Apr 15

Question Everything

Feeding Da Brain

In 90s you would say: “I am a programmer”. Some would reply with “o.. k”. More insightful would reply with a question “which programming language?”.

21st century.. socially accepted terminology has changed a bit, now you would say “I am a developer”. Some would ask “which programming language?”. More insightful would reply with a question “which out of these 42 languages do you use the most?”

The greatest thing about using several at the same time is that feeling of constant adjustment as I jump between the languages. It feels like my brain goes through exuberant synaptogenesis over and over again building those new formations.

   What's for dinner today honey?
   Asynchronous brain refactoring with a gentle touch of "mental polish"

With all these new synapses, I came to love the fact that something that seemed so holy and “crystal right” before, now gets questioned and can easily be dismissed. Was it wrong all along? No. Did it change? No. So what changed then? Well.. perception did.

Inmates of the “Gang of Four” Prison

Design patterns are these “ways” of doing things that cripple new programmers, and imprison many senior ones. Instead of having an ability to think freely, we have all these “software standard patterns” which mostly have to do with limitations of “technology at time”.

Take big guys, like C++ / Java / C#, while they have many great features and ideas, these languages have horrible story of “behavior and state”: you always have to guard something. Whether it is from multiple threads, or from other people misusing it. The languages themselves promote reuse vs. decoupling: i.e. “let’s inherit that behavior”, etc..

So how do we overcome these risks and limitations? Simple: let’s create dozens of “ways” that all developers will follow to fight this together. Oh, yea, and let’s make it industry standard, call them patterns, teach them in schools, and select people by how well they can “apply” these patterns to “any” problem at hand.

Not all developers bought into this cult of course. Here is Peter Norvig’s notes from 1996, where he “dismisses” 16 out of 23 patterns from Gang of Four, by just using functions, types, modules, etc.

Builder Pattern vs. Immutable Data Structures

Builder pattern makes sense unless.. several things. There is a great “Builders vs. Option Maps” short post that talks about builder patter limitations:

* Builders are too verbose
* Builders are not data structures
* Builders are mutable
* Builders can’t easily compose
* Builders are order dependent

Due to mutable data structures (in Java/C#/alike) Builders still make sense for things like Google protobufs with simple (i.e. primitive) types, but for most cases where immutable things need to be created it is best to use immutable data structures for the above reasons.

While jumping between the languages, I often need to create things in Clojure that are implemented in Java with Builders. This is not always easy, especially when Builders rely on external state or/and when Builders need to be passed around (i.e. to achieve a certain level of composition).

Let’s say I need to create a notification client that, by design (on the Java side of things), takes some initial state (i.e. an external system connection props), and then event handlers (callbacks) are registered on it one by one, before it gets built, i.e. builds a final, immutable notification client:

SomeClassBuilder builder = SomeClass.newBuilder()
                             .setState( state )
                             .setAnotherThing( thing );
builder.register( notificationHandlerOne );
builder.register( notificationHandlerTwo );
builder.register( notificationHandlerN );

In case you need to decouple “register events” logic from this monolithic piece above, you would pass that builder to the caller that would pass it down the chain. It is something that seems “normal” to do (at least to a “9 to 5” developer), since methods with side effects do not really raise any eyebrows in OO languages. In fact most of methods in those languages have side effects.

I quite often see people designing builders such as the one above (with lots of external state), and when I need to use them in Clojure, it becomes very apparent that the above is not well designed:

;; creates a "mutable" builder..
(defn- make-bldr [state thing]
  (-> (SomeClass/newBuilder)
      (.withState state)
      (.withAnotherThing thing)))
;; wraps "builder.register(foo)" into a composable function
(defn register-event-handler! [bldr handler]
    ;; in case handler is just a Clojure function wrap it with something ".register" will accept
    (.register bldr handler))
(defn notification-client [state thing]
  (let [bldr (make-bldr state thing)]
    ;; ... do things that would call "register-event-handler!" passing them the "bldr"
    (.build bldr)))

Things that immediately feel “off” are: returning a mutable builder from “make-bldr”, mutating that builder in “register-event-handler!”, and returning that mutated builder back.. Not something common in Clojure at all.

Again the goal is to “decouple logic to register events from notification client creation“, if both can happen at the same time, the above Builder would work.

