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The Apache Commons Codec package contains simple encoder and decoders for various formats such as Base64 and Hexadecimal. In addition to these widely used encoders and decoders, the codec package also maintains a collection of phonetic encoding utilities.

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/*   Copyright 2004 The Apache Software Foundation
 *
 *   Licensed 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.xmlbeans.impl.common;

public class Mutex
{
    private Thread  owner      = null;  // Owner of mutex, null if nobody
    private int     lock_count = 0;

    /**
     * Acquire the mutex. The mutex can be acquired multiple times
     * by the same thread, provided that it is released as many
     * times as it is acquired. The calling thread blocks until
     * it has acquired the mutex. (There is no timeout).
     */
    public synchronized void acquire() throws InterruptedException
    {
        while (tryToAcquire() == false)
        {
            wait();
        }
    }

    /**
     * Attempts to acquire the mutex. Returns false (and does not
     * block) if it can't get it.
     */

    public synchronized boolean tryToAcquire()
    {
        // Try to get the mutex. Return true if you got it.

        if( owner == null )
        {
            owner = Thread.currentThread();
            lock_count = 1;
            return true;
        }

        if( owner == Thread.currentThread() )
        {
            ++lock_count;
            return true;
        }

        return false;
    }

    /**
     * Release the mutex. The mutex has to be released as many times
     * as it was acquired to actually unlock the resource. The mutex
     * must be released by the thread that acquired it
     *
     * @throws IllegalStateException (a RuntimeException) if a thread
     *      other than the current owner tries to release the mutex.
     */

    public synchronized void release()
    {
        if (owner != Thread.currentThread())
            throw new IllegalStateException("Thread calling release() doesn't own mutex");

        if (--lock_count <= 0)
        {
            owner = null;
            notify();
        }
    }
}