import java.util.concurrent.Semaphore;

/**

 *

 * JDK文档使用备注：<br>

 * Semaphores are often used to restrict the number of threads than

 * can access some (physical or logical) resource. For example, here is a class

 * that uses a semaphore to control access to a pool of items:

 *

 */

public class Pool {

    private static final int MAX_AVAILABLE = 100;

    private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);

    public Object getItem() throws InterruptedException {

        available.acquire();

        return getNextAvailableItem();

    }

    public void putItem(Object x) {

        if (markAsUnused(x))

            available.release();

    }

    // Not a particularly efficient data structure; just for demo

    protected Object[] items = new Object[MAX_AVAILABLE];

    protected boolean[] used = new boolean[MAX_AVAILABLE];

    protected synchronized Object getNextAvailableItem() {

        for (int i = 0; i < MAX_AVAILABLE; ++i) {

            if (!used[i]) {

                used[i] = true;

                return items[i];

            }

        }

        return null; // not reached

    }

    protected synchronized boolean markAsUnused(Object item) {

        for (int i = 0; i < MAX_AVAILABLE; ++i) {

            if (item == items[i]) {

                if (used[i]) {

                    used[i] = false;

                    return true;

                } else

                    return false;

            }

        }

        return false;

    }

}