Tomcat example source code file (SemaphoreValve.java)

This example Tomcat source code file (SemaphoreValve.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Tomcat tags/keywords

interruptedexception, io, ioexception, ioexception, lifecycle, lifecycleexception, lifecycleexception, lifecyclesupport, response, response, semaphore, servlet, servletexception, servletexception, string, threading, threads, valvebase

The Tomcat SemaphoreValve.java source code

/*
 * 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.catalina.valves;


import java.io.IOException;
import java.util.concurrent.Semaphore;

import javax.servlet.ServletException;

import org.apache.catalina.Lifecycle;
import org.apache.catalina.LifecycleException;
import org.apache.catalina.LifecycleListener;
import org.apache.catalina.connector.Request;
import org.apache.catalina.connector.Response;
import org.apache.catalina.util.LifecycleSupport;
import org.apache.catalina.util.StringManager;


/**
 * <p>Implementation of a Valve that limits concurrency.

 *
 * <p>This Valve may be attached to any Container, depending on the granularity
 * of the concurrency control you wish to perform.</p>
 *
 * @author Remy Maucherat
 * @version $Revision: 467222 $ $Date: 2006-10-24 05:17:11 +0200 (mar., 24 oct. 2006) $
 */

public class SemaphoreValve
    extends ValveBase
    implements Lifecycle {


    // ----------------------------------------------------- Instance Variables


    /**
     * The descriptive information related to this implementation.
     */
    private static final String info =
        "org.apache.catalina.valves.SemaphoreValve/1.0";


    /**
     * The string manager for this package.
     */
    private StringManager sm =
        StringManager.getManager(Constants.Package);


    /**
     * Semaphore.
     */
    protected Semaphore semaphore = null;
    

    /**
     * The lifecycle event support for this component.
     */
    protected LifecycleSupport lifecycle = new LifecycleSupport(this);


    /**
     * Has this component been started yet?
     */
    private boolean started = false;


    // ------------------------------------------------------------- Properties

    
    /**
     * Concurrency level of the semaphore.
     */
    protected int concurrency = 10;
    public int getConcurrency() { return concurrency; }
    public void setConcurrency(int concurrency) { this.concurrency = concurrency; }
    

    /**
     * Fairness of the semaphore.
     */
    protected boolean fairness = false;
    public boolean getFairness() { return fairness; }
    public void setFairness(boolean fairness) { this.fairness = fairness; }
    

    /**
     * Block until a permit is available.
     */
    protected boolean block = true;
    public boolean getBlock() { return block; }
    public void setBlock(boolean block) { this.block = block; }
    

    /**
     * Block interruptibly until a permit is available.
     */
    protected boolean interruptible = false;
    public boolean getInterruptible() { return interruptible; }
    public void setInterruptible(boolean interruptible) { this.interruptible = interruptible; }
    

    // ------------------------------------------------------ Lifecycle Methods


    /**
     * Add a lifecycle event listener to this component.
     *
     * @param listener The listener to add
     */
    public void addLifecycleListener(LifecycleListener listener) {

        lifecycle.addLifecycleListener(listener);

    }


    /**
     * Get the lifecycle listeners associated with this lifecycle. If this
     * Lifecycle has no listeners registered, a zero-length array is returned.
     */
    public LifecycleListener[] findLifecycleListeners() {

        return lifecycle.findLifecycleListeners();

    }


    /**
     * Remove a lifecycle event listener from this component.
     *
     * @param listener The listener to add
     */
    public void removeLifecycleListener(LifecycleListener listener) {

        lifecycle.removeLifecycleListener(listener);

    }


    /**
     * Prepare for the beginning of active use of the public methods of this
     * component.  This method should be called after <code>configure(),
     * and before any of the public methods of the component are utilized.
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    public void start() throws LifecycleException {

        // Validate and update our current component state
        if (started)
            throw new LifecycleException
                (sm.getString("semaphoreValve.alreadyStarted"));
        lifecycle.fireLifecycleEvent(START_EVENT, null);
        started = true;

        semaphore = new Semaphore(concurrency, fairness);

    }


    /**
     * Gracefully terminate the active use of the public methods of this
     * component.  This method should be the last one called on a given
     * instance of this component.
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that needs to be reported
     */
    public void stop() throws LifecycleException {

        // Validate and update our current component state
        if (!started)
            throw new LifecycleException
                (sm.getString("semaphoreValve.notStarted"));
        lifecycle.fireLifecycleEvent(STOP_EVENT, null);
        started = false;

        semaphore = null;

    }

    
    // --------------------------------------------------------- Public Methods


    /**
     * Return descriptive information about this Valve implementation.
     */
    public String getInfo() {
        return (info);
    }


    /**
     * Do concurrency control on the request using the semaphore.
     *
     * @param request The servlet request to be processed
     * @param response The servlet response to be created
     *
     * @exception IOException if an input/output error occurs
     * @exception ServletException if a servlet error occurs
     */
    public void invoke(Request request, Response response)
        throws IOException, ServletException {

        if (controlConcurrency(request, response)) {
            boolean shouldRelease = true;
            try {
                if (block) {
                    if (interruptible) {
                        try {
                            semaphore.acquire();
                        } catch (InterruptedException e) {
                            shouldRelease = false;
                            permitDenied(request, response);
                            return;
                        }  
                    } else {
                        semaphore.acquireUninterruptibly();
                    }
                } else {
                    if (!semaphore.tryAcquire()) {
                        shouldRelease = false;
                        permitDenied(request, response);
                        return;
                    }
                }
                getNext().invoke(request, response);
            } finally {
                if (shouldRelease) {
                    semaphore.release();
                }
            }
        } else {
            getNext().invoke(request, response);
        }

    }

    
    /**
     * Subclass friendly method to add conditions.
     */
    public boolean controlConcurrency(Request request, Response response) {
        return true;
    }
    

    /**
     * Subclass friendly method to add error handling when a permit isn't granted.
     */
    public void permitDenied(Request request, Response response)
        throws IOException, ServletException {
    }
    

}