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();

Negating a Predicate

You can use the not method on the PredicateBuilder to negate a predicate.

First we import the not static, so it makes our route nice and easy to read:

import static org.apache.camel.builder.PredicateBuilder.not

And then we can use it to enclose an existing predicate and negate it as the example shows:

from("direct:start")
    .choice()
        .when(not(header("username").regex("goofy|pluto"))).to("mock:people")
        .otherwise().to("mock:animals")
    .end();

Compound Predicates

You can also create compound predicates using boolean operators such as and, or, not and many others.

Currently this feature is only available in the Java-based DSLs, but not in the Spring nor Blueprint DSLs.

Using the PredicateBuilder class, you can combine predicates from different Expression Languages based on logical operators and comparison operators:

  • not, and, or

  • isNull, isNotNull

  • isEqualTo, isGreaterThan, isLessThan

  • startsWith, endsWith

  • in ("any of X predicates stands true")

Additionally, with PredicateBuilder you can create Regular Expressions and use them as predicates, applying them to the result of an expression, e.g. PredicateBuilder.regex(header("foo"), "\d{4}") applies the regular expression to the header = foo.

Combining different Expression Languages is also possible, e.g.:

PredicateBuilder.and(XPathBuilder.xpath("/bookings/flights"), simple("${property.country = 'Spain'}"))

The sample below demonstrates further use cases:

// We define 3 predicates based on some user roles
// we have static imported and/or from org.apache.camel.builder.PredicateBuilder

// First we have a regular user that is just identified having a username header
Predicate user = header("username").isNotNull();

// The admin user must be a user AND have a admin header as true
Predicate admin = and(user, header("admin").isEqualTo("true"));

// And God must be an admin and (either have type god or a special message containing Camel Rider)
Predicate god = and(admin, or(body().contains("Camel Rider"), header("type").isEqualTo("god")));

// As you can see with the predicates above we can stack them to build compound predicates

// In our route below we can create a nice content based router based on the predicates we
// have defined. Then the route is easy to read and understand.
// We encourage you to define complex predicates outside the fluent router builder as
// it will just get a bit complex for humans to read
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

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