---
title: Writing a Java Extension
slug: ps/help/en_us/vtm/jdev/writing-a-java-extension
docTags: 
createdAt: 2026-08-02T09:24:25.548Z
---

This chapter describes the function of Java classes and Servlet APIs, describes how a Java Extension is used with a Servlet API, how to create TrafficScript functions using Java Extensions, and how to write (compile) and debug Java Extensions.

## Java Classes and Servlet APIs

Java Extensions generate server responses, modify requests to backend servers, or alter responses from other servers.

To use the Servlet API, you must create a Java class that extends either the GenericServlet or one of its subclasses, such as the HttpServlet class.

Here’s an example of a simple HTTP Servlet:

```bash
import java.io.*;
import javax.servlet.ServletException;
import javax.servlet.http.*;

public class MyServlet extends HttpServlet
{
public void doGet( HttpServletRequest req, HttpServletResponse res )
throws ServletException, IOException
{
res.setContentType( "text/plain" );
PrintWriter out = res.getWriter();
out.println( "Hello World!" );
}
}
```

This is a standard Servlet that prints the phrase “Hello World!” whenever the Servlet is used.

The method doGet() is overridden from HttpServlet and is used whenever an HTTP GET request/response (depending if the Java Servlet is called in a TrafficScript response/request rule) is received. An identical function called doPost does the same for HTTP POST messages.

:::hint{type="info"}
By default, the doGet/doPost methods display the error message “HTTP method POST/GET is not supported by this URL”. To avoid this error, override the doGet/doPost methods.
:::