Chapter 13. Introduction

Abstract

This chapter provides an overview of all the expression languages supported by Apache Camel.

13.1. Overview of the Languages

Table of expression and predicate languages

Table 13.1, “Expression and Predicate Languages” gives an overview of the different syntaxes for invoking expression and predicate languages.

Table 13.1. Expression and Predicate Languages

LanguageStatic MethodFluent DSL MethodXML ElementAnnotationArtifact

See Bean Integration in the Apache Camel Development Guide on the customer portal.

bean()

EIP().method()

method

@Bean

Camel core

Chapter 14, Constant

constant()

EIP().constant()

constant

@Constant

Camel core

Chapter 15, EL

el()

EIP().el()

el

@EL

camel-juel

Chapter 17, Groovy

groovy()

EIP().groovy()

groovy

@Groovy

camel-groovy

Chapter 18, Header

header()

EIP().header()

header

@Header

Camel core

Chapter 19, JavaScript

javaScript()

EIP().javaScript()

javaScript

@JavaScript

camel-script

Chapter 20, JoSQL

sql()

EIP().sql()

sql

@SQL

camel-josql

Chapter 21, JsonPath

None

EIP().jsonpath()

jsonpath

@JsonPath

camel-jsonpath

Chapter 22, JXPath

None

EIP().jxpath()

jxpath

@JXPath

camel-jxpath

Chapter 23, MVEL

mvel()

EIP().mvel()

mvel

@MVEL

camel-mvel

Chapter 24, The Object-Graph Navigation Language(OGNL)

ognl()

EIP().ognl()

ognl

@OGNL

camel-ognl

Chapter 25, PHP

php()

EIP().php()

php

@PHP

camel-script

Chapter 26, Exchange Property

property()

EIP().property()

property

@Property

Camel core

Chapter 27, Python

python()

EIP().python()

python

@Python

camel-script

Chapter 28, Ref

ref()

EIP().ref()

ref

N/A

Camel core

Chapter 29, Ruby

ruby()

EIP().ruby()

ruby

@Ruby

camel-script

Chapter 30, The Simple Language/Chapter 16, The File Language

simple()

EIP().simple()

simple

@Simple

Camel core

Chapter 31, SpEL

spel()

EIP().spel()

spel

@SpEL

camel-spring

Chapter 32, The XPath Language

xpath()

EIP().xpath()

xpath

@XPath

Camel core

Chapter 33, XQuery

xquery()

EIP().xquery()

xquery

@XQuery

camel-saxon

13.2. How to Invoke an Expression Language

Prerequisites

Before you can use a particular expression language, you must ensure that the required JAR files are available on the classpath. If the language you want to use is not included in the Apache Camel core, you must add the relevant JARs to your classpath.

If you are using the Maven build system, you can modify the build-time classpath simply by adding the relevant dependency to your POM file. For example, if you want to use the Ruby language, add the following dependency to your POM file:

<dependency>
  <groupId>org.apache.camel</groupId>
  <artifactId>camel-groovy</artifactId>
  <!-- Use the same version as your Camel core version -->
  <version>${camel.version}</version>
</dependency>

If you are going to deploy your application in a Red Hat JBoss Fuse OSGi container, you also need to ensure that the relevant language features are installed (features are named after the corresponding Maven artifact). For example, to use the Groovy language in the OSGi container, you must first install the camel-groovy feature by entering the following OSGi console command:

karaf@root> features:install camel-groovy
Note

If you are using an expression or predicate in the routes, refer the value as an external resource by using resource:classpath:path or resource:file:path. For example, resource:classpath:com/foo/myscript.groovy.

Camel on EAP deployment

The camel-groovy component is supported by the Camel on EAP (Wildfly Camel) framework, which offers a simplified deployment model on the Red Hat JBoss Enterprise Application Platform (JBoss EAP) container.

Approaches to invoking

As shown in Table 13.1, “Expression and Predicate Languages”, there are several different syntaxes for invoking an expression language, depending on the context in which it is used. You can invoke an expression language:

As a static method

Most of the languages define a static method that can be used in any context where an org.apache.camel.Expression type or an org.apache.camel.Predicate type is expected. The static method takes a string expression (or predicate) as its argument and returns an Expression object (which is usually also a Predicate object).

