How do I obtain ServletContext of another application?

The ServletContext.getContext(String uripath) enable us to access servlet context of another web application deployed on the same application server. A configuration need to be added to enable this feature.

In the example below we will forward the request from the current application to the /otherapp/hello.jsp page. We place a string in the request object attribute of the current application and going to show it in the hello.jsp page.

package org.kodejava.example.servlet;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(urlPatterns = {"/context"})
public class GetAnotherContextServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        // Get ServletContext of another application on the same Servlet
        // container. This allow us to forward request to another application
        // on the same application server.
        ServletContext ctx = request.getServletContext().getContext("/otherapp");

        // Set a request attribute and forward to hello.jsp page on another 
        // context.
        request.setAttribute("MESSAGE", "Hello There!");
        RequestDispatcher dispatcher = ctx.getRequestDispatcher("/hello.jsp");
        dispatcher.forward(request, response);
    }
}

To enable this feature in Tomcat we need to enable the crossContext attribute by setting the value to true, the default value is false. Update the server.xml file to add the following configuration.

...
<Context path="/webapp" debug="0" reloadable="true" crossContext="true"/>
...

How do I define a servlet with @WebServlet annotation?

Annotations is one new feature introduces in the Servlet 3.0 Specification. Previously to declare servlets, listeners or filters we must do it in the web.xml file. Now, with the new annotations feature we can just annotate servlet classes using the @WebServlet annotation.

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

@WebServlet(
        name = "HelloAnnotationServlet",
        urlPatterns = {"/hello", "/helloanno"},
        asyncSupported = false,
        initParams = {
                @WebInitParam(name = "name", value = "admin"),
                @WebInitParam(name = "param1", value = "value1"),
                @WebInitParam(name = "param2", value = "value2")
        }
)
public class HelloAnnotationServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html");

        PrintWriter out = response.getWriter();
        out.write("<html><head><title>WebServlet Annotation</title></head>");
        out.write("<body>");
        out.write("<h1>Servlet Hello Annotation</h1>");
        out.write("<hr/>");
        out.write("Welcome " + getServletConfig().getInitParameter("name"));
        out.write("</body></html>");
        out.close();
    }
}

After you’ve deploy the servlet you’ll be able to access it either using the /hello or /helloanno url.

The table below give a brief information about the attributes accepted by the @WebServlet annotation and their purposes.

ATTRIBUTE DESCRIPTION
name The servlet name, this attribute is optional.
description The servlet description and it is an optional attribute.
displayName The servlet display name, this attribute is optional.
urlPatterns An array of url patterns use for accessing the servlet, this attribute is required and should at least register one url pattern.
asyncSupported Specifies whether the servlet supports asynchronous processing or not, the value can be true or false.
initParams An array of @WebInitParam, that can be used to pass servlet configuration parameters. This attribute is optional.
loadOnStartup An integer value that indicates servlet initialization order, this attribute is optional.
smallIcon A small icon image for the servlet, this attribute is optional.
largeIcon A large icon image for the servlet, this attribute is optional.

How do I create zip file in Servlet for download?

The example below is a servlet that shows you how to create a zip file and send the generated zip file for user to download. The compressing process is done by the zipFiles method of this class.

For a servlet to work you need to configure it in the web.xml file of your web application which can be found after the code snippet below.

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.util.zip.ZipEntry;
import java.util.zip.ZipOutputStream;

public class ZipDownloadServlet extends HttpServlet {
    public static final String FILE_SEPARATOR = System.getProperty("file.separator");

    protected void doPost(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        try {
            // The path below is the root directory of data to be
            // compressed.
            String path = getServletContext().getRealPath("data");

            File directory = new File(path);
            String[] files = directory.list();

            // Checks to see if the directory contains some files.
            if (files != null && files.length > 0) {

                // Call the zipFiles method for creating a zip stream.
                byte[] zip = zipFiles(directory, files);

