Simple ESB with Camel and Groovy

I'm not sure if writing an ESB service can be considered fun - but when using Groovy and Camel together it comes pretty close. Camel provedes a nice Java based DSL which makes the routing very clean and easy to follow. When you mix in the power of Groovy you get a very elegant solution.

With Groovy you can say a closure implements an interface. This allows us to embed Groovy code inline as a Processor in a Camel route.

    from("jetty:http://localhost:8080/hello")
    .process({Exchange exchange ->
      exchange.out.setBody "Hello World"
    } as Processor)

We can go one step futher by taking advantage of Groovy's dynamic metaClass to create our own method which takes a closure directly. The following adds a new method 'x' to the RouteDefinition class.

RouteDefinition.metaClass.x = {Closure c -> ((RouteDefinition)delegate).process(c as Processor)}

Our route definition now becomes much more elegant

    from("jetty:http://localhost:8080/hello")
    .x {Exchange ex -> ex.out.setBody "Hello"}
    .x {Exchange ex -> ex.out.body = ex.in.body + " World"}

Add in some Grab annotations from Groovy's Grape extension to automatically download the Camel libraries and the whole thing becomes a single, small, self-contained script.

#!/usr/bin/env groovy

@Grab(group="org.apache.camel", module="camel-core", version="2.5.0")
@Grab(group="org.apache.camel", module="camel-jetty", version="2.5.0")
import org.apache.camel.CamelContext
import org.apache.camel.impl.DefaultCamelContext
import org.apache.camel.builder.RouteBuilder
import org.apache.camel.Processor
import org.apache.camel.Exchange
import org.apache.camel.model.RouteDefinition

RouteDefinition.metaClass.x = {Closure c -> ((RouteDefinition)delegate).process(c as Processor)}

CamelContext cxt = new DefaultCamelContext()

cxt.addRoutes(new RouteBuilder(){
  void configure(){
    from("jetty:http://localhost:8080/hello")
    .x {Exchange ex -> ex.out.setBody "Hello"}
    .x {Exchange ex -> ex.out.body = ex.in.body + " World"}
  }
})

cxt.start()