OnJava8-Examples/concurrency/Pool.java

// concurrency/Pool.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
// Using a Semaphore inside a Pool, to restrict
// the number of tasks that can use a resource.
import java.util.concurrent.*;
import java.util.*;

public class Pool<T> {
  private int size;
  private List<T> items = new ArrayList<>();
  private volatile boolean[] checkedOut;
  private Semaphore available;
  public Pool(Class<T> classObject, int size) {
    this.size = size;
    checkedOut = new boolean[size];
    available = new Semaphore(size, true);
    // Load pool with objects that can be checked out:
    for(int i = 0; i < size; ++i)
      try {
        // Assumes a no-arg constructor:
        items.add(classObject.newInstance());
      } catch(InstantiationException |
              IllegalAccessException e) {
        throw new RuntimeException(e);
      }
  }
  public T checkOut() throws InterruptedException {
    available.acquire();
    return getItem();
  }
  public void checkIn(T x) {
    if(releaseItem(x))
      available.release();
  }
  private synchronized T getItem() {
    for(int i = 0; i < size; ++i)
      if(!checkedOut[i]) {
        checkedOut[i] = true;
        return items.get(i);
      }
    return null; // Semaphore prevents reaching here
  }
  private synchronized boolean releaseItem(T item) {
    int index = items.indexOf(item);
    if(index == -1) return false; // Not in the list
    if(checkedOut[index]) {
      checkedOut[index] = false;
      return true;
    }
    return false; // Wasn't checked out
  }
}