Lease.java

/*
 * Lease.java May 2004
 *
 * Copyright (C) 2004, Niall Gallagher <niallg@users.sf.net>
 *
 * 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.simpleframework.util.lease;

import java.util.concurrent.TimeUnit;

/**
 * The <code>Lease</code> object is used to keep a keyed resource active. This
 * provides a very simple lease that can be used to track the activity of a
 * resource or system. Keeping track of activity allows resources to be
 * maintained until such time that they are no longer required, allowing the
 * server to clean up any allocated memory, file descriptors, or other such
 * data.
 * 
 * @author Niall Gallagher
 */
public interface Lease<T> {

    /**
     * Determines the duration remaining before the lease expires. The expiry is
     * given as the number of <code>TimeUnit</code> seconds remaining before the
     * lease expires. If this value is negative it should be assumed that the
     * lease has expired.
     * 
     * @param unit
     *            this is the time unit used for the duration
     * 
     * @return the duration remaining within this lease instance
     * 
     * @exception Exception
     *                if the expiry could not be acquired
     */
    long getExpiry(TimeUnit unit) throws LeaseException;

    /**
     * This ensures that the leased resource is maintained for the specified
     * number of <code>TimeUnit</code> seconds. Allowing the duration unit to be
     * specified enables the lease system to maintain a resource with a high
     * degree of accuracy. The accuracy of the leasing system is dependent on
     * how long it takes to clean the resource associated with the lease.
     * 
     * @param duration
     *            this is the length of time to renew for
     * @param unit
     *            this is the time unit used for the duration
     * 
     * @exception Exception
     *                if the lease could not be renewed
     */
    void renew(long duration, TimeUnit unit) throws LeaseException;

    /**
     * This will cancel the lease and release the resource. This has the same
     * effect as the <code>renew</code> method with a zero length duration. Once
     * this has been called the <code>Cleaner</code> used should be notified
     * immediately. If the lease has already expired this throws an exception.
     * 
     * @exception Exception
     *                if the expiry has been passed
     */
    void cancel() throws LeaseException;

    /**
     * Provides the key for the resource that this lease represents. This can be
     * used to identify the resource should the need arise. Also, this provides
     * a convenient means of identifying leases when using or storing it as an
     * <code>Object</code>.
     * 
     * @return this returns the key for the resource represented
     */
    T getKey();

}