Tuesday, November 19, 2013

Tutorial on Spring Remoting with JMS


Thanks
This tutorial was kindly donated to Apache Camel by Martin Gilday.

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.

Prerequisites

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

Distribution

This sample is distributed with the Camel distribution as examples/camel-example-spring-jms.

About

This tutorial is a simple example that demonstrates more the fact how well Camel is seamless integrated with Spring to leverage the best of both worlds. This sample is client server solution using JMS messaging as the transport. The sample has two flavors of servers and also for clients demonstrating different techniques for easy communication.
The Server is a JMS message broker that routes incoming messages to a business service that does computations on the received message and returns a response.
The EIP patterns used in this sample are:
PatternDescription
Message ChannelWe need a channel so the Clients can communicate with the server.
MessageThe information is exchanged using the Camel Message interface.
Message TranslatorThis 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.
Message EndpointIt should be easy to send messages to the Server from the the clients. This is archived with Camels powerful Endpoint pattern that even can be more powerful combined with Spring remoting. The tutorial have clients using each kind of technique for this.
Point to Point ChannelWe using JMS queues so there are only one receive of the message exchange
Event Driven ConsumerYes the JMS broker is of course event driven and only reacts when the client sends a message to the server.
We use the following Camel components:
ComponentDescription
ActiveMQWe use Apache ActiveMQ as the JMS broker on the Server side
BeanWe use the bean binding to easily route the messages to our business service. This is a very powerful component in Camel.
FileIn the AOP enabled Server we store audit trails as files.
JMSUsed for the JMS messaging

Create the Camel Project

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

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.
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-core</artifactId>
</dependency>
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-jms</artifactId>
</dependency>
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-spring</artifactId>
</dependency>
<dependency>
    <groupId>org.apache.activemq</groupId>
    <artifactId>activemq-camel</artifactId>
</dependency>
<dependency>
    <groupId>org.apache.activemq</groupId>
    <artifactId>activemq-pool</artifactId>
</dependency>
As we use spring xml configuration for the ActiveMQ JMS broker we need this dependency:
<dependency>
    <groupId>org.apache.xbean</groupId>
    <artifactId>xbean-spring</artifactId>
</dependency>

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.
public interface Multiplier {
 
    /**
     * Multiplies the given number by a pre-defined constant.
     *
     * @param originalNumber The number to be multiplied
     * @return The result of the multiplication
     */
    int multiply(int originalNumber);
 
}
And the implementation of this service is:
@Service(value = "multiplier")
public class Treble implements Multiplier {
 
    public int multiply(final int originalNumber) {
        return originalNumber * 3;
    }
 
}
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

public class ServerRoutes extends RouteBuilder {
 
    @Override
    public void configure() throws Exception {
        // route from the numbers queue to our business that is a spring bean registered with the id=multiplier
        // Camel will introspect the multiplier bean and find the best candidate of the method to invoke.
        // You can add annotations etc to help Camel find the method to invoke.
        // As our multiplier bean only have one method its easy for Camel to find the method to use.
        from("jms:queue:numbers").to("multiplier");
 
        // Camel has several ways to configure the same routing, we have defined some of them here below
 
        // as above but with the bean: prefix
        //from("jms:queue:numbers").to("bean:multiplier");
 
        // beanRef is using explicit bean bindings to lookup the multiplier bean and invoke the multiply method
        //from("jms:queue:numbers").beanRef("multiplier", "multiply");
 
        // the same as above but expressed as a URI configuration
        //from("jms:queue:numbers").to("bean:multiplier?methodName=multiply");
    }
 
}
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.
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.
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:
<context:component-scan base-package="org.apache.camel.example.server"/>
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.
<camel:camelContext id="camel-server">
  <camel:package>org.apache.camel.example.server</camel:package>
  
  
  <camel:jmxAgent id="agent" createConnector="true"/>
</camel:camelContext>
The ActiveMQ JMS broker is also configured in this xml file. We set it up to listen on TCP port 61610.
<broker:broker useJmx="true" persistent="false" brokerName="myBroker">
  <broker:transportConnectors>
    
    <broker:transportConnector name="vm" uri="vm://myBroker"/>
    
    <broker:transportConnector name="tcp" uri="tcp://localhost:${tcp.port}"/>
  </broker:transportConnectors>
</broker:broker>
As this examples uses JMS then Camel needs a JMS component that is connected with the ActiveMQ broker. This is configured as shown below:
<bean id="jms" class="org.apache.activemq.camel.component.ActiveMQComponent">
  <property name="brokerURL" value="vm://myBroker"/>
</bean>
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.
component-scanDefines the package to be scanned for Spring stereotype annotations, in this case, to load the "multiplier" bean
camel-contextDefines the package to be scanned for Camel routes. Will find the ServerRoutes class and create the routes contained within it
jms beanCreates the Camel JMS component

Run the Server

The Server is started using the org.apache.camel.spring.Main class that can start camel-spring application out-of-the-box. The Server can be started in several flavors:
  • as a standard java main application - just start the org.apache.camel.spring.Main class
  • using maven jave:exec
  • using camel:run
In this sample as there are two servers (with and without AOP) we have prepared some profiles in maven to start the Server of your choice.
The server is started with:
mvn compile exec:java -PCamelServer

