Apache Camel not returning http response after JSLT Transformation

Question

I am trying to get a http response out of the JSLT Transformer within Apache Camel.

Like in the given code sample

rest("/transform")
.post("/start")
.route()
.to("jslt:transform.jslt").transform().body

I want to return the body after the transformation.

But I always get an exception like

2022/03/23 14:51:09,842 [ERROR] [CamelHttpTransportServlet] - Error processing request
java.lang.IllegalStateException: getWriter() has already been called for this response

After adding a logger I can confirm that the jslt transformation into the body is working fine.

Plain responses like

rest("/say")
.get("/hello").route().transform().constant("Hello World");

work. I think it has to do with the implementation of the JSLT Transform.

Answer 1

It depends on your actual use case but assuming that you post a json payload to your endpoint then what you want to achieve can be done as next assuming that you use camel-undertow:

// configure rest-dsl
restConfiguration()
    // to use undertow component and run on port 8080
    .component("undertow").port(8080);
// The endpoint /transform/start calls the route `direct:jslt`
rest("/transform")
    .post("/start").consumes("application/json").to("direct:jslt");
from("direct:jslt")
    // Convert the payload to a String as only String or InputStream 
    // are supported by the component jslt
    .convertBodyTo(String.class)
    // Transform the body using the template transform.jslt available from 
    // the classpath
    .to("jslt:transform.jslt");

The result is directly set in the body of your message so it is what the rest endpoint will return back to the client in its body.