Predicates
Camel supports a pluggable interface called Predicate![]()
which can be used to integrate a dynamic predicate into Enterprise Integration Patterns such as when using the Message Filter or Content Based Router.
A Predicate is being evaluated to a boolean value so the result is either true or false. This makes Predicate so powerful as it is often used to control the routing of message in which path they should be routed.
A simple example is to route an Exchange based on a header value:
from("jms:queue:order")
.choice()
.when().header("type").isEqualTo("widget").to("bean:widgetOrder")
.when().header("type").isEqualTo("wombat").to("bean:wombatOrder")
.otherwise()
.to("bean:miscOrder")
.end();
In the route above the Predicate is the header("type").isEqualTo("widget") as its constructed as an _expression_ that is evaluated as a Predicate. To do this the various Builder classes helps us here to create a nice and fluent syntax. isEqualTo is a builder method that returns a Predicate based on the input.
Sometimes the fluent builders can get long and a bit complex to read, then you can just define your predicate outside the route and then just refer to the predicate in the route:
Predicate isWidget = header("type").isEqualTo("widget")
And then you can refer to it in the route as:
from("jms:queue:order")
.choice()
.when(isWidget).to("bean:widgetOrder")
.when(isWombat).to("bean:wombatOrder")
.otherwise()
.to("bean:miscOrder")
.end();
Compound Predicates
You can also create compound predicates using boolean operators such as and, or, not and many others.
The sample below demonstrates this:
Predicate user = header("username").isNotNull();
Predicate admin = and(user, header("admin").isEqualTo("true"));
Predicate god = and(admin, or(body().contains("Camel Rider"), header("type").isEqualTo("god")));
from("direct:start").choice()
.when(god).to("mock:god")
.when(admin).to("mock:admin")
.when(user).to("mock:user")
.otherwise().to("mock:guest")
.end();
Extensible Predicates
Camel supports extensible Predicates using multiple Languages; the following languages are supported out of the box
Most of these languages is also supported used as Annotation Based _expression_ Language.
You can easily write your own plugin predicate by implementing the Predicate interface![]()
.
There are also a number of helper builders available such as the PredicateBuilder class![]()
Using Predicates in your IDE
To use different _expression_ and predicates in your IDE you need to perform a static import of the builder class for the language(s) you wish to use.
See Also
