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>