Preface

This tutorial aims to guide the reader through the stages of creating a project which uses Camel to facilitate the routing of messages from a JMS queue to a Spring service. The route works in a synchronous fashion returning a response to the client.

TODOs

• Embellish the tutorial with more detail, explaining at each stage what is happening.
• Link to specific sections of Camel documentation when referring to components.
• Attach completed example project.
• Explain in more detail what is happening when the JMS component is being defined in camel-server.xml
• Show how logging can be introduced to monitor exchange body contents.
• Detail how time-outs can be configured.
• Show how to catch lost messages.
• Show how to use a wiretap.
• Can we make this ActiveMQ embedded so that it does not require the reader to download and start it manually?

Prerequisites

This tutorial uses Maven to setup the Camel project and for dependencies for artifacts.

Create the Camel Project

mvn archetype:create -DgroupId=org.example -DartifactId=CamelWithJmsAndSpring

Update the POM with Dependencies

First we need to have dependencies for the core Camel jars and its spring and jms components.

As we use spring xml configuration for the ActiveMQ JMS broker we need this dependency:

TODO: AOP dependencies

Writing the Server

Create the Spring Service

For this example the Spring service (= our business service) on the server will be a simple multiplier which trebles in the received value.

And the implementation of this service is:

Notice that this class has been annotated with the @Service spring annotation. This ensures that this class is registered as a bean in the registry with the given name multiplier.

Define the Camel Routes

This defines a Camel route from the JMS queue named numbers to the Spring bean named multiplier. Camel will create a consumer to the JMS queue which forwards all received messages onto the the Spring bean, using the method named multiply.

Configure Spring

The Spring config file is placed under META-INF/spring as this is the default location used by the Camel Maven Plugin, which we will later use to run our server.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:context="http://www.springframework.org/schema/context"
	xmlns:camel="http://activemq.apache.org/camel/schema/spring"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
		http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
		http://www.springframework.org/schema/context
		http://schemas.leadx.com/spring/spring-context-2.5.xsd
		http://activemq.apache.org/camel/schema/spring
		http://activemq.apache.org/camel/schema/spring/camel-spring-1.3.0.xsd">
		
	<context:component-scan base-package="org.example.server" />

	<camel:camelContext id="camel">
		<camel:package>org.example.server</camel:package>
	</camel:camelContext>

	<bean id="jms" class="org.apache.camel.component.jms.JmsComponent">
		<property name="connectionFactory">
			<bean class="org.apache.activemq.ActiveMQConnectionFactory">
				<property name="brokerURL" value="tcp://localhost:61616" />
			</bean>
		</property>
	</bean>

</beans>

Run the Server

public class CamelServer {

  public static void main(final String[] args) {
    ApplicationContext context = new ClassPathXmlApplicationContext("camel-server.xml");
  }

}

The main method can then be executed to start the server.

Creating the Client

We will initially create a client by directly using CamelTemplate. We will later create a client which uses Spring remoting to hide the fact that messaging is being used.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:camel="http://activemq.apache.org/camel/schema/spring"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
		http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
		http://activemq.apache.org/camel/schema/spring
		http://activemq.apache.org/camel/schema/spring/camel-spring-1.3-SNAPSHOT.xsd">

	<camel:camelContext id="camel" />
	<camel:template id="camelTemplate" />
	
	<bean id="jms" class="org.apache.camel.component.jms.JmsComponent">
		<property name="connectionFactory">
			<bean class="org.apache.activemq.ActiveMQConnectionFactory">
				<property name="brokerURL" value="tcp://localhost:61616" />
			</bean>
		</property>
	</bean>
</beans>

The client will not use the Camel Maven Plugin so the Spring XML has been placed in src/main/resources so not to conflict with the server configs.

public class CamelClient {

  public static void main(final String[] args) {

    ApplicationContext context = new ClassPathXmlApplicationContext("camel-client.xml");
    CamelTemplate<JmsExchange> camelTemplate = (CamelTemplate) context.getBean("camelTemplate");

    int response = (Integer)camelTemplate.sendBody("jms:queue:numbers",
      ExchangePattern.InOut,
      22
    );

    Assert.assertEquals(66, response);
    System.out.println(response);

  }
}

The CamelTemplate is retrieved from a Spring ApplicationContext and used to manually place a message on the "numbers" JMS queue. The exchange pattern (ExchangePattern.InOut) states that the call should be synchronous, and that we will receive a response. We then assert that the response is three times the value of the original.