                // Sends the response back to the user / browser. The
                // content for zip file type is "application/zip". We
                // also set the content disposition as attachment for
                // the browser to show a dialog that will let user 
                // choose what action will he do to the sent content.
                ServletOutputStream sos = response.getOutputStream();
                response.setContentType("application/zip");
                response.setHeader("Content-Disposition", "attachment; filename="DATA.ZIP"");

                sos.write(zip);
                sos.flush();
            }
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * Compress the given directory with all its files.
     */
    private byte[] zipFiles(File directory, String[] files) throws IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ZipOutputStream zos = new ZipOutputStream(baos);
        byte bytes[] = new byte[2048];

        for (String fileName : files) {
            FileInputStream fis = new FileInputStream(directory.getPath() + 
                ZipDownloadServlet.FILE_SEPARATOR + fileName);
            BufferedInputStream bis = new BufferedInputStream(fis);

            zos.putNextEntry(new ZipEntry(fileName));

            int bytesRead;
            while ((bytesRead = bis.read(bytes)) != -1) {
                zos.write(bytes, 0, bytesRead);
            }
            zos.closeEntry();
            bis.close();
            fis.close();
        }
        zos.flush();
        baos.flush();
        zos.close();
        baos.close();

        return baos.toByteArray();
    }
}

The web.xml configuration:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
         id="WebApp_ID" version="2.5">
    <servlet>
        <servlet-name>ZipDownloadServlet</servlet-name>
        <servlet-class>org.kodejava.example.servlet.ZipDownloadServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>ZipDownloadServlet</servlet-name>
        <url-pattern>/zipservlet</url-pattern>
    </servlet-mapping>
</web-app>

How do I get web application context path?

The context path always comes first in a request URI. The path starts with a “/” character but does not end with a “/” character. When I have a web application with the URL like http://localhost:8080/myapps then /myapps is the context path.

For servlets in the default (root) context, this method returns "" (empty string).

package org.kodejava.example.servlet;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import java.io.IOException;
import java.io.PrintWriter;

public class ContextPathDemo extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse res)
            throws ServletException, IOException {

        // HttpServletRequest.getContextPath() returns the portion 
        // of the request URI that indicates the context of the 
        // request.
        String contextPath = req.getContextPath();

        PrintWriter pw = res.getWriter();
        pw.print("Context Path: " + contextPath);
    }
}

Register your servlet in the web.xml file and map it to the ctxpath as the url-pattern. Let say you’ve deployed your servlet into a web application named webapp then you can access your servlet using the following url: http://localhost:8080/webapp/ctxpath.

You’ll get the following information in your browser:

Context Path: /webapp

How do I get servlet request URL information?

In the example below we extract information about the request object path information. We extract the protocol user, server and and its assigned port number. We extract our application context path, servlet path, path info and the query string information. If we combaine all the information below we’ll get someting equals to the request.getRequestURL().

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class ServletUrlInformation extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doPost(request, response);
    }

    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response)
            throws ServletException, IOException {
        // Getting servlet request URL
        String url = request.getRequestURL().toString();

        // Getting servlet request query string.
        String queryString = request.getQueryString();

        // Getting request information without the hostname.
        String uri = request.getRequestURI();

        // Below we extract information about the request object path
        // information.
        String scheme = request.getScheme();
        String serverName = request.getServerName();
        int portNumber = request.getServerPort();
        String contextPath = request.getContextPath();
        String servletPath = request.getServletPath();
        String pathInfo = request.getPathInfo();
        String query = request.getQueryString();

        response.setContentType("text/html");
        PrintWriter pw = response.getWriter();
        pw.print("Url: " + url + "<br/>");
        pw.print("Uri: " + uri + "<br/>");
        pw.print("Scheme: " + scheme + "<br/>");
        pw.print("Server Name: " + serverName + "<br/>");
        pw.print("Port: " + portNumber + "<br/>");
        pw.print("Context Path: " + contextPath + "<br/>");
        pw.print("Servlet Path: " + servletPath + "<br/>");
        pw.print("Path Info: " + pathInfo + "<br/>");
        pw.print("Query: " + query);
    }
}

