In Java, Servlet wrappers are objects that provide a way to modify the behavior of a Servlet or a request/response object. They work by "wrapping" or encapsulating the original object and adding new functionality to it.

Servlet wrappers are often used in web applications to implement additional security features or to modify the behavior of a request or response. Here are some common examples of Servlet wrappers:

1. HttpServletRequestWrapper: Wraps an existing HttpServletRequest object and provides methods for modifying the request parameters or headers.
2. HttpServletResponseWrapper: Wraps an existing HttpServletResponse object and provides methods for modifying the response headers or content.
3. FilterChainWrapper: Wraps an existing FilterChain object and provides methods for adding or removing filters from the chain.

To use a Servlet wrapper, you create a new class that extends one of the wrapper classes and overrides the methods that you want to modify. Here's an example of a simple HttpServletRequestWrapper that adds an additional parameter to the request:

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

public class MyHttpServletRequestWrapper extends HttpServletRequestWrapper {

    public MyHttpServletRequestWrapper(HttpServletRequest request) {
        super(request);
    }
    
    public String getParameter(String name) {
        String value = super.getParameter(name);
        if (name.equals("myParam")) {
            return "myValue";
        }
        return value;
    }
}

In this example, we create a new class MyHttpServletRequestWrapper that extends HttpServletRequestWrapper. We override the getParameter() method to add an additional parameter to the request if the parameter name is "myParam".

To use this wrapper, you need to replace the original HttpServletRequest object with an instance of your wrapper class. You can do this in a Servlet or a Filter by calling the getRequest() method on the ServletRequest object:

public void doGet(HttpServletRequest request, HttpServletResponse response) {
    HttpServletRequest wrappedRequest = new MyHttpServletRequestWrapper(request);
    // Use the wrapped request in your code
}

In this example, we create a new MyHttpServletRequestWrapper object and pass the original HttpServletRequest object to its constructor. We can then use the wrappedRequest object in our code instead of the original HttpServletRequest.

Servlet wrappers provide a powerful mechanism for modifying the behavior of Servlets and requests in a web application. By creating wrapper classes that add new functionality to the original objects, you can implement a wide range of features, such as additional security checks, request/response logging, or data transformation.