THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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This tutorial was kindly donated to Apache Camel by Martin Gilday.
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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.toc
Prerequisites
This tutorial uses Maven to setup the Camel project and for dependencies for artifacts.
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Pattern | Description |
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We need a channel so the Clients can communicate with the server. | |
The information is exchanged using the Camel Message interface. | |
This is where Camel shines as the message exchange between the Server and the Clients are text based strings with numbers. However our business service uses int for numbers. So Camel can do the message translation automatically. | |
It should be easy to send messages to the Server from the the clients. This is achieved with Camels Camel's powerful Endpoint pattern that even can be more powerful combined with Spring remoting. The tutorial has clients using each kind of technique for this. | |
The client and server exchanges exchange data using point to point using a JMS queue. | |
The JMS broker is event driven and is invoked when the client sends a message to the server. |
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Component | Description |
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We use Apache ActiveMQ as the JMS broker on the Server side | |
We use the bean binding to easily route the messages to our business service. This is a very powerful component in Camel. | |
In the AOP enabled Server we store audit trails as files. | |
Used for the JMS messaging |
Create the Camel Project
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For the purposes of the tutorial a single Maven project will be used for both the client and server. Ideally you would break your application down into the appropriate components.
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Update the POM with Dependencies
First we need to have dependencies for the core Camel jars, its spring, jms components, and finally ActiveMQ as the message broker.
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As we use spring xml configuration for the ActiveMQ JMS broker we need this dependency:
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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.
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And the implementation of this service is:
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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
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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.
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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.
First we need to do the standard scheme declarations in the top. In the camel-server.xml we are using spring beans as the default bean: namespace and springs context:. For configuring ActiveMQ we use broker: and for Camel we of course have camel:. Notice that we don't use version numbers for the camel-spring schema. At runtime the schema is resolved in the Camel bundle. If we use a specific version number such as 1.4 then its IDE friendly as it would be able to import it and provide smart completion etc. See Xml Reference for further details.
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We use Spring annotations for doing IoC dependencies and its component-scan features comes to the rescue as it scans for spring annotations in the given package name:
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Camel will of course not be less than Spring in this regard so it supports a similar feature for scanning of Routes. This is configured as shown below.
Notice that we also have enabled the JMXAgent so we will be able to introspect the Camel Server with a JMX Console.
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The ActiveMQ JMS broker is also configured in this xml file. We set it up to listen on TCP port 61610.
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As this examples uses JMS then Camel needs a JMS component that is connected with the ActiveMQ broker. This is configured as shown below:
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Notice: The JMS component is configured in standard Spring beans, but the gem is that the bean id can be referenced from Camel routes - meaning we can do routing using the JMS Component by just using jms: prefix in the route URI. What happens is that Camel will find in the Spring Registry for a bean with the id="jms". Since the bean id can have arbitrary name you could have named it id="jmsbroker" and then referenced to it in the routing as from="jmsbroker:queue:numbers).to("multiplier");
We use the vm protocol to connect to the ActiveMQ server as its embedded in this application.
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We will initially create a client by directly using ProducerTemplate. We will later create a client which uses Spring remoting to hide the fact that messaging is being used.
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The client will not use the Camel Maven Plugin so the Spring XML has been placed in src/main/resources to not conflict with the server configs.
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And the CamelClient source code:
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The ProducerTemplate is retrieved from a Spring ApplicationContext and used to manually place a message on the "numbers" JMS queue. The requestBody method will use the exchange pattern InOut, which states that the call should be synchronous, and that the caller expects a response.
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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.
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The snippet above only illustrates the different and how Camel easily can setup and use Spring Remoting in one line configurations.
The proxy 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. The 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.
And the Java client source code:
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Again, the client is similar to the original client, but with some important differences.
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Okay enough talk, show me the code!
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Switching to a different component is just a matter of using the correct endpoint. So if we had defined a TCP endpoint as: "mina:tcp://localhost:61610" then its just a matter of getting hold of this endpoint instead of the JMS and all the rest of the java code is exactly the same.
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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.
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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.
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Camel has extensive support for JMX and allows us to inspect the Camel Server at runtime. As we have enabled the JMXAgent in our tutorial we can fire up the jconsole and connect to the following service URI: service:jmx:rmi:///jndi/rmi://localhost:1099/jmxrmi/camel. Notice that Camel will log at INFO level the JMX Connector URI:
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In the screenshot below we can see the route and its performance metrics:
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