For example, to implement a content-based router that processes messages in XML format, you could route messages based on the value of the /order/address/countryCode element, as follows:

from("SourceURL")
  .choice
    .when(xpath("/order/address/countryCode = 'us'"))
      .to("file://countries/us/")
    .when(xpath("/order/address/countryCode = 'uk'"))
      .to("file://countries/uk/")
    .otherwise()
      .to("file://countries/other/")
  .to("TargetURL");

As a fluent DSL method

The Java fluent DSL supports another style of invoking expression languages. Instead of providing the expression as an argument to an Enterprise Integration Pattern (EIP), you can provide the expression as a sub-clause of the DSL command. For example, instead of invoking an XPath expression as filter(xpath("Expression")), you can invoke the expression as, filter().xpath("Expression").

For example, the preceding content-based router can be re-implemented in this style of invocation, as follows:

from("SourceURL")
  .choice
    .when().xpath("/order/address/countryCode = 'us'")
      .to("file://countries/us/")
    .when().xpath("/order/address/countryCode = 'uk'")
      .to("file://countries/uk/")
    .otherwise()
      .to("file://countries/other/")
  .to("TargetURL");

As an XML element

You can also invoke an expression language in XML, by putting the expression string inside the relevant XML element.

For example, the XML element for invoking XPath in XML is xpath (which belongs to the standard Apache Camel namespace). You can use XPath expressions in a XML DSL content-based router, as follows:

<from uri="file://input/orders"/>
<choice>
  <when>
    <xpath>/order/address/countryCode = 'us'</xpath>
    <to uri="file://countries/us/"/>
  </when>
  <when>
    <xpath>/order/address/countryCode = 'uk'</xpath>
    <to uri="file://countries/uk/"/>
  </when>
  <otherwise>
    <to uri="file://countries/other/"/>
  </otherwise>
</choice>

Alternatively, you can specify a language expression using the language element, where you specify the name of the language in the language attribute. For example, you can define an XPath expression using the language element as follows:

<language language="xpath">/order/address/countryCode = 'us'</language>

As an annotation

Language annotations are used in the context of bean integration . The annotations provide a convenient way of extracting information from a message or header and then injecting the extracted data into a bean’s method parmeters.

For example, consider the bean, myBeanProc, which is invoked as a predicate of the filter() EIP. If the bean’s checkCredentials method returns true, the message is allowed to proceed; but if the method returns false, the message is blocked by the filter. The filter pattern is implemented as follows:

// Java
MyBeanProcessor myBeanProc = new MyBeanProcessor();

from("SourceURL")
  .filter().method(myBeanProc, "checkCredentials")
  .to("TargetURL");

The implementation of the MyBeanProcessor class exploits the @XPath annotation to extract the username and password from the underlying XML message, as follows:

// Java
import org.apache.camel.language.XPath;

public class MyBeanProcessor {
    boolean void checkCredentials(
        @XPath("/credentials/username/text()") String user,
        @XPath("/credentials/password/text()") String pass
    ) {
        // Check the user/pass credentials...
        ...
    }
}

The @XPath annotation is placed just before the parameter into which it gets injected. Notice how the XPath expression explicitly selects the text node, by appending /text() to the path, which ensures that just the content of the element is selected, not the enclosing tags.

As a Camel endpoint URI

Using the Camel Language component, you can invoke a supported language in an endpoint URI. There are two alternative syntaxes.

To invoke a language script stored in a file (or other resource type defined by Scheme), use the following URI syntax:

language://LanguageName:resource:Scheme:Location[?Options]

Where the scheme can be file:, classpath:, or http:.

For example, the following route executes the mysimplescript.txt from the classpath:

from("direct:start")
  .to("language:simple:classpath:org/apache/camel/component/language/mysimplescript.txt")
  .to("mock:result");

To invoke an embedded language script, use the following URI syntax:

language://LanguageName[:Script][?Options]

For example, to run the Simple language script stored in the script string:

String script = URLEncoder.encode("Hello ${body}", "UTF-8");
from("direct:start")
  .to("language:simple:" + script)
  .to("mock:result");

For more details about the Language component, see Language in the Apache Camel Component Reference Guide.