DDevelopment & Design

Easier and nicer JMS

JMS seems like a hostile ground. It has all it’s quirks and strange behaviours. A couple of defining standards plus esoteric brokers, queues and topics.

At work, we mainly use open source Jms solutions, namely Apache ActiveMQ. This one is usually bundled with Apache Servicemix, as a message broker for this particular ESB. As there are some minor caveats in this scennerio, I’d like to describe here some guidelines for getting to running JMS queues.

Treat this post as a quick cheat sheet with the most common things about JMS I tend to forget :)

Minor glitches encountered during work with embedded broker led to some thoughts about switching to external broker. This is how I configure SMX and AcviteMQ.

Necessary steps:

  • change apache-servicemix/conf/servicemix.properties activemq.port to sth else than standard, for example 61626
  • change apache-activemq/conf/activemq.xml with this settings:
    • change port, the service listens on:
    • setup separate JMX instance:
  • the nicest tool I found for browsing queues and topics is Hermes JMS. Sample config, that connects Hermes to ActiveMQ instance is on the picture below: HermesJMS to ActiveMQ connection config
  • sending simple messages with Hermes is basic, but what if you need to set some headers, send bulk messages, etc. Easy, just use Hermes xml format. Look like this code snippet below and is rather self-explanatory: 
    
    
        
            
            
<![CDATA[
  
    
      105
      1235
    
  
]]>
  • since we use lots of Apache Camel to consume messages, here is a simple way to start broker in your tests:
    • start a broker 
              BrokerService broker = new org.apache.activemq.broker.BrokerService();
        broker.setBrokerName("AMQ-1");
        broker.addConnector("tcp://localhost:51616");
        broker.setPersistent(false);
        broker.start();

      Notice it has persistance disabled.

    • initialize Camel’s JMS component: 
          ctx.removeComponent("jms");
    ctx.addComponent("jms", ActiveMQComponent.activeMQComponent("tcp://localhost:51616"));
    • if you want to pass messages to reference endpoints, (like ref:input), use this wrapper method: 
      private JmsEndpoint createJmsEndpoint(String endpoint) throws JMSException {
        ActiveMQComponent amqc = (ActiveMQComponent) ctx.getComponent("jms");
        JmsEndpoint endp = JmsEndpoint.newInstance(new ActiveMQTopic(endpoint), amqc);
        return endp;
}

createJmsEndpoint("ESB/XYZ")

These are all the tricks I’ve got for now! But if you know some other good tools that handle JMS, feel free to comment! Got more advices, again, comment!

— Previous article

Licealista zwycięzcą ITPW

Next article —

<!--:en-->Turing completeness II<!--:-->

You May Also Like
DDevelopment & Design

Private fields and methods are not private in groovy

I used to code in Java before I met groovy. Like most of you, groovy attracted me with many enhancements. This was to my surprise to discover that method visibility in groovy is handled different than Java!

Consider this example:

class Person {
private String name
public  String surname

private Person() {}

private String signature() { "${name?.substring(0, 1)}. $surname" }

public String toString() { "I am $name $surname" }
}

How is this class interpreted with Java?

  1. Person has private constructor that cannot be accessed
  2. Field "name" is private and cannot be accessed
  3. Method signature() is private and cannot be accessed

Let's see how groovy interpretes Person:

public static void main(String[] args) {
def person = new Person()   // constructor is private - compilation error in Java
println(person.toString())

    person.@name = 'Mike'       // access name field directly - compilation error in Java
println(person.toString())

    person.name = 'John'        // there is a setter generated by groovy
println(person.toString())

    person.@surname = 'Foo'     // access surname field directly
println(person.toString())

    person.surname = 'Bar'      // access auto-generated setter
println(person.toString())

println(person.signature()) // call private method - compilation error in Java
}

I was really astonished by its output:

I am null null
I am Mike null
I am John null
I am John Foo
I am John Bar
J. Bar

As you can see, groovy does not follow visibility directives at all! It treats them as non-existing. Code compiles and executes fine. It's contrary to Java. In Java this code has several errors, pointed out in comments.

I've searched a bit on this topic and it seems that this behaviour is known since version 1.1 and there is a bug report on that: http://jira.codehaus.org/browse/GROOVY-1875. It is not resolved even with groovy 2 release. As Tim Yates mentioned in this Stackoverflow question: "It's not clear if it is a bug or by design". Groovy treats visibility keywords as a hint for a programmer.

I need to keep that lesson in mind next time I want to make some field or method private!