Before running the client be sure that both the ActiveMQ broker and the CamelServer are running.

Using Spring Remoting

Spring Remoting "eases the development of remote-enabled services". It does this by allowing you to invoke remote services through your regular Java interface, masking that a remote service is being called.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:camel="http://activemq.apache.org/camel/schema/spring"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
		http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
		http://activemq.apache.org/camel/schema/spring
		http://activemq.apache.org/camel/schema/spring/camel-spring-1.3-SNAPSHOT.xsd">

	<camel:camelContext id="camel" />
	
	<camel:proxy
		id="multiplier"
		serviceInterface="org.example.server.Multiplier"
		serviceUrl="jms:queue:numbers"
	/>
	
	<bean id="jms" class="org.apache.camel.component.jms.JmsComponent">
		<property name="connectionFactory">
			<bean class="org.apache.activemq.ActiveMQConnectionFactory">
				<property name="brokerURL" value="tcp://localhost:61616" />
			</bean>
		</property>
	</bean>
</beans>

First we create a new Spring config file. This has a few changes made from camel-client.xml. Firstly the Camel template has been removed, as it will not be used. Secondly a proxy is defined. This will create a proxy service bean for you to use to make the remote invocations. The serviceInterface property details which Java interface is to be implemented by the proxy. serviceUrl defines where messages sent to this proxy bean will be directed. Here we define the JMS endpoint with the "numbers" queue we used when working with Camel template directly. The value of the id property is the name that will be the given to the bean when it is exposed through the Spring ApplicationContext. We will use this name to retrieve the service in our client. I have named the bean multiplierProxy simply to highlight that it is not the same multiplier bean as is being used by CamelServer. They are in completely independent contexts and have no knowledge of each other. As you are trying to mask the fact that remoting is being used in a real application you would generally not include proxy in the name.

public class CamelClientRemoting {

  public static void main(final String[] args) {

    ApplicationContext context = new ClassPathXmlApplicationContext("camel-client-remoting.xml");
    Multiplier multiplier = (Multiplier) context.getBean("multiplierProxy");

    int response = multiplier.multiply(22);

    Assert.assertEquals(66, response);
    System.out.println(response);
  }

}

Again, the client is similar to the original client, but with some important differences.

1. The Spring context is created with the new camel-client-remoting.xml
2. We retrieve the proxy bean instead of a CamelTemplate. In a non-trivial example you would have the bean injected as in the standard Spring manner.
3. The multiply method is then called directly. In the client we are now working to an interface. There is no mention of Camel or JMS inside our Java code.

Using the Camel Maven Plugin

The Camel Maven Plugin allows you to run your Camel routes directly from Maven. This negates the need to create a host application, as we did with Camel server, simply to start up the container. This can be very useful during development to get Camel routes running quickly.

<build>
  <plugins>
    <plugin>
      <groupId>org.apache.camel</groupId>
      <artifactId>camel-maven-plugin</artifactId>
    </plugin>      
  </plugins>
</build>

All that is required is a new plugin definition in your Maven POM. As we have already placed our Camel config in the default location (camel-server.xml has been placed in META-INF/spring/) we do not need to tell the plugin where the route definitions are located. Simply run mvn camel:run.

TODO: Testing

TODO: Detail how to unit and integration test this example.

See Also

• Spring Remoting with JMS Example on Amin Abbaspour's Weblog