Servlet - Learn Java by Examples

How do I share object or data between users in web application?

By I Wayan Saryada in Servlet Last modified: May 16, 2023 0 Comment

In a web application there are different type of scope where we can store object or data. There are a page, request, session and application scope.

To share data between users of the web application we can put a shared object in application scope which can be done by calling setAttribute() method of the ServletContext. By this way data can then be accessing by other users by calling the getAttribute() method of the ServletContext.

Let’s see the example code in a simple servlet.

package org.kodejava.servlet;

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(name = "SharedObjectServlet", urlPatterns = "/shared-object")
public class ApplicationContextScopeAttribute extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        ServletContext context = this.getServletContext();
        context.setAttribute("HELLO.WORLD", "Hello World 123");
    }
}

And here is what we code in the JSP page to access it.

<%= request.getServletContext().getAttribute("HELLO.WORLD") %>

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

How do I get my web application real path?

By I Wayan Saryada in Servlet Last modified: May 16, 2023 3 Comments

This code helps you to get the physical path where your web application is deployed on the server. It may be useful, so you can for instance read or write files on the server. Please be aware that this method will only work when your web application is deployed in an exploded way, if it was deployed in a war format the getRealPath() method just return null.

package org.kodejava.servlet;

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;
import java.io.PrintWriter;

@WebServlet(name = "RealPathServlet", urlPatterns = "/real-path")
public class GetWebApplicationPathServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String path = getServletContext().getRealPath("/");
        PrintWriter writer = response.getWriter();
        writer.println("Application path: " + path);
    }
}

If you tried access the servlet you’ll get an output like:

Application path: F:\Wayan\Kodejava\kodejava-example\kodejava-servlet\target\kodejava-servlet\

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

How do I check if parameter is exists in servlet request?

By I Wayan Saryada in Servlet Last modified: May 16, 2023 0 Comment

ServletRequest or HttpServletRequest object has a map object that maps parameter name and its value. By accessing this map we can check if a parameter was passed in servlet request. Let’s see the example below.

package org.kodejava.servlet;

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(name = "ParameterCheck", urlPatterns = "/parameter-check")
public class ParameterCheck extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse res)
            throws ServletException, IOException {

        // Check if username parameter exists
        if (req.getParameterMap().containsKey("username")) {
            String username = req.getParameter("username");
            System.out.println("username = " + username);
        }

        // Check if password parameter exists
        if (req.getParameterMap().containsKey("password")) {
            String password = req.getParameter("password");
            System.out.println("password = " + password);
        }
    }
}

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

How do I get parameter names from servlet request?

By I Wayan Saryada in Servlet Last modified: May 16, 2023 0 Comment

This example show you how to obtain parameter name from servlet request. By calling request.getParameterNames() you will get an Enumeration object by iterating this object you can get the parameter names.

package org.kodejava.servlet;

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

@WebServlet(name = "ParameterName", urlPatterns = "/parameter-names")
public class ParameterName extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        PrintWriter pw = response.getWriter();

        // Let's obtains parameters name here!
        Enumeration<String> enumeration = request.getParameterNames();
        while (enumeration.hasMoreElements()) {
            String parameterName = enumeration.nextElement();
            pw.println("Parameter = " + parameterName);
        }
        pw.close();
    }
}

When you call the servlet and pass some parameters you get the parameter name echoed on the web browser.

http://localhost:8080/parameter-names?txid=001&item=10&price=1000

Parameter = txid
Parameter = item
Parameter = price

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

How do I read request parameter from servlet?

By I Wayan Saryada in Servlet Last modified: May 16, 2023 0 Comment

When creating an application with Java servlet most of the time we will work with the request and response object. From the request object we can read the parameter submitted by the user’s browser either through an HTTP GET or POST method.

Basically what you need to know is when you try to get the passed parameter from inside the servlet you can call the request.getParameter(paramName) where the paramName is the name of parameter that you want to read from the servlet request object.

In this example I’ll show you how to read the parameter to process user action in a very simple login servlet. In this example we’ll create a login form, a JSP page that accept user input for a username and password.

<%@ page contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %>
<html lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Login Page</title>
</head>
<body>
<form id="loginForm" action="${pageContext.request.contextPath}/login" method="post">
    <label for="username">Username</label>
    <input id="username" type="text" name="username"/>
    <label for="password">Password</label>
    <input id="password" type="password" name="password"/>
    <input type="submit" value="Login"/>
</form>
</body>
</html>

In this form you’ll have to input box for a username and password. You also have a submit button for executing the login process. Now we have the form, let’s create the login servlet.

package org.kodejava.servlet;

import javax.servlet.Servlet;
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 = "LoginServlet", urlPatterns = "/login")
public class LoginServlet extends HttpServlet implements Servlet {
    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response) throws IOException {
        doLogin(request, response);
    }

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

    protected void doLogin(HttpServletRequest request,
                           HttpServletResponse response) throws IOException {
        // Here we read the parameters from servlet request
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        PrintWriter pw = response.getWriter();
        if (username != null && username.equals("administrator")
                && password != null && password.equals("secret")) {
            // authentication accepted!
            pw.println("Success!");
        } else {
            // authentication denied!
            pw.println("Denied!");
        }
        pw.close();
    }
}

Now you have everything, you can deploy the application on your servlet container, for example Apache Tomcat. Access your login page in the following address:

http://localhost:8080/login.jsp

You can also access the servlet directly from the following url:

http://localhost:8080/login

To pass the username and password information you can append the parameter like:

http://localhost:8080/login?username=administrator&password=secret

This will call the servlet and validate your login information.

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>