Writing The Clients

This sample has three clients demonstrating different Camel techniques for communication
  • CamelClient using the ProducerTemplate for Spring template style coding
  • CamelRemoting using Spring Remoting
  • CamelEndpoint using the Message Endpoint EIP pattern using a neutral Camel API

Client Using The ProducerTemplate

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.
<camel:camelContext id="camel-client">
  <camel:template id="camelTemplate"/>
</camel:camelContext>
<bean id="jms" class="org.apache.activemq.camel.component.ActiveMQComponent">
  <property name="brokerURL" value="tcp://localhost:${tcp.port}"/>
</bean>
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.
camelContextThe Camel context is defined but does not contain any routes
templateThe ProducerTemplate is used to place messages onto the JMS queue
jms beanThis initialises the Camel JMS component, allowing us to place messages onto the queue
And the CamelClient source code:
public static void main(final String[] args) throws Exception {
    System.out.println("Notice this client requires that the CamelServer is already running!");
 
    ApplicationContext context = new ClassPathXmlApplicationContext("camel-client.xml");
 
    // get the camel template for Spring template style sending of messages (= producer)
    ProducerTemplate camelTemplate = context.getBean("camelTemplate", ProducerTemplate.class);
 
    System.out.println("Invoking the multiply with 22");
    // as opposed to the CamelClientRemoting example we need to define the service URI in this java code
    int response = (Integer)camelTemplate.sendBody("jms:queue:numbers", ExchangePattern.InOut, 22);
    System.out.println("... the result is: " + response);
 
    System.exit(0);
}
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.
Before running the client be sure that both the ActiveMQ broker and the CamelServer are running.

Client 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.
<camel:proxy
  id="multiplierProxy"
  serviceInterface="org.apache.camel.example.server.Multiplier"
  serviceUrl="jms:queue:numbers"/>
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. 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:
public static void main(final String[] args) {
    System.out.println("Notice this client requires that the CamelServer is already running!");
 
    ApplicationContext context = new ClassPathXmlApplicationContext("camel-client-remoting.xml");
    // just get the proxy to the service and we as the client can use the "proxy" as it was
    // a local object we are invoking. Camel will under the covers do the remote communication
    // to the remote ActiveMQ server and fetch the response.
    Multiplier multiplier = context.getBean("multiplierProxy", Multiplier.class);
 
    System.out.println("Invoking the multiply with 33");
    int response = multiplier.multiply(33);
    System.out.println("... the result is: " + response);
 
    System.exit(0);
}
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 ProducerTemplate. 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.

Client Using Message Endpoint EIP Pattern

This client uses the Message Endpoint EIP pattern to hide the complexity to communicate to the Server. The Client uses the same simple API to get hold of the endpoint, create an exchange that holds the message, set the payload and create a producer that does the send and receive. All done using the same neutral Camel API for all the components in Camel. So if the communication was socket TCP based you just get hold of a different endpoint and all the java code stays the same. That is really powerful.
Okay enough talk, show me the code!
public static void main(final String[] args) throws Exception {
    System.out.println("Notice this client requires that the CamelServer is already running!");
 
    ApplicationContext context = new ClassPathXmlApplicationContext("camel-client.xml");
    CamelContext camel = context.getBean("camel-client", CamelContext.class);
 
    // get the endpoint from the camel context
    Endpoint endpoint = camel.getEndpoint("jms:queue:numbers");
 
    // create the exchange used for the communication
    // we use the in out pattern for a synchronized exchange where we expect a response
    Exchange exchange = endpoint.createExchange(ExchangePattern.InOut);
    // set the input on the in body
    // must be correct type to match the expected type of an Integer object
    exchange.getIn().setBody(11);
 
    // to send the exchange we need an producer to do it for us
    Producer producer = endpoint.createProducer();
    // start the producer so it can operate
    producer.start();
 
    // let the producer process the exchange where it does all the work in this oneline of code
    System.out.println("Invoking the multiply with 11");
    producer.process(exchange);
 
    // get the response from the out body and cast it to an integer
    int response = exchange.getOut().getBody(Integer.class);
    System.out.println("... the result is: " + response);
 
    // stop and exit the client
    producer.stop();
    System.exit(0);
}
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.

Run the Clients

The Clients is started using their main class respectively.
  • as a standard java main application - just start their main class
  • using maven jave:exec
In this sample we start the clients using maven:
mvn compile exec:java -PCamelClient
mvn compile exec:java -PCamelClientRemoting
mvn compile exec:java -PCamelClientEndpoint
Also see the Maven pom.xml file how the profiles for the clients is defined.

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.
pom.xml
  
    
      org.apache.camel
      camel-maven-plugin
    
  
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.

Using Camel JMX

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:
...
DefaultInstrumentationAgent    INFO  JMX connector thread started on service:jmx:rmi:///jndi/rmi://claus-acer:1099/jmxrmi/camel
...
In the screenshot below we can see the route and its performance metrics:

See Also

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