setBody() call in a Camel route specified in Java DSL appears to have no effect

Issue

Consider this piece of Java code, which is intended to create a Camel route that generates a new message body on each invocation:

public class MyRouteBuilder extends RouteBuilder {
    static int counter = 0;
    public void configure() throws Exception {
        from("direct:start")
                .setBody (simple ("Hello World, " + counter++))
                .to ("log://foo")
    }
}

The purpose of the static variable counter is just to keep a count of the number of invocations of the route, and provide a different message body on each one. In practice, the message body will be generated by some business logic.

It might be thought that each new invocation would result in a different message body, which in this example would follow a simple sequence like this:

Hello World 0
Hello World 1
Hello World 2
....

In practice, however, the message body is always Hello World 0. Subsequent invocations of the route do not change the body.