In Java Servlets, URL rewriting is a technique used to append session information to URLs. This is typically done in order to implement session tracking in web applications. When a web application uses URL rewriting, a session ID is appended to each URL and this session ID is used to identify the user's session when the request is sent back to the server.

Here's an example of how to use URL rewriting in a Servlet:

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class MyServlet extends HttpServlet {
    
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Get the current session or create a new one
        HttpSession session = request.getSession(true);
        
        // Set a session attribute
        session.setAttribute("attributeName", "attributeValue");
        
        // Create a URL with the session ID appended
        String url = response.encodeURL("page.jsp");
        
        // Redirect the client to the new URL
        response.sendRedirect(url);
    }
}

In this example, we get the current session or create a new one using the getSession() method. We then set a session attribute using the setAttribute() method. We create a URL with the session ID appended to it using the encodeURL() method and redirect the client to the new URL using the sendRedirect() method.

Here's an example of how to retrieve session information from a URL in a Servlet:

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class MyServlet extends HttpServlet {
    
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Get the session ID from the URL
        String sessionID = request.getRequestedSessionId();
        
        // Get the session associated with the session ID
        HttpSession session = request.getSession(false);
        
        // Get a session attribute
        String attributeValue = (String) session.getAttribute("attributeName");
        
        // Send the attribute value to the client
        response.getWriter().println("Attribute value: " + attributeValue);
    }
}

In this example, we retrieve the session ID from the URL using the getRequestedSessionId() method. We get the session associated with the session ID using the getSession() method with the false parameter to indicate that we do not want to create a new session if one does not exist. We then retrieve a session attribute using the getAttribute() method and send the attribute value to the client using the getWriter() method.

This is an example of how to use URL rewriting in a Servlet to implement session tracking in a web application.