- Define HTTP request parameters
- Define URL structure and query string
- Write a servlet to get a request parameter from the query string
- Write a servlet to generate a dynamic response based on request parameter values
We will continue working with the GreetServlet from the previous lesson.
So far the servlet behaves like a static web page because it always generates the same HTML content as a response. We are going to update the servlet to receive data from the client request and generate a dynamic response using the data.
A query string is a part of a URL that assigns values to specified parameters.
A query string is the part of a URL that follows a question mark ?. For example:
- Given the URL:
http://localhost:8080/greet?name=Bola. - The query string is
name=Bola, which consists of:- a parameter
name. - a value
Bola.
- a parameter
A query string may contain multiple parameters that will be separated
with an ampersand &. For example: http://localhost:8080/area?width=6&height=2.
The parameters would be width and height, with values 6 and 2.
The HttpServletRequest interface provides methods for accessing request information.
HttpServletRequest extends the ServletRequst interface, which provides a method
getParameter(String parameter) that returns the value
of a parameter from the query string.
We can update the doGet() method to retrieve the name from the query string using the getParameter() method:
// Get the name parameter from the query string
String name = req.getParameter("name");We then update the response to include the name:
out.println(String.format("<h1>Greetings %s!</h1>", name));Edit GreetServlet to get the name parameter from the query string, and generate
the response to include the name in the greeting:
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(urlPatterns = "/greet")
public class GreetServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// Get the name parameter from the query string
String name = req.getParameter("name");
// Prepare the server response
resp.setContentType("text/html; charset=UTF-8");
PrintWriter out = resp.getWriter();
out.println("<html>");
out.println("<head><title>Servlet Example</title></head>");
out.println("<body>");
out.println(String.format("<h1>Greetings %s!</h1>", name));
out.println("</body>");
out.println("</html>");
}
}Save the class, then stop and restart the Tomcat9 plugin.
- Stop executing the web application by pressing the red stop icon on the Maven toolbar.
- Double-click on tomcat9:run to restart the web application.
Test the servlet by passing different names through the query string:
| URL | Server Response |
|---|---|
| http://localhost:8080/greet?name=Dani | Greetings Dani! |
| http://localhost:8080/greet?name=Nur | Greetings Nur! |
Congratulations, you just wrote a dynamic web application that generates a unique response based on the client data passed in the request!
We will create another servlet class named AreaServlet that will
take two parameters containing integer values for width and height.
The servlet will calculate the area based on the parameter values and
generate a response.
- Create a new class in the
javadirectory namedAreaServlet. - Edit the class to extend
HttpServlet, define the@WebServleturl pattern, and override thedoGet()method as shown:
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(urlPatterns = "/area")
public class AreaServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// Get the width parameter from the query string, convert to integer
Integer width = Integer.parseInt(req.getParameter("width"));
Integer height = Integer.parseInt(req.getParameter("height"));
// Calculate the area using the request data
int area = width * height;
// Prepare the server response
resp.setContentType("text/html; charset=UTF-8");
PrintWriter out = resp.getWriter();
out.println("<html>");
out.println("<head><title>Servlet Example</title></head>");
out.println("<body>");
out.println(String.format("<h1>Area = %d</h1>", area));
out.println("</body>");
out.println("</html>");
}
}Note that the getParameter method returns a string.
Therefore, we need to use Integer.parseInt() to convert the string to an integer.
// Get the width parameter from the query string, convert to integer
Integer width = Integer.parseInt(req.getParameter("width"));
Integer height = Integer.parseInt(req.getParameter("height"));We will need to pass two parameters to the query string. Multiple parameters
are separated with an ampersand &, for example http://localhost:8080/area?width=5&height=2.
Make sure you save the AreaServlet class, then stop and restart the Tomcat9 plugin.
- Stop executing the web application by pressing the red stop icon on the Maven toolbar.
- Double-click on tomcat9:run to restart the web application.
Test the servlet by passing different width and height values through the query string. Note that you can pass the parameters in any order:
| URL | Server Response |
|---|---|
| http://localhost:8080/area?width=5&height=2 | Area = 10 |
| http://localhost:8080/area?height=7&width=4 | Area = 28 |
The HttpServletRequest provides methods for accessing parameters of a request.
The type of the request determines where the parameters come from. Typically, a GET
request takes the parameters from the query string.
- Given the URL:
http://localhost:8080/greet?name=Bola. - The query string is the part after the question mark:
name=Bola. - We can get the
nameparameter with the method callreq.getParameter("name").
Our servlet can then generate a dynamic response to include the parameter value.