[[Predicate-Predicates]]
= Predicates

Camel supports a pluggable interface called
https://www.javadoc.io/doc/org.apache.camel/camel-api/current/org/apache/camel/Predicate.html[Predicate]
which can be used to integrate a dynamic predicate into
xref:{eip-vc}:eips:enterprise-integration-patterns.adoc[Enterprise Integration
Patterns] such as when using the xref:{eip-vc}:eips:filter-eip.adoc[Message Filter]
or xref:{eip-vc}:eips:content-based-router-eip.adoc[Content Based Router].

A Predicate is being evaluated to a boolean value so the result is
either `true` or `false`. This makes xref:predicate.adoc[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 xref:exchange.adoc[Exchange] based on a
header value:

[source,java]
----
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 xref:predicate.adoc[Predicate] is the
`header("type").isEqualTo("widget")` as its constructed as an
xref:expression.adoc[Expression] that is evaluated as a
xref:predicate.adoc[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 xref:predicate.adoc[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:

[source,java]
----
Predicate isWidget = header("type").isEqualTo("widget");
----

And then you can refer to it in the route as:

[source,java]
----
from("jms:queue:order")
   .choice()
      .when(isWidget).to("bean:widgetOrder")
      .when(isWombat).to("bean:wombatOrder")
   .otherwise()
      .to("bean:miscOrder")
   .end();
----

[[Predicate-NegatingaPredicate]]
== 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:

[source,java]
----
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:

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

[[Predicate-CompoundPredicates]]
== 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
https://www.javadoc.io/doc/org.apache.camel/camel-support/current/org/apache/camel/support/builder/PredicateBuilder.html[`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.:

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

The sample below demonstrates further use cases:

[source,java]
----
// 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();
----

[[Predicate-ExtensiblePredicates]]
== Extensible Predicates

Camel supports extensible Predicates using multiple
xref:languages.adoc[Languages]; the following languages are supported
out of the box

* xref:components:languages:bean-language.adoc[Bean Language] for using Java for expressions
* xref:components:languages:constant-language.adoc[Constant]
* xref:components:languages:header-language.adoc[Header]
* xref:components:languages:jsonpath-language.adoc[JSonPath]
* xref:components:languages:mvel-language.adoc[Mvel]
* xref:components:languages:ognl-language.adoc[OGNL]
* xref:components:languages:ref-language.adoc[Ref Language]
* xref:components:languages:exchangeProperty-language.adoc[ExchangeProperty]
** xref:components:languages:groovy-language.adoc[Groovy]
* xref:components:languages:simple-language.adoc[Simple]
** xref:components:languages:file-language.adoc[File Language]
* xref:components:languages:spel-language.adoc[Spring Expression Language]
* xref:components:languages:tokenize-language.adoc[Tokenizer]
* xref:components:languages:xpath-language.adoc[XPath]
* xref:components:languages:xquery-language.adoc[XQuery]

Most of these languages is also supported used as
xref:parameter-binding-annotations.adoc[Annotation Based
Expression Language].

You can easily write your own plugin predicate by implementing the
https://github.com/apache/camel/blob/master/core/camel-api/src/main/java/org/apache/camel/Predicate.java[Predicate
interface].

There are also a number of helper builders available such as the
https://github.com/apache/camel/blob/master/core/camel-core/src/main/java/org/apache/camel/builder/PredicateBuilder.java[PredicateBuilder
class]
