resourcepool.java

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/**
 *  
 *  Concurrency utilities (JSR-166) example
 **/
package semaphore;

import java.net.*;
import java.io.*;
import java.util.concurrent.*;

/**
 *  Use a Semaphore to control the usage of a pool of resources.  If a
 *  consumer requests a resource when all are being used the consumer 
 *  will block until aresource is returned and made available
 **/
public class ResourcePool {
  private int poolSize;
  private Semaphore available;
  private Integer[] resources;
  private boolean[] used;

  /**
   *  Constructor
   *
   * @param poolSize Size of fixed pool of resources
   **/
  public ResourcePool(int poolSize) {
    this.poolSize = poolSize;

    /*  Create a pool of resources (for this example just a set of Integer
     *  objects.  Create a new semaphore to control access to the resources
     */
    available = new Semaphore(poolSize);

    used = new boolean[poolSize];
    resources = new Integer[poolSize];

    for (int i = 0; i < poolSize; i++)
      resources[i] = new Integer(i);
  }

  /**
   *  Get a resource.  If all are currently being used this will block
   *  until one is returned to the pool.  This is a synchronised method 
   *  to make the code fully thread safe.
   *
   * @return The reource to use
   **/
  public Integer getResource() {
    try {
      available.acquire();
    } catch (InterruptedException ie) {
      // Ignore
    }

    for (int i = 0; i < poolSize; i++) {
      if (used[i] == false) {
        /*  If the method was not synchronised multiple threads could get to
         *  this point and think that they both had the same resource
         */
        used[i] = true;
        return resources[i];
      }
    }

    return null;
  }

  /**
   *  Return a resource to the pool
   *
   * @param resource The resource being returned to the pool
   **/
  public void putResource(Integer resource) {
    /*  Note use of auto-unboxing  */
    used[resource] = false;
    available.release();
  }
}