Register the servlet in the web.xml file and define the url-pattern to urlinfo in the servlet-mapping. When you access this servlet using the following url http://localhost:8080/urlinfo?x=1&y=1, you’ll get the following output on your browser:

Url: http://localhost:8080/urlinfo
Uri: /urlinfo
Scheme: http
Server Name: localhost
Port: 8080
Context Path: 
Servlet Path: /urlinfo
Path Info: null
Query: x=1&y=1

How do I read servlet context initilization parameters?

package org.kodejava.example.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

import javax.servlet.Servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ContextInitParameter extends HttpServlet implements Servlet {

    public ContextInitParameter() {
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();

        // Get an instance of ServletContext
        ServletContext context = getServletContext();

        // To read context initialization parameter we can call context.getInitParameter() 
        // and pass the name of initialization parameter to be read. If the named
        // parameter does not exists the returned value will be null. 
        //
        // In this example we read an initialization parameter called LOG.PATH
        String logPath = context.getInitParameter("LOG.PATH");
        writer.println("Log Path: " + logPath + "<br/>");

        // Reads all the name of servlet's initialization parameters. If the
        // servlet doesn't have any an empty enumeration will be returned.
        Enumeration enumeration = context.getInitParameterNames();
        while (enumeration.hasMoreElements()) {
            String paramName = (String) enumeration.nextElement();
            String paramValue = context.getInitParameter(paramName);               

            writer.println("Context Init Param: [" + paramName + " = " + paramValue + "]<br/>");
        }
    }
}

How do I know session last access time?

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;

public class SessionLastAccessTime extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        HttpSession session = request.getSession();
        Date date = new Date(session.getLastAccessedTime());

        PrintWriter writer = response.getWriter();
        writer.println("Last accessed time: " + date);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doPost(request, response);
    }
}

This servlet will return a result like:

Last accessed time: Thu Jan 03 00:47:34 ICT 2008

How do I send a response status in servlet?

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class ResponseStatus extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doPost(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // All response status is defined in the HttpServletResponse class. We
        // can then use these constants value to return process status to the
        // browser.
        response.setContentType("text/html");

        // Let say this servlet only handle request for page name inputForm. So
        // when user request for other page name error page not found 404 will
        // be returned, other wise it will be 200 which mean OK.
        String page = request.getParameter("page");
        if (page != null && page.equals("inputForm")) {
            response.setStatus(HttpServletResponse.SC_OK);
        } else {
            response.sendError(HttpServletResponse.SC_NOT_FOUND, "The requested page ["
                    + page + "] not found.");
        }
    }
}

Here is a list of all available status code that are defined in the HttpServletResponse class.

