Camel - extend Java DSL?

Question

I've got a repeating pattern in my routes - a certain Processor needs the same 3 Headers set every time I call it, so I've got the following code in my routes about 10+ times:

.whatever()
.setHeader("foo1", "bar1")
.setHeader("foo2", "bar2")
.setHeader("foo3", "bar3")
.processRef("processorBazThatNeedsHeaders")
.whatever()

The headers are populated differently every time, so abstracting this out into a subroute doesn't really buy me anything.

What I love to be able to do is subclass RouteDefinition to have another method in my DSL that would allow me to do this:

.whatever()
.bazProcessor("bar1", "bar2", "bar3")
.whatever()

and in 'bazProcessor', set the headers and call the processor.

I've tried to do this but it seems that it's only possible with some serious probably-not-future-proof surgery, and it seems that others have had similar luck.

I need them to be set as headers as opposed to passing them as parameters directly to the processor because the values are also used after the processor for routing.

Is there some hidden facility to achieve something like this?

Answer 1

By subclassing the RouteDefinition your extension will only be visible direct after from(...). This could be a limitation if you would like to use the DSL extension for example after the filter(...) DSL.

A simpler approach would be to encapsulate the logic somewhere, and use it in a class that implements the org.apache.camel.Processor interface, and then call an overload of .process(...), or bean(...) in the route to use the logic. You will be actually very closed to a DSL extension if you use a meaningful name for the Processor instance or a method, that returns that Processor instance. Here is an example of the suggested approach. At the end, your code could look like:

.whatever()
.process(setTheHeadersForBaz)
.whatever()

Just for reference: if you really need to do a DSL, there is a project that extends the Camel DSL based on Groovy. I guess a Scala way based on the Camel Scala DSL could be also an option.

Answer 2

Though slightly irrelevant, following is an example of extending Scala DSL.

We can create an implicit methods to DSL trait via an implicit class.

object DSLImplicits {
  implicit class RichDSL(val dsl: DSL) {
    def get = dsl.setHeader(Exchange.HTTP_METHOD, _ => HttpMethods.GET.name)

    def post = dsl.setHeader(Exchange.HTTP_METHOD, _ => HttpMethods.POST.name)
  }
}

And use it like this.

import DSLImplicits.RichDSL
//----------------------------
from("someWhere")
  //Do some processing
  .get.to("http://somewhere.com")

More details @ http://siliconsenthil.in/blog/2013/07/11/apache-camel-with-scala-extending-dsl/