Java Servlet API 4.0.1 Source Code Files are important if you want to compile them with different JDK releases.

You can get Java Servlet API 4.0.1 Source Code Files from Servlet API Maven Website in the "4.0.1/" folder.

You can also browse Java Servlet API 4.0.1 Source Code files below:

⏎ javax/servlet/http/HttpFilter.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 *
 * Copyright (c) 1997-2017 Oracle and/or its affiliates. All rights reserved.
 *
 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  You
 * may not use this file except in compliance with the License.  You can
 * obtain a copy of the License at
 * https://oss.oracle.com/licenses/CDDL+GPL-1.1
 * or LICENSE.txt.  See the License for the specific
 * language governing permissions and limitations under the License.
 *
 * When distributing the software, include this License Header Notice in each
 * file and include the License file at LICENSE.txt.
 *
 * GPL Classpath Exception:
 * Oracle designates this particular file as subject to the "Classpath"
 * exception as provided by Oracle in the GPL Version 2 section of the License
 * file that accompanied this code.
 *
 * Modifications:
 * If applicable, add the following below the License Header, with the fields
 * enclosed by brackets [] replaced by your own identifying information:
 * "Portions Copyright [year] [name of copyright owner]"
 *
 * Contributor(s):
 * If you wish your version of this file to be governed by only the CDDL or
 * only the GPL Version 2, indicate your decision by adding "[Contributor]
 * elects to include this software in this distribution under the [CDDL or GPL
 * Version 2] license."  If you don't indicate a single choice of license, a
 * recipient has the option to distribute your version of this file under
 * either the CDDL, the GPL Version 2 or to extend the choice of license to
 * its licensees as provided above.  However, if you add GPL Version 2 code
 * and therefore, elected the GPL Version 2 license, then the option applies
 * only if the new code is made subject to such option by the copyright
 * holder.
 */

package javax.servlet.http;

import java.io.IOException;
import javax.servlet.FilterChain;
import javax.servlet.GenericFilter;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;


/**
 *
 * <p>Provides an abstract class to be subclassed to create
 * an HTTP filter suitable for a Web site. A subclass of
 * <code>HttpFilter</code> should override {@link #doFilter(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, javax.servlet.FilterChain) }.</p>
 *
 * <p>Filters typically run on multithreaded servers,
 * so be aware that a filter must handle concurrent
 * requests and be careful to synchronize access to shared resources.
 * Shared resources include in-memory data such as
 * instance or class variables and external objects
 * such as files, database connections, and network 
 * connections.
 * See the
 * <a href="https://docs.oracle.com/javase/tutorial/essential/concurrency/">
 * Java Tutorial on Multithreaded Programming</a> for more
 * information on handling multiple threads in a Java program.
 *
 * @author  Various
 *
 * @since Servlet 4.0
 */

public abstract class HttpFilter extends GenericFilter
{
    
    /**
     * <p>Does nothing, because this is an abstract class.</p>
     * 
     * @since 4.0
     */

    public HttpFilter() { }

    /**
     *
     * <p>The <code>doFilter</code> method of the Filter is called by the
     * container each time a request/response pair is passed through the
     * chain due to a client request for a resource at the end of the chain.
     * The FilterChain passed in to this method allows the Filter to pass
     * on the request and response to the next entity in the chain.  There's no need to
     * override this method.</p>
     * 
     * <p>The default implementation inspects the incoming {@code req} and {@code res}
     * objects to determine if they are instances of {@link HttpServletRequest}
     * and {@link HttpServletResponse}, respectively.  If not, a {@link ServletException} is thrown.
     * Otherwise, the protected {@link #doFilter(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, javax.servlet.FilterChain)}
     * method is called.</p>
     *
     * @param req   a {@link ServletRequest} object that
     *                  contains the request the client has made
     *                  of the filter
     *
     * @param res  a {@link ServletResponse} object that
     *                  contains the response the filter sends
     *                  to the client
     * 
     * @param chain     the <code>FilterChain</code> for invoking the next filter or the resource
     * 
     * @throws IOException   if an input or output error is 
     *                              detected when the filter handles
     *                              the request
     *
     * @throws ServletException  if the request for the could not be handled or 
     * either parameter is not an instance of the respective {@link HttpServletRequest}
     * or {@link HttpServletResponse}.
     *
     * @since Servlet 4.0
     */
    @Override
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
        if (!(req instanceof HttpServletRequest &&
                res instanceof HttpServletResponse)) {
            throw new ServletException("non-HTTP request or response");
        }

        this.doFilter((HttpServletRequest)req, (HttpServletResponse)res, chain);
    }
    
    /**
     *
     * <p>The <code>doFilter</code> method of the Filter is called by the
     * container each time a request/response pair is passed through the
     * chain due to a client request for a resource at the end of the chain.
     * The FilterChain passed in to this method allows the Filter to pass
     * on the request and response to the next entity in the chain.</p>
     * 
     * <p>The default implementation simply calls {@link FilterChain#doFilter}</p>
     *
     * @param req   a {@link HttpServletRequest} object that
     *                  contains the request the client has made
     *                  of the filter
     *
     * @param res  a {@link HttpServletResponse} object that
     *                  contains the response the filter sends
     *                  to the client
     * 
     * @param chain     the <code>FilterChain</code> for invoking the next filter or the resource
     * 
     * @throws IOException   if an input or output error is 
     *                              detected when the filter handles
     *                              the request
     *
     * @throws ServletException  if the request for the could not be handled
     *
     * @since Servlet 4.0
     */
    protected void doFilter(HttpServletRequest req, HttpServletResponse res, FilterChain chain) throws IOException, ServletException {
        chain.doFilter(req, res);
    }
    
}

⏎ javax/servlet/http/HttpFilter.java

Or download all of them as a single archive file:

File name: javax.servlet-api-4.0.1-sources.jar
File size: 203006 bytes
Release date: 2018-04-20
Download 

⇒ Download javax.servlet-api-3.0.1.jar

⇐ Download javax.servlet-api-4.0.1.jar

⇑ Downloading servlet-api.jar - Java Servlet API

⇑⇑ FAQ for Java Servlet API