STATUS CODE DESCRIPTION
SC_ACCEPTED Status code (202) indicating that a request was accepted for processing, but was not completed
SC_BAD_GATEWAY Status code (502) indicating that the HTTP server received an invalid response from a server it consulted when acting as a proxy or gateway
SC_BAD_REQUEST Status code (400) indicating the request sent by the client was syntactically incorrect
SC_CONFLICT Status code (409) indicating that the request could not be completed due to a conflict with the current state of the resource
SC_CONTINUE Status code (100) indicating the client can continue
SC_CREATED Status code (201) indicating the request succeeded and created a new resource on the server
SC_EXPECTATION_FAILED Status code (417) indicating that the server could not meet the expectation given in the Expect request header
SC_FORBIDDEN Status code (403) indicating the server understood the request but refused to fulfill it
SC_FOUND Status code (302) indicating that the resource reside temporarily under a different URI
SC_GATEWAY_TIMEOUT Status code (504) indicating that the server did not receive a timely response from the upstream server while acting as a gateway or proxy
SC_GONE Status code (410) indicating that the resource is no longer available at the server and no forwarding address is known
SC_HTTP_VERSION_NOT_SUPPORTED Status code (505) indicating that the server does not support or refuses to support the HTTP protocol version that was used in the request message
SC_INTERNAL_SERVER_ERROR Status code (500) indicating an error inside the HTTP server which prevented it from fulfilling the request
SC_LENGTH_REQUIRED Status code (411) indicating that the request cannot be handled without a defined Content-Length
SC_METHOD_NOT_ALLOWED Status code (405) indicating that the method specified in the Request-Line is not allowed for the resource identified by the Request-URI
SC_MOVED_PERMANENTLY Status code (301) indicating that the resource has permanently moved to a new location, and that future references should use a new URI with their requests
SC_MOVED_TEMPORARILY Status code (302) indicating that the resource has temporarily moved to another location, but that future references should still use the original URI to access the resource
SC_MULTIPLE_CHOICES Status code (300) indicating that the requested resource corresponds to any one of a set of representations, each with its own specific location
SC_NO_CONTENT Status code (204) indicating that the request succeeded but that there was no new information to return
SC_NON_AUTHORITATIVE_INFORMATION Status code (203) indicating that the meta information presented by the client did not originate from the server
SC_NOT_ACCEPTABLE Status code (406) indicating that the resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request
SC_NOT_FOUND Status code (404) indicating that the requested resource is not available
SC_NOT_IMPLEMENTED Status code (501) indicating the HTTP server does not support the functionality needed to fulfill the request
SC_NOT_MODIFIED Status code (304) indicating that a conditional GET operation found that the resource was available and not modified
SC_OK Status code (200) indicating the request succeeded normally
SC_PARTIAL_CONTENT Status code (206) indicating that the server has fulfilled the partial GET request for the resource
SC_PAYMENT_REQUIRED Status code (402) reserved for future use
SC_PRECONDITION_FAILED Status code (412) indicating that the precondition given in one or more of the request-header fields evaluated to false when it was tested on the server
SC_PROXY_AUTHENTICATION_REQUIRED Status code (407) indicating that the client MUST first authenticate itself with the proxy
SC_REQUEST_ENTITY_TOO_LARGE Status code (413) indicating that the server is refusing to process the request because the request entity is larger than the server is willing or able to process
SC_REQUEST_TIMEOUT Status code (408) indicating that the client did not produce a request within the time that the server was prepared to wait
SC_REQUEST_URI_TOO_LONG Status code (414) indicating that the server is refusing to service the request because the Request-URI is longer than the server is willing to interpret
SC_REQUESTED_RANGE_NOT_SATISFIABLE Status code (416) indicating that the server cannot serve the requested byte range
SC_RESET_CONTENT Status code (205) indicating that the agent SHOULD reset the document view which caused the request to be sent
SC_SEE_OTHER Status code (303) indicating that the response to the request can be found under a different URI
SC_SERVICE_UNAVAILABLE Status code (503) indicating that the HTTP server is temporarily overloaded, and unable to handle the request
SC_SWITCHING_PROTOCOLS Status code (101) indicating the server is switching protocols according to Upgrade header
SC_TEMPORARY_REDIRECT Status code (307) indicating that the requested resource resides temporarily under a different URI
SC_UNAUTHORIZED Status code (401) indicating that the request requires HTTP authentication
SC_UNSUPPORTED_MEDIA_TYPE Status code (415) indicating that the server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method
SC_USE_PROXY Status code (305) indicating that the requested resource MUST be accessed through the proxy given by the Location field

How do I read servlet init parameter?

When configuring a servlet in the web.xml file we can define some init parameters in the servlet configuration section. This init parameter can be used to define where a configuration file of our application is stored. This simple servlet below shows how to obtain these init parameters value.

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class InitParameterServlet extends javax.servlet.http.HttpServlet
        implements javax.servlet.Servlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        doPost(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        // Get application configuration path
        String configPath = getInitParameter("configPath");

        System.out.println("Configuration Path: " + configPath);
    }
}

How do I read cookie in Servlet?

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class ReadCookieExample extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();

        Cookie[] cookies = request.getCookies();
        for (int i = 0; i < cookies.length; i++) {
            writer.println("Name: " + cookies[i].getName() + "; Value: " + cookies[i].getValue());            
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

An example result of the servlet above is:

Name: JSESSIONID; Value: A1BF9D70A238CB3359946ABA8FC3212D