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

How to Capture and Log HTTP Servlet Response Content

Introduction

Logging both request and response content is essential for comprehensive server-side debugging and monitoring. While accessing the request parameters through a filter is straightforward, retrieving the response content can be more challenging. This article explains how to create a filter that intercepts and copies the servlet response for logging purposes.

Creating a Filter to Capture Response Content

To capture the response content, we need to create a custom Filter that wraps the HttpServletResponse object and overrides the getOutputStream() and getWriter() methods. These methods are responsible for providing streams to write response data. By overriding them, we can create a custom stream that intercepts the data and copies it into a buffer.

Here's an example of a filter that accomplishes this task:

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

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

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

Customizing the Response Wrapper

The HttpServletResponseCopier class wraps the original HttpServletResponse and creates a custom ServletOutputStream that intercepts the data. The overridden getOutputStream() and getWriter() methods create and return the ServletOutputStreamCopier instance.

public class HttpServletResponseCopier extends HttpServletResponseWrapper {

    private ServletOutputStreamCopier copier;

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

    @Override
    public PrintWriter getWriter() {
        if (copier == null) {
            copier = new ServletOutputStreamCopier(getResponse().getOutputStream());
        }
        return new PrintWriter(new OutputStreamWriter(copier, getResponse().getCharacterEncoding()), true);
    }
}

Intercepting and Copying Response Data

The ServletOutputStreamCopier class overrides the write(int b) method to intercept the data written to the response. It writes the data to the original stream while also copying it into a ByteArrayOutputStream for later retrieval.

public class ServletOutputStreamCopier extends ServletOutputStream {

    private ByteArrayOutputStream copy;

    @Override
    public void write(int b) {
        outputStream.write(b);
        copy.write(b);
    }

    public byte[] getCopy() {
        return copy.toByteArray();
    }
}

Retrieving the Captured Response

After the request-response cycle has completed, the filter retrieves the captured response content from the ServletOutputStreamCopier's getCopy() method. This data can then be used for logging or any other desired purpose.

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