I've created a filter to in my java webserver (appengine actually) that logs the parameters of an incoming request. I'd also like to log the resulting response that my webserver writes. Although I have access to the response object, I'm not sure how to get the actual string/content response out of it.
Any ideas?
You need to create a Filter
wherein you wrap the ServletResponse
argument with a custom HttpServletResponseWrapper
implementation wherein you override the getOutputStream()
and getWriter()
to return a custom ServletOutputStream
implementation wherein you copy the written byte(s) in the base abstract OutputStream#write(int b)
method. Then, you pass the wrapped custom HttpServletResponseWrapper
to the FilterChain#doFilter()
call instead and finally you should be able to get the copied response after the the call.
In other words, the Filter
:
@WebFilter("/*")
public class ResponseLogger implements Filter {
@Override
public void init(FilterConfig config) throws ServletException {
// NOOP.
}
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {
if (response.getCharacterEncoding() == null) {
response.setCharacterEncoding("UTF-8"); // Or whatever default. UTF-8 is good for World Domination.
}
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())); // Do your logging job here. This is just a basic example.
}
}
@Override
public void destroy() {
// NOOP.
}
}
The custom HttpServletResponseWrapper
:
public class HttpServletResponseCopier extends HttpServletResponseWrapper {
private ServletOutputStream outputStream;
private PrintWriter writer;
private ServletOutputStreamCopier copier;
public HttpServletResponseCopier(HttpServletResponse response) throws IOException {
super(response);
}
@Override
public ServletOutputStream getOutputStream() throws IOException {
if (writer != null) {
throw new IllegalStateException("getWriter() has already been called on this response.");
}
if (outputStream == null) {
outputStream = getResponse().getOutputStream();
copier = new ServletOutputStreamCopier(outputStream);
}
return copier;
}
@Override
public PrintWriter getWriter() throws IOException {
if (outputStream != null) {
throw new IllegalStateException("getOutputStream() has already been called on this response.");
}
if (writer == null) {
copier = new ServletOutputStreamCopier(getResponse().getOutputStream());
writer = new PrintWriter(new OutputStreamWriter(copier, getResponse().getCharacterEncoding()), true);
}
return writer;
}
@Override
public void flushBuffer() throws IOException {
if (writer != null) {
writer.flush();
} else if (outputStream != null) {
copier.flush();
}
}
public byte[] getCopy() {
if (copier != null) {
return copier.getCopy();
} else {
return new byte[0];
}
}
}
The custom ServletOutputStream
:
public class ServletOutputStreamCopier extends ServletOutputStream {
private OutputStream outputStream;
private ByteArrayOutputStream copy;
public ServletOutputStreamCopier(OutputStream outputStream) {
this.outputStream = outputStream;
this.copy = new ByteArrayOutputStream(1024);
}
@Override
public void write(int b) throws IOException {
outputStream.write(b);
copy.write(b);
}
public byte[] getCopy() {
return copy.toByteArray();
}
}