In Clojure it would just be a map. All data structures in Clojure are immutable, so there would be no intermediate mutable holder/builder, it would always be an immutable map. When all handlers are registered, this map would be passed to a function that would create a notification client with these handlers and start it with “state” and “thing”.

Mocking Suspicions

Another synapse formation, that was created from using many languages at the same time, convinced me that if I have to use а mock to test something, that something needs a close look, and would potentially welcome refactoring.

The most common case for mocking is:

A method of a component "A" takes a component "B" that depends on a component "C".

So if I want to test A’s method, I can just mock B and not to worry about C.

The flaw here is:

"B" that depends on a component "C".

These things are extremely beneficial to question. I used to use Spring a lot, and when I did, I loved it. Learned from it, taught it to others, and had a great sense of accomplishment when high complexity could be broken down to simple pieces and re wired together with Spring.

Time went on, and in Erlang or Clojure, or even Groovy for that matter, I used Spring less and less. I still use it for all my Java work, but not as much. So if 10 years ago:

"B" that depends on a component "C".

was a way of life, now, every time I see it, I ask why?. Does “B” have to depend on “C”? Can “B” be stateless and take “C” whenever it needed it, rather that be injected with it and carry that state burden?

If before “B” was:

public class B {
  private final C c;
  public B ( C c ) {
    this.c = c;
  public Profit doBusiness() {
    return new Profit( c.doYourBusiness() + 42 );

Can it instead just be:

public final class B {
  public static Profit doBusiness( C c ) {
    return new Profit( c.doYourBusiness() + 42 );

In most cases it can! It really can, the problem is not enough of us question that dependency, but we should.

This does not mean “B” no longer depends on “C”, it means something more: there is no “B” (“there is no spoon..”) as it just becomes a module, which does not need to be passed around as an instance. The only thing that is left from “B” is “doBusiness( C c )”. Do we need to mock “C” now? Can it, its instance disappear the way “B” did? Most likely, and even if it can’t for whatever reason (i.e. someone else’s code), I did question it, and so should you.

The more synapse formations I go through the better I learn to question pretty much everything. It is fun, and it pays back with beautiful revelations. I love my brain :)

Jul 11

Having Cluster Fun @ Chariot Solutions

The best way to experiment with distributed computing is to have a distributed cluster of things to play with. One approach would of course be to spin off multiple Amazon EC2 instances, which would be wise and pretty cheap:

Micro instances provide 613 MB of memory and support 32-bit and 64-bit platforms on both Linux and Windows. Micro instance pricing for On-Demand instances starts at $0.02 per hour for Linux and $0.03 per hour for Windows”

However some problems are better solved/simulated by having real, “touchable” hardware, that would have real dedicated disks, dedicated cores, RAM, and would only share any kind of state with other nodes over network. Easier said that done though.. Do you have a dozen of spare (same spec’ed) PCs laying around?

But what if you had an awesome training room with, let’s say, 10 iMacs? That would look something like:

Chariot Solutions Training Room

This is in fact the real deal => “Chariot Solutions Training Room“, which is usually occupied by people learning about Scala, Clojure, Hadoop, Spring, Hibernate, Maven, Rails, etc..

So once upon a time, in after training hours, we decided to run some distributed simulations. As we were passing by the training room, we had a thought: “It’s Friday night, and as any other creatures, these beautiful machines would definitely like to hang out together”…

Cluster at Chariot Solutions

This is one of this night’s highlights: a MongoDB playground. The same Friday night we played with Riak, Cassandra, RabbitMQ and vanilla distributed Erlang. As you can imagine iMacs had a lot of fun in a process pumping data in and out via 10 Gigabit switch. And we geeked out like real men!

Mar 11

Spring Batch: CommandLineJobRunner to Run Multiple Jobs

Sometimes this requirement may jump at you: “I still want to have them as several jobs, but I want no operational overhead, and need to run them in sequence via a single launch…”.

So if you were using CommandLineJobRunner to launch these jobs, it would only be natural to aggregate those calls under your own e.g. “CommandLineMultipleJobRunner” that just delegates its calls to “CommandLineJobRunner” for each job in a list..

The only caveat is the default SystemExiter which is set to JvmSystemExiter, which makes it imposible to make more than one launch using a “CommandLineJobRunner”, since “JvmSystemExiter” does:

public void exit(int status) {

Fortunately, there is a static accessor to override this behavior:

// disabling a default "JvmSystemExiter" to be able to run several times
CommandLineJobRunner.presetSystemExiter( new SystemExiter() { public void exit( int status ) {} } );

Now, call “CommandLineJobRunner.main( String[] args )” as many times as you’d like.

