Servlet chaining in Java is a technique where one servlet forwards a request to another servlet for processing, and the second servlet can forward the request to a third servlet, and so on. This allows for the creation of modular and reusable web applications, where each servlet performs a specific task and forwards the request to the next servlet in the chain for further processing.

To implement servlet chaining in Java, a servlet can use the RequestDispatcher interface to forward a request to another servlet. The target servlet can then forward the request to the next servlet in the chain, and so on. This can be done by passing the request and response objects to the next servlet's RequestDispatcher object.

For example, consider a simple scenario where there are three servlets, each of which performs a specific task. The first servlet handles user authentication, the second servlet processes the user's request, and the third servlet generates a response. The first servlet can forward the request to the second servlet, which can then forward the request to the third servlet for generating a response.

The following code snippet shows how the first servlet can forward a request to the second servlet:

RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/SecondServlet");
dispatcher.forward(request, response);

In this example, the getRequestDispatcher() method of the ServletContext object is used to obtain a RequestDispatcher object for the second servlet, which has a URL pattern of "/SecondServlet". The forward() method is then called on the dispatcher object, passing in the original request and response objects.

Similarly, the second servlet can forward the request to the third servlet by obtaining a RequestDispatcher object for the third servlet and calling its forward() method.

In summary, servlet chaining in Java is a powerful technique for building modular and reusable web applications. It allows for the separation of concerns between different servlets, each of which performs a specific task, and enables the easy reuse of servlets in different parts of a web application.