How to Capture and Log HTTP Servlet Response Output Stream Content in Java?

Capturing and Logging HTTP Servlet Response Output Stream Content

In Java webservers, you may encounter situations where you need to log both the incoming request parameters and the resulting server response. This article provides a solution to obtain the HTTP servlet response output stream content for logging purposes.

To achieve this, you can utilize a filter to wrap the HttpServletResponse object with a custom implementation called HttpServletResponseWrapper. Within this wrapper, we override the getOutputStream() and getWriter() methods to return a customized ServletOutputStream implementation, ServletOutputStreamCopier.

The ServletOutputStreamCopier wraps the original output stream and copies the written bytes to a ByteArrayOutputStream, accessible as byte[] through the getCopy() method.

Once the custom response wrapper is in place, we pass it to the FilterChain#doFilter() call. After the request-response cycle completes, we retrieve the copied response content from the ServletOutputStreamCopier for logging.

Here's an example implementation of such a filter:

public class ResponseLogger implements Filter {

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) {
        if (response.getCharacterEncoding() == null) {
            response.setCharacterEncoding("UTF-8");
        }

        HttpServletResponseCopier responseCopier = new HttpServletResponseCopier((HttpServletResponse) response);

        chain.doFilter(request, responseCopier);
        responseCopier.flushBuffer();
        byte[] copy = responseCopier.getCopy();
        System.out.println(new String(copy, response.getCharacterEncoding()));
    }
}

This filter implementation captures the response output as a byte array, allowing you to handle logging or any other necessary processing.

The code snippets for HttpServletResponseCopier and ServletOutputStreamCopier are also provided to complete the solution.

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