If you have a choice, think about creating a single multi step job instead, but if you don’t have a luxury, “CommandLineMultipleJobRunner” should now be not all that hard to implement.

Batch Away! :)

Oct 10

Running Spring AspectJ Tests with Maven

You need to use a real AspectJ aspect with Spring, where you would like to inject some dependencies into this aspect as a Spring container starts up.

Here is an AspectJ aspect:

public class VeryImportantAspect {
    public void setSomeDependency( SomeDependency dependency ) {
        this.dependency = dependency;

Now let’s define it as a bean and inject “someDependency”:

<bean id="veryImportantAspect" 
         <property name="someDependency" ref="someDependency"/>

Notice factory-method=”aspectOf” that tells Spring to use a real AspectJ ( not Spring AOP ) aspect to create this bean. In reality, if the “VeryImportantAspect” was already woven in, it will have an “aspectOf” method.

Hence if you get this error at some point:

No matching factory method found: factory method 'aspectOf()'

That would mean that the aspect was not woven by AspectJ weaver.

We also need to tell Spring to use load time weaving:


If you run with the above configuration without doing anything, you would get a following error:

ClassLoader does NOT provide an 'addTransformer(ClassFileTransformer)' method

That tells you ( well not explicitly :) ) to include a “spring-instrument.jar” javaagent.

So at this point two things need to be added: AspectJ weaver and Spring Instrument agent. Build / Continues integration server does not use any IDE, so it is not as simple as downloading AspectJ plugin for IDEA / Eclipse and including VM arguments in Run Configurations… It is simpler: just add the two in your POM file.

Adding an AspectJ plugin:

                 <goal>compile</goal>       <!-- use this goal to weave all your main classes -->
                 <goal>test-compile</goal>  <!-- use this goal to weave all your test classes -->

Adding a Spring Instrument agent to participate in a test execution:


And all that work, just too see a couple of lines from Maven:

[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------

Sep 10

Money Making Project

For the past several years most of my projects were based on Spring and Hibernate. And what I keep seeing is how people struggle to understand how the two are working together.

So going from project to project, I have to repeat the same spiel over and over again to get people up to speed.

Since I believe that the most effective way to LEARN is to DO, I create simple examples for people to play with / to complete / to analyze, etc..

So “Money Making Project” is one of such examples. It comfortably lives at github’s luxury apartments, so anybody can “git clone” / “fork” and play with it.

Besides making money, this project also demos things such as:

A way to structure a project

Maven based structure ( hence can be easily used by gradle ). Configuration and property files organized under “META-INF/conf”, “META-INF/props”, etc..

A way to separate Spring configs

“tx-spring-config.xml”, “persistence-spring-config.xml”, “service-spring-config.xml”, “repository-spring-config.xml” and “application-context.xml” that includes them all

Properties via PropertyPlaceholderConfigurer

<context:property-placeholder location="classpath:META-INF/props/env.properties"/>

Hibernate overall configuration file

That is injected into AnnotationSessionFactoryBean

Hibernate Named Queries

That are linked to the Hibernate overall config

<mapping resource="META-INF/conf/hibernate/mapping/startup-bank-named-queries.xml"/>

Spring’s DAO / Hibernate Exception Translation

Via @Repository and “PersistenceExceptionTranslationPostProcessor”

Simple CRUD Repository

public interface MoneyRepository {
    public void make( MoneyRoll moneyRoll );        // C
    public MoneyRoll find( Long id );                       // R
    public void update( MoneyRoll moneyRoll );     // U
    public void takeOut( MoneyRoll moneyRoll );    // D

with a Hibernate based implementation

Transaction Management with Spring AOP

Declarative, on a Service Level, using “aop:config”, “tx:advice” namespaces

Spring Testing

With SpringJUnit4ClassRunner, ContextConfiguration, etc..

Using Embeded in-memory H2 Database for Testing

<jdbc:embedded-database id="dataSource" type="H2"/>

Hibernate Logging

Most useful hibernate “log4j.logger” properties

Demoing how important Transaction is for Hibernate Sessions

“HibernateSessionNotBoundToThreadIntegrationTest” for the second (after LazyInitializationException ) most common Hibernate exception: “No Hibernate Session bound to thread, and configuration does not allow creation of non-transactional one here”