How to Capture and Log HTTP Servlet Response Output Stream Content for Analysis?

Capturing and Logging HTTP Servlet Response Output Stream Content for Analysis

In the realm of web development, logging incoming and outgoing HTTP requests and responses is crucial for debugging and auditing purposes. To enhance your request logging capabilities, consider capturing the actual response content generated by your servlet.

To accomplish this, create a custom Filter that wraps the ServletResponse object. Override the getOutputStream() and getWriter() methods to return a custom ServletOutputStream implementation that duplicates written content.

The following code demonstrates the extended Filter:

@WebFilter("/*")
public class ResponseLogger implements Filter {

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {
        HttpServletResponseCopier responseCopier = new HttpServletResponseCopier((HttpServletResponse) response);

        try {
            chain.doFilter(request, responseCopier);
            responseCopier.flushBuffer();
        } finally {
            // Perform logging operations with the captured response content.
        }
    }
}

Within the custom HttpServletResponseWrapper, you'll need to override the getOutputStream() and getWriter() methods to return a custom ServletOutputStream implementation that captures the written data.

public class HttpServletResponseCopier extends HttpServletResponseWrapper {

    private ServletOutputStream outputStream;
    private ServletOutputStreamCopier copier;

    @Override
    public ServletOutputStream getOutputStream() throws IOException {
        if (outputStream == null) {
            outputStream = getResponse().getOutputStream();
            copier = new ServletOutputStreamCopier(outputStream);
        }

        return copier;
    }
}

Finally, the custom ServletOutputStream implementation provides the override for writing data:

public class ServletOutputStreamCopier extends ServletOutputStream {

    private ByteArrayOutputStream copy;

    @Override
    public void write(int b) throws IOException {
        super.write(b);
        copy.write(b);
    }
}

With this setup, you'll have access to the duplicated response content for analysis or logging, ensuring complete visibility into your server's HTTP interactions.

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