Getting ready

The Java code for this recipe is located in the org.camelcookbook.routing.routingslip package. The Spring XML files are located under src/main/resources/META-INF/spring and are prefixed with routingSlip.

How to do it...

Create a route with a routingSlip statement. The Expression within that statement must return a String or list of Strings, where each String identifies an endpoint URI to route the message to in sequence.

In these examples, the Routing Slip uses the contents of the message header myRoutingSlipHeader, which could either have been set with the original message or set by the previous route steps:

In the XML DSL, this logic is written as:

<route>
  <from uri="direct:start"/>
  <routingSlip>
    <header>myRoutingSlipHeader</header>
  </routingSlip>
</route>

In the Java DSL, the same thing is expressed as:

from("direct:start")
  .routingSlip(header("myRoutingSlipHeader"));

How it works...

The Routing Slip uses a Camel Expression to determine the list of endpoints to route to. This Expression is evaluated only once, and the message is routed in sequence to one or more endpoints specified in that list.

If the intent of the Expression is to return a list of endpoint URIs that will be called in sequence, the Expression may separate the URIs with a delimiter which by default is a comma (,). The separator can be changed using the uriDelimiter attribute of the routingSlip element.

If you find that you are trying to route to an endpoint that does not currently exist, and do not want this condition to be considered an error, you can configure the ignoreInvalidEndpoints option to true (defaults to false) to tell Camel to ignore the ResolveEndpointFailedException that would normally be thrown.

There's more...

We can use Camel's Bean Injection capabilities to put everything into the Java class using Java Annotations. The @RoutingSlip annotation provides the same information to Camel as the routingSlip element within a route. The @Consume annotation tells Camel to create a new route consuming on the provided endpoint URI no need to provide a separate route definition.

@Consume(uri = "direct:routingSlipAnnotated")
@RoutingSlip(delimiter = ",")
public List<String> routeMe(String body, 
    @Headers Map<String, Object> headers) {
  //...
}

This pattern can also be considered a variation on the Recipient List EIP. In the Recipient List, an Expression is also used to determine a list of endpoint URIs; however, in that pattern each endpoint receives its own copy of the original message.

See also