lock.java

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/***********************************************************************
 * Copyright (c) 2000-2004 The Apache Software Foundation.             *
 * All rights reserved.                                                *
 * ------------------------------------------------------------------- *
 * 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.james.util;

import java.util.Hashtable;

/**
 * Provides Lock functionality
 *
 */
public class Lock {
    /**
     * An internal hash table of keys to locks
     */
    private Hashtable locks = new Hashtable();

    /**
     * Check to see if the object is locked
     *
     * @param key the Object on which to check the lock
     * @return true if the object is locked, false otherwise
     */
    public boolean isLocked(final Object key) {
        return (locks.get(key) != null);
    }

    /**
     * Check to see if we can lock on a given object.
     *
     * @param key the Object on which to lock
     * @return true if the calling thread can lock, false otherwise
     */
    public boolean canI(final Object key) {
        Object o = locks.get( key );

        if (null == o || o == this.getCallerId()) {
            return true;
        }

        return false;
    }

    /**
     * Lock on a given object.
     *
     * @param key the Object on which to lock
     * @return true if the locking was successful, false otherwise
     */
    public boolean lock(final Object key) {
        Object theLock;

        synchronized(this) {
            theLock = locks.get(key);

            if (null == theLock) {
                locks.put(key, getCallerId());
                return true;
            } else if (getCallerId() == theLock) {
                return true;
            } else {
                return false;
            }
        }
    }

    /**
     * Release the lock on a given object.
     *
     * @param key the Object on which the lock is held
     * @return true if the unlocking was successful, false otherwise
     */
    public boolean unlock(final Object key) {
        Object theLock;
        synchronized (this) {
            theLock = locks.get(key);

            if (null == theLock) {
                return true;
            } else if (getCallerId() == theLock) {
                locks.remove(key);
                return true;
            } else {
                return false;
            }
        }
    }

    /**
     * Private helper method to abstract away caller ID.
     *
     * @return the id of the caller (i.e. the Thread reference)
     */
    private Object getCallerId() {
        return Thread.currentThread();
    }
}