper. The id is the value returned
   * from put(). If there is no processor by that id, or it has been garbage collected, null is
   * returned.
   */
  public Object retrieve(int id) {
    Object o = null;
    if (this.useWeakRefs) {
      final WeakReference<?> ref = (WeakReference<?>) this.map.get(id);
      if (ref != null) {
        o = ref.get();
        if (o == null) {
          // Clean up
          this.map.remove(id, ref);
        }
      }
    } else {
      o = this.map.get(id);
    }
    // System.out.println("ProcessorKeeper.retrieve(" + int + ") returning " + processor);
    return o;
  }

  /**
   * Remove the processor with the given id. It's okay if no processor with that id exists.
   */
  public void remove(int id) {
    map.remove(id);
  }

}