🧠 What Are Servlet Wrapper Classes?

Wrapper classes are special decorators that wrap around the original HttpServletRequest, HttpServletResponse, or ServletInputStream/ServletOutputStream.

They allow you to:

• Intercept or modify behavior
• Read, change, or log request/response data
• Implement security, compression, caching, etc.

In short: wrapper = original object + extra behavior (without modifying the original object itself).

🎯 Main Servlet Wrapper Classes (from Java EE / Jakarta EE)

Wrapper Class	Wraps	Purpose
HttpServletRequestWrapper	HttpServletRequest	Modify or read the incoming request
HttpServletResponseWrapper	HttpServletResponse	Modify or decorate the outgoing response
ServletRequestWrapper	ServletRequest (more generic)	Modify a basic servlet request
ServletResponseWrapper	ServletResponse (more generic)	Modify a basic servlet response
ServletInputStream (custom wrapper)	Servlet input stream	If you want to intercept raw request body
ServletOutputStream (custom wrapper)	Servlet output stream	If you want to capture or compress response

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🔥 Let’s See Quick Examples:

1. HttpServletRequestWrapper

Used when you want to override some methods from HttpServletRequest.

✅ Example:
Suppose you want to add a fake header to all requests.

public class MyRequestWrapper extends HttpServletRequestWrapper {
    public MyRequestWrapper(HttpServletRequest request) {
        super(request);
    }

    @Override
    public String getHeader(String name) {
        if ("X-Fake-Header".equals(name)) {
            return "FakeHeaderValue";
        }
        return super.getHeader(name);
    }
}

Then inside a Filter:

MyRequestWrapper wrappedRequest = new MyRequestWrapper((HttpServletRequest) request);
chain.doFilter(wrappedRequest, response);

2. HttpServletResponseWrapper

Used when you want to capture or modify the outgoing response.

✅ Example:
Suppose you want to capture response content.

public class MyResponseWrapper extends HttpServletResponseWrapper {
    private StringWriter capture;

    public MyResponseWrapper(HttpServletResponse response) {
        super(response);
        capture = new StringWriter();
    }

    @Override
    public PrintWriter getWriter() {
        return new PrintWriter(capture);
    }

    public String getCapturedData() {
        return capture.toString();
    }
}

You can then analyze the response later in your filter.

3. ServletInputStream / ServletOutputStream Wrappers

These are lower-level and you use them if you want to:

• Read raw POST body multiple times (normally it can only be read once).
• Compress output (like GZIP).
• Encrypt/decrypt stream data.

You would usually extend ServletInputStream or ServletOutputStream and override methods like read(), write(), etc.

🚀 Why Are Wrappers Useful?

• Security: e.g., sanitize inputs (XSS protection).
• Logging: log request and response bodies.
• Compression: wrap response with GZIPOutputStream.
• Modification: inject or change data on the fly.
• Caching: cache results of slow operations.

You often use wrappers inside a Filter, because filters are perfect for request/response interception.

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⚡ Quick Summary:

You Need	Use This Wrapper
Modify incoming request	HttpServletRequestWrapper
Modify outgoing response	HttpServletResponseWrapper
Modify raw input stream	Custom ServletInputStream subclass
Modify raw output stream	Custom ServletOutputStream subclass
Modify generic (non-HTTP) requests/responses	ServletRequestWrapper / ServletResponseWrapper