001    /*
002     * Copyright 2012 UnboundID Corp.
003     *
004     * This program is free software; you can redistribute it and/or modify
005     * it under the terms of the GNU General Public License (GPLv2 only)
006     * or the terms of the GNU Lesser General Public License (LGPLv2.1 only)
007     * as published by the Free Software Foundation.
008     *
009     * This program is distributed in the hope that it will be useful,
010     * but WITHOUT ANY WARRANTY; without even the implied warranty of
011     * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
012     * GNU General Public License for more details.
013     *
014     * You should have received a copy of the GNU General Public License
015     * along with this program; if not, see <http://www.gnu.org/licenses>.
016     */
017    
018    package com.unboundid.scim.wink;
019    
020    
021    
022    import java.util.concurrent.Semaphore;
023    
024    
025    
026    /**
027     * A semaphore that can be dynamically resized.
028     */
029    public final class AdjustableSemaphore extends Semaphore
030    {
031      private static final long serialVersionUID = -2211345506986838529L;
032    
033    
034    
035      /**
036       * The maximum number of permits provided by this semaphore.
037       */
038      private int maxPermits;
039    
040    
041    
042      /**
043       * Create a new instance of this semaphore.
044       *
045       * @param maxPermits The initial number of permits.
046       */
047      public AdjustableSemaphore(final int maxPermits)
048      {
049        super(maxPermits);
050        this.maxPermits = maxPermits;
051      }
052    
053    
054    
055      /**
056       * Retrieves the maximum number of permits.
057       * @return  The maximum number of permits.
058       */
059      public int getMaxPermits()
060      {
061        return maxPermits;
062      }
063    
064    
065    
066      /**
067       * {@inheritDoc}
068       */
069      @Override
070      public int availablePermits()
071      {
072        // Do not return a negative number of permits. This could happen if
073        // there are threads waiting to acquire a permit and the maximum number
074        // of permits is reduced.
075        final int availablePermits = super.availablePermits();
076        return availablePermits >= 0 ? availablePermits : 0;
077      }
078    
079    
080    
081      /**
082       * Set the maximum number of permits.
083       *
084       * @param maxPermits The maximum number of permits. Must be greater than zero.
085       */
086      public synchronized void setMaxPermits(final int maxPermits)
087      {
088        if (maxPermits < 1)
089        {
090          throw new IllegalArgumentException();
091        }
092    
093        int delta = maxPermits - this.maxPermits;
094        if (delta == 0)
095        {
096          return;
097        }
098        else if (delta > 0)
099        {
100          release(delta);
101        }
102        else
103        {
104          reducePermits(-delta);
105        }
106    
107        this.maxPermits = maxPermits;
108      }
109    }