java/org/apache/tomcat/dbcp/pool2/impl/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.tomcat.dbcp.pool2.impl;

import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;

/**
 * Provides a shared idle object eviction timer for all pools. This class is
 * currently implemented using {@link ScheduledThreadPoolExecutor}. This
 * implementation may change in any future release. This class keeps track of
 * how many pools are using it. If no pools are using the timer, it is cancelled.
 * 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>
 * 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>
 * This class is intended to be thread-safe.
 *
 * @since 2.0
 */
class EvictionTimer {

    /** Executor instance */
    private static ScheduledThreadPoolExecutor executor; //@GuardedBy("EvictionTimer.class")

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


    /**
     * @since 2.4.3
     */
    @Override
    public String toString() {
        final StringBuilder builder = new StringBuilder();
        builder.append("EvictionTimer []");
        return builder.toString();
    }


    /**
     * Add the specified eviction task to the timer. Tasks that are added with a
     * call to this method *must* call {@link #cancel(BaseGenericObjectPool.Evictor,long,TimeUnit)}
     * 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(
            final BaseGenericObjectPool<?>.Evictor task, final long delay, final long period) {
        if (null == executor) {
            executor = new ScheduledThreadPoolExecutor(1, new EvictorThreadFactory());
            executor.setRemoveOnCancelPolicy(true);
        }
        final ScheduledFuture<?> scheduledFuture =
                executor.scheduleWithFixedDelay(task, delay, period, TimeUnit.MILLISECONDS);
        task.setScheduledFuture(scheduledFuture);
    }

    /**
     * Remove the specified eviction task from the timer.
     *
     * @param task      Task to be cancelled
     * @param timeout   If the associated executor is no longer required, how
     *                  long should this thread wait for the executor to
     *                  terminate?
     * @param unit      The units for the specified timeout
     */
    static synchronized void cancel(
            final BaseGenericObjectPool<?>.Evictor task, final long timeout, final TimeUnit unit) {
        task.cancel();
        if (executor.getQueue().size() == 0) {
            executor.shutdown();
            try {
                executor.awaitTermination(timeout, unit);
            } catch (final InterruptedException e) {
                // Swallow
                // Significant API changes would be required to propagate this
            }
            executor.setCorePoolSize(0);
            executor = null;
        }
    }

    /**
     * Thread factory that creates a thread, with the context classloader from this class.
     */
    private static class EvictorThreadFactory implements ThreadFactory {

        @Override
        public Thread newThread(final Runnable r) {
            final Thread t = new Thread(null, r, "commons-pool-evictor-thread");

            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                @Override
                public Void run() {
                    t.setContextClassLoader(EvictorThreadFactory.class.getClassLoader());
                    return null;
                }
            });

            return t;
        }
    }
}