EvictionTimer.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.commons.pool.impl;

import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Timer;
import java.util.TimerTask;

/**
 * <p>
 * Provides a shared idle object eviction timer for all pools. This class wraps
 * the standard {@link Timer} and keeps track of how many pools are using it.
 * If no pools are using the timer, it is canceled. This prevents a thread
 * being left running which, in application server environments, can lead to
 * memory leads and/or prevent applications from shutting down or reloading
 * cleanly.
 * </p>
 * <p>
 * This class has package scope to prevent its inclusion in the pool public API.
 * The class declaration below should *not* be changed to public.
 * </p>
 */
class EvictionTimer {
    
    /** Timer instance */
    private static Timer _timer; //@GuardedBy("this")
    
    /** Static usage count tracker */
    private static int _usageCount; //@GuardedBy("this")

    /** Prevent instantiation */
    private EvictionTimer() {
        // Hide the default constuctor
    }

    /**
     * Add the specified eviction task to the timer. Tasks that are added with a
     * call to this method *must* call {@link #cancel(TimerTask)} to cancel the
     * task to prevent memory and/or thread leaks in application server
     * environments.
     * @param task      Task to be scheduled
     * @param delay     Delay in milliseconds before task is executed
     * @param period    Time in milliseconds between executions
     */
    static synchronized void schedule(TimerTask task, long delay, long period) {
        if (null == _timer) {
            // Force the new Timer thread to be created with a context class
            // loader set to the class loader that loaded this library
            ClassLoader ccl = AccessController.doPrivileged(
                    new PrivilegedGetTccl());
            try {
                AccessController.doPrivileged(new PrivilegedSetTccl(
                        EvictionTimer.class.getClassLoader()));
                _timer = new Timer(true);
            } finally {
                AccessController.doPrivileged(new PrivilegedSetTccl(ccl));
            }
        }
        _usageCount++;
        _timer.schedule(task, delay, period);
    }

    /**
     * Remove the specified eviction task from the timer.
     * @param task      Task to be scheduled
     */
    static synchronized void cancel(TimerTask task) {
        task.cancel();
        _usageCount--;
        if (_usageCount == 0) {
            _timer.cancel();
            _timer = null;
        }
    }
    
    /** 
     * {@link PrivilegedAction} used to get the ContextClassLoader
     */
    private static class PrivilegedGetTccl implements PrivilegedAction<ClassLoader> {

        /** 
         * {@inheritDoc}
         */
        public ClassLoader run() {
            return Thread.currentThread().getContextClassLoader();
        }
    }

    /** 
     * {@link PrivilegedAction} used to set the ContextClassLoader
     */
    private static class PrivilegedSetTccl implements PrivilegedAction<ClassLoader> {

        /** ClassLoader */
        private final ClassLoader cl;

        /**
         * Create a new PrivilegedSetTccl using the given classloader
         * @param cl ClassLoader to use
         */
        PrivilegedSetTccl(ClassLoader cl) {
            this.cl = cl;
        }

        /** 
         * {@inheritDoc}
         */
        public ClassLoader run() {
            Thread.currentThread().setContextClassLoader(cl);
            return null;
        }
    }

}