JPA provides essentially 2 types of locking mechanisms to help synchronize access to entities. Both mechanisms prevent a scenario, where 2 transactions overwrite data of each other without knowing it.

By entity locking, we typically want to prevent following scenario with 2 parallel transactions:

  1. Adam’s transaction reads data X
  2. Barbara’s transaction reads data X
  3. Adam’s transaction modifies data X, and changes it to XA
  4. Adam’s transaction writes data XA
  5. Barbara’s transaction modifies data X and changes it to XB
  6. Barbara’s transaction writes data XB

As a result, changes done by Adam are completely gone and overwritten by Barbara without her even noticing. A scenario like this is sometimes called dirty-read. Obviously, a desired result is that Adam writes XA, and Barbara is forced to review XA changes before writing XB.

How Optimistic Locking works


Read in Slovak language: Štruktúra modernej Java EE aplikácie (Structure of modern Java EE application).


Anybody I don’t like, read this! :

com.superframework.core.base.Object object = new com.superframework.core.base.Object()

Sometimes one cannot avoid this rubbish in Java, even today. I do not wish my enemies to read such code, not in my code I want to be proud of!

I wonder how many times I have asked myself why Java is so complicated to read and write? Why I have to keep writing so many characters and lines of code to express a simple repetitive task? It’s appeared to me like Java language designers keep torturing developers by forcing us to use constructs invented 15+ years ago without an alternative.

But this one is simply an outrage. (more…)

Ever wondered why Eclipse/Netbeans keeps pausing for a while every now an then? Especially right at the time when you want to show something in the code to your dear colleages? It feelt embarrassing and awkward, didn’t it?

I found out that most of the time the IDE pauses because of Garbage Collector execution. The subtle little element in design of JVM, which usually does great job in relieving us developers from worrying about memory consumption, and most people are just happy that it does its job well and ignore it most of the time. However, the consequences of running Garbage Collector may surprise us if we simply ignore it. (more…)

After having the pleasure of building my code around CDI for couple of years, it feels very natural to use it to structure my code according to well-known patterns. CDI is a dependency injection mechanism designed to be used within Java EE application servers, and this could be perceived as a disadvantage. However, I want to show that it can be used and has great potential also in a Java SE application.

What is great about CDI is that it is much more than an injection mechanism. On top of this it provides also an elegant and powerful event passing mechanism. This feature can be nicely combined with Swing to build a GUI application based on MVC pattern.

It is really possible to efficiently combine CDI and Swing framework to build a Java GUI application rapidly and with a clear structure.


The day 2 started earlier than the day before. A bit too early for me. While hurrying to catch the beginning of the first presentation, however, a stranger with a big suitcase passed by me in an even greater hurry, a bit confused about which way to take. I grinned to myself as I recollected the familiar face from the speakers’ section of Geecon web page. Anyway, speakers are only human too…

Although latest and greatest themes in Java world these days are reactive programming, alternative languages, HTML5 and microservices, I decided to stay close to the ground at first. I chose jBMP presentation to get an update on how thinggus are moving in the old Java enterprise waters.
jBMP looks like a vivid yet mature project and it is promisingly evolving under the RedHat umbrella. In fact, the team have a strategy to focus on knowledge, business goals, their visibility and continuous improvement. Does that ring a bell? To me, that sounds quite close to what agile principles adhere to.

OK, enough business, we all want some fun too, right? And the next presentation certainly was about how to make fun and even conquer the world with home-made devices. (more…)

Eventually, a famous Java conference came to Prague. After planning to travel to Krakow some time, it was suddenly to be here at my finger tips. As a fresh freelancer, not yet fixed to any long-term project, I decided to invest the money to inhale the atmosphere of a big conference and enjoy presence of all the people interested in Java and all the tech stack around it. So here I was to see and listen to Neal Ford – the one from ThoughtWorks, so adored for their Radar. I came alone, as freelancers often tend to, expecting to meet some familiar faces from my previous jobs. What followed was much beyond my expectations, as so many of them started suddenly popping up. Great feeling to meet so many pleasant people after 2 years spent away from Prague.

“So this is the water!” – I told to myself while Neal was talking about continuous delivery, acceptance tests and all the supportive things aiding in software development, wiring together the effort of developers, testers, operations and project managers. Warm feeling rose even higher when the very book I’m currently reading appeared on one of Neal’s slides. He hit the nail again, when he stated that meta-work is more fun than real work. And it is often not productive at all, when the fun is not guided well. Damn, that is so true. Even though claiming to seek improvement in my daily work, the biggest power to drive me into my pet projects is the desire to do fun stuff to complement boring daily work. Boy, that’s a relief that I’m not the only one having problems with lack of fun! But then, banks are not ACID? Again, not ACID?? Then, how the hell can I be sure to get my salary on my account at the end of the day? Wait, banks are running auditing jobs after working hours to put everything in order. ACID is slow and inefficient in real world. Eventual consistency is enough in most cases. But remember to schedule the auditing jobs, guys, just in case. Immutable database – an oxymoron, sure? No, Datomic is here to keep track of all the past changes. The SQL is dead, long live NoSQL! :) Jokes aside, now I seriously started thinking that some NoSQL solutions have really something to offer. Read NoSQL distilled if you want to know more…


When tried to tune our near-production application, we came to problem when a single entity reference When tried to tune our near-production application, we came to problem when a single entity reference (not a collection) is loaded lazily. We used inheritance with this entity and hibernate JPA provider (as probably any other provider) inserts a proxy object into referencing entity instead of reference to real object loaded from database (as it is not yet loaded).

We came to a problem because we were casting super class to subclasses using instanceof. The problem is that proxy class is already a subclass and cannot be cast to a sibling class. It is a proxy only to super class instance, but it’s not possible to access methods of superclasses directly. The proxy never gets converted to real subclass. The problem is specified here, here and here. All described solutions depend on hibernate non-standard API to retrieve deproxied instance.

However, after lots of thinking…

… I found a solution to deproxy a class using standard Java and JPA API. Tested with hibernate, but does not require hibernate as a dependency and should work with all JPA providers.

Only one requirement – its necessary to modify parent class (Address) and add a simple helper method.

General idea: add helper method to parent class which returns itself. when method called on proxy, it will forward the call to real instance and return this real instance.

Implementation is a little bit more complex, as hibernate recognizes that proxied class returns itself and still returns proxy instead of real instance. Workaround is to wrap returned instance into a simple wrapper class, which has different class type than the real instance.

In code:


To cast Address proxy to real subclass, use following:


List of elements in persistence.xml


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