How to put breakpoint in class without having it's source?

Depending on your luck, you can do this with a decompiler. You'll have to place the breakpoint in the appropriate line (which, alas, might not contain "breakpointable" code)

The better way to do this is to create a ServletResponse wrapper. Incidentally, yesterday I created such a thing (with a slightly different purpose), so here it is:

public class DebugFilter implements Filter { public void init(FilterConfig filterConfig) {} public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { chain.doFilter(request, new ResponseWrapper((HttpServletResponse) response)); } public void destroy() {} } class ResponseWrapper extends HttpServletResponseWrapper { public ResponseWrapper(HttpServletResponse response) { super(response); } @Override public PrintWriter getWriter() throws IOException { return super.getWriter(); // breakpoint here } // Override whichever methods you like } 

I'm afraid you will need the source code if you want this to work.

For debugging, you need readable code + line numbers that match this code. None of these items are included in the class files