Java DSL

Apache Camel offers a Java based DSL.

In the Java DSL you create a route by extending the RouteBuilder class, and implementing the configure method.

Java DSL example

This is best illustrate by an example. In the code below we create a new class called MyRouteBuilder that extends the org.apache.camel.builder.RouteBuilder from Camel.

In the configure method the Java DSL is at our disposal.

import org.apache.camel.builder.RouteBuilder;

/**
 * A Camel Java DSL Router
 */
public class MyRouteBuilder extends RouteBuilder {

    /**
     * Let's configure the Camel routing rules using Java code...
     */
    public void configure() {

        // here is a sample which processes the input files
        // (leaving them in place - see the 'noop' flag)
        // then performs content based routing on the message using XPath
        from("file:src/data?noop=true")
            .choice()
                .when(xpath("/person/city = 'London'"))
                    .to("file:target/messages/uk")
                .otherwise()
                    .to("file:target/messages/others");
    }

}

In the configure method we can define Camel Routes.

In the example above we have a single route, which pickup files (the from).

from("file:src/data?noop=true")

Then we use the Content Based Router EIP (the choice) to route the message whether the person is from London or not.

.choice()
    .when(xpath("/person/city = 'London'"))
        .to("file:target/messages/uk")
    .otherwise()
        .to("file:target/messages/others");

Routes using Java lambda style

Camel now supports to define Camel routes in Java DSL using Lambda style. This can be beneficial for microservices or serverless where you may want to quickly define a few routes.

For example using lambda style you can define a Camel route that takes messages from Kafka and send to JMS in a single line of code:

rb -> rb.from("kafka:cheese").to("jms:queue:foo");

There is a bit more to this as the lambda route must be coded in a Java method that returns an instance of LambdaRouteBuilder. See more at the LambdaRouteBuilder documentation.

More Details

For more details see DSL, Routes, and Processor.

The Java DSL under the hood

As mentioned in the Getting Started guide, you can use Camel’s Java DSL in a way that almost looks like a DSL. For instance:

Note: comments afterwards explain some of the constructs used in the example.

Example of Camel’s "Java DSL"

RouteBuilder builder = new RouteBuilder() {
    public void configure() {
        from("queue:a").filter(header("foo").isEqualTo("bar")).to("queue:b");

        from("queue:c").choice()
                .when(header("foo").isEqualTo("bar")).to("queue:d")
                .when(header("foo").isEqualTo("cheese")).to("queue:e")
                .otherwise().to("queue:f");
    }
};
CamelContext myCamelContext = new DefaultCamelContext();
myCamelContext.addRoutes(builder);

The first line in the above example creates an object which is an instance of an anonymous subclass of RouteBuilder with the specified configure() method.

The CamelContext.addRoutes(RouterBuilder builder) method invokes builder.setContext(this) – so the RouteBuilder object knows which CamelContext object it is associated with – and then invokes builder.configure(). The body of configure() invokes methods such as from(), filter(), choice(), when(), isEqualTo(), otherwise() and to().

The RouteBuilder.from(String uri) method invokes getEndpoint(uri) on the CamelContext associated with the RouteBuilder object to get the specified Endpoint and then puts a FromBuilder wrapper around this Endpoint. The FromBuilder.filter(Predicate predicate) method creates a FilterProcessor object for the Predicate (that is, condition) object built from the header("foo").isEqualTo("bar") expression. In this way, these operations incrementally build up a Route object (with a RouteBuilder wrapper around it) and add it to the CamelContext instance associated with the RouteBuilder.

More Information

See Lambda Route Builder for creating a routing rule using the DSL, using Java lambda style.