How to diagnose "org.restlet.data.Parameter cannot be cast to org.restlet.data.Header" error in Restlet 2.3.5?

Question

I'm using Restlet 2.3.5 in my application. When the GET request handler of a certain server resource is invoked, I get the following error:

[10:26:04] [Restlet-860541310/WARN]: Nov 29, 2015 10:26:04 AM org.restlet.engine.adapter.ServerAdapter addResponseHeaders
WARNING: Exception intercepted while adding the response headers
java.lang.ClassCastException: org.restlet.data.Parameter cannot be cast to org.restlet.data.Header
    at org.restlet.engine.header.HeaderUtils.addExtensionHeaders(HeaderUtils.java:226)
    at org.restlet.engine.header.HeaderUtils.addResponseHeaders(HeaderUtils.java:653)
    at org.restlet.engine.adapter.ServerAdapter.addResponseHeaders(ServerAdapter.java:83)
    at org.restlet.engine.adapter.ServerAdapter.commit(ServerAdapter.java:184)
    at org.restlet.engine.adapter.HttpServerHelper.handle(HttpServerHelper.java:144)
    at org.restlet.engine.connector.HttpServerHelper$1.handle(HttpServerHelper.java:64)
    at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:77)
    at sun.net.httpserver.AuthFilter.doFilter(AuthFilter.java:83)
    at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:80)
    at sun.net.httpserver.ServerImpl$Exchange$LinkHandler.handle(ServerImpl.java:677)
    at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:77)
    at sun.net.httpserver.ServerImpl$Exchange.run(ServerImpl.java:649)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:745)

The problem is that this exception is thrown outside of the code that I wrote (I added logging statements in my server resource and according to them, the exception is thrown somewhere else).

As a result, I get 500 response (internal server error), even though my server resource sends correct data back to the client.

How can I find out, what exactly is causing this error?

Answer 1

This could be caused when trying to add custom headers in Restlet. When initializing the corresponding map, you would use a map (Series in Restlet) of Parameter instead of Header...

You can use something like that:

Series<Header> responseHeaders = (Series<Header>) 
response.getAttributes().get(HeaderConstants.ATTRIBUTE_HEADERS); 
if (responseHeaders == null) { 
    responseHeaders = new Series(Header.class); 
    response.getAttributes().put(
            HeaderConstants.ATTRIBUTE_HEADERS, responseHeaders); 
}
responseHeaders.add(new Header("X-MyHeader", "value")); 

Hope it helps you, Thierry