If I understand you correctly, you want to log the response body? That’s a pretty expensive task, but if that’s the business requirement…
As @duffymo pointed, a Filter
is a suitable place for this. You can capture the response body by replacing the passed-in ServletResponse
with a HttpServletResponseWrapper
implementation which replaces the HttpServletResponse#getWriter()
with an own implementation which copies the response body into some buffer. After continuing the filter chain with the replaced response, just log the copy.
Here’s a kickoff example how the doFilter()
method can look like:
public void doFilter(ServletRequest request, final ServletResponse response, FilterChain chain) throws IOException, ServletException {
final CopyPrintWriter writer = new CopyPrintWriter(response.getWriter());
chain.doFilter(request, new HttpServletResponseWrapper((HttpServletResponse) response) {
@Override public PrintWriter getWriter() {
return writer;
}
});
logger.log(writer.getCopy());
}
Here’s how the CopyPrintWriter
can look like:
public class CopyPrintWriter extends PrintWriter {
private StringBuilder copy = new StringBuilder();
public CopyPrintWriter(Writer writer) {
super(writer);
}
@Override
public void write(int c) {
copy.append((char) c); // It is actually a char, not an int.
super.write(c);
}
@Override
public void write(char[] chars, int offset, int length) {
copy.append(chars, offset, length);
super.write(chars, offset, length);
}
@Override
public void write(String string, int offset, int length) {
copy.append(string, offset, length);
super.write(string, offset, length);
}
public String getCopy() {
return copy.toString();
}
}
Map this filter on an url-pattern
for which you’d like to log responses for. Keep in mind that binary/static content like images, CSS, JS files and so on won’t be logged this way. You’d like to exclude them by using a specific enough url-pattern
, e.g. *.jsp
or just on the servlet-name
of the servlet in question. If you want to log binary/static content anyway (for which I don’t see any benefit), then you need to replace the HttpServletResponse#getOutputStream()
the same way as well.