Question

I am trying to implement AJAX Crawling for my Webpage.

To get a feeling I created a new GWT Project with sample code.

I created a filter

public final class CrawlServlet implements Filter {
    @Override
      public void doFilter(ServletRequest request, ServletResponse response,
          FilterChain chain) throws IOException {

    PrintWriter out = response.getWriter();

    if (request.toString().contains("_escaped_fragment=")) {
        out.write("yo");
    } else {
        try {
            chain.doFilter(request, response);
            return;
        } catch (ServletException e) {
            e.printStackTrace();
        }
    }
  }`

I get the correct response if browse this site: http://127.0.0.1:8888/URLFilter.html?gwt.codesvr=127.0.0.1:9997?_escaped_fragment=key=#!yo
and I see the GWT Sample Application if my URL is not escaped_fragment. But when I hit ‘Send’ (calling the RPC) I get a IllegalStatementException.

[WARN] Exception while dispatching incoming RPC call
java.lang.IllegalStateException: WRITER

at test.server.CrawlServlet.doFilter(CrawlServlet.java:32)

(this is the chain.doFilter(request, response);

My web.xml

 <!-- Servlets --> 

 <filter>
    <filter-name>crawlServlet</filter-name>
    <filter-class>test.server.CrawlServlet</filter-class>
  </filter>

  <filter-mapping>
    <filter-name>crawlServlet</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>


  <servlet>
    <servlet-name>greetServlet</servlet-name>
    <servlet-class>test.server.GreetingServiceImpl</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>greetServlet</servlet-name>
    <url-pattern>/urlfilter/greet</url-pattern>
  </servlet-mapping>

As i haven’t found much of tutorials/example regarding filter/rpc in GWT I gladly appreciate any help.

Thanks

Answer 1

PrintWriter out = response.getWriter();

Is your problem – you are getting a reference to the writer in the filter, then the servlet (or another filter) is using getOutputStream() to do its own work. From the javadocs for getWriter:

 * <p>Either this method or {@link #getOutputStream} may be called
 * to write the body, not both.

…

 * @exception IllegalStateException
 *          if the <code>getOutputStream</code>
 *          method has already been called for this 
 *          response object

The standard approach is to only use the writer/output stream if no other filter/servlet can possible do any writing – this includes even calling the getter. If you always want to write something, whether or not a later filter/servlet in the chain will also respond, then wrap the current response object up in something like a HttpServletResponseWrapper, possibly with a custom output stream or writer so that others can continue.