Tuesday, November 22, 2016

Twitterfeed #1

Twitterfeed is the collection of news that I find via Twitter. I have no particular system or a method on how do I pick the news. Neither do I have a predefined period for grouping the news. It is neither daily or weekly or monthly - it is all just random. Enjoy! :)

Java 10 is now officially a project
IntelliJ IDEA 2016.3, my favourite IDE, was released!!! Yay!
CLion 2016.3, a IDE for C/C++ development was released
Rider, a new IDE for .NET is now publicly available via EAP
Akka 2.4.14 was released
Ceylon 1.3.1 was released
Fedora 25 was released

Heinz Kabutz teaches how to implement our own ArrayList in less than 10 minutes
Martin Kleppmann talks about conflict resolution for eventual consistency
Yegor Bugayenko rants about software architects
Roland Kuhn writes about understanding distribution

Wednesday, May 18, 2016

Hello World with JBoss Modules

JBoss Modules is quite an interesting project that powers JBoss application server and some other projects in JBoss ecosystem. However, I was surprised to find out that there isn't much you can find about Modules on the webs. Documentation is... bad half-done, not that many tutorials, no good examples of how you could use this awesome library in your project. The best you can find is the description on how to apply JBoss Modules within the application server. (sad panda)

I was looking for the simplest "Hello World" example and couldn't find it. Well, why not create one myself then? 

Downloading JBoss Modules

A surprising fact is that you won't find JBoss Modules in the list of upstream projects at jboss.orgThe first option is to download the jboss-modules.jar from Bintray or Maven Central. And the second option is to build it from sources

Oh, ok, one more option (not the best one) is to download the application server that includes jboss-modules.jar, e.g. WildFly.

Hello World

Ahh, the good old "Hello World" :) The main application class is as follows:

public class Main {
  public static void main(String[] args) {
     new Hello().say();

So we have a dependency, the Hello class, that will reside in a different module:

public class Hello {
  public void say(){

So to mimic the modules we first have to compile both classes and assemble corresponding JARs. Plus, a proper directory layout is expected by JBoss Modules to resolve the artefacts.

Main class belongs to 'app' module, and Hello class belongs to 'hello' module. Each module requires module.xml descriptor. This part is somewhat documented actually. Also the 'main' directory that you see within each module's directory structure is actually a version (!). 
A version slot identifier is an arbitrary string; thus one can use just about any system they wish for organization.  If not otherwise specified, the version slot identifier defaults to "main".
Here's the module.xml for the app module:

<?xml version="1.0" encoding="UTF-8"?>
<module xmlns="urn:jboss:module:1.5" name="app">
  <main-class name="Main"/>

    <resource-root path="main.jar"/>

        <module name="hello"/>

It specifies the main class (i.e. Main), the reference to the actual JAR that will be used in this module's classpath, and a dependency - the 'hello' module.

Same module.xml for the 'hello' module:

<?xml version="1.0" encoding="UTF-8"?>
<module xmlns="urn:jboss:module:1.5" name="hello">
    <resource-root path="hello.jar"/>

Voila! Now we can execute our brand new modular "Hello World" app:

> java -jar jboss-modules-1.5.1.Final.jar -mp mods app

The format for the command is as follows. First, java -jar jboss-modules.jar is used to bootstrap the environment; -mp mods, and the 'app' parameter is the name of the application module that should be executed.

This example isn't really practical, but at least it gives a hint on how to get started with JBoss Modules. Hopefully, one day, the documentation for this awesome project will be complete and there will be a bit more tutorials for different the use cases.

Friday, February 12, 2016

HotSwap vs hot deploy

It is the year 2016 and one still has to explain that HotSwap and hot deploy in Java IDEs is not the same thing.

Stackoverflow is full of questions about avoiding restarts of Java applications. So of the answers suggests that “Eclipse can update code without restarting the application” or “IntelliJ IDEA can hot update running applications” or “NetBeans automatically updates running code in debugger”. But the ultimate solution for this problem is JRebel, of course.

UPD: You can also read about various solutions to the redeployment problem in my Stackoverflow answer.

One fundamental thing that people don’t understand is that it is not even the capability of an IDE to be able to update applications. The IDE is just a medium -- it only triggers the update, and then the runtime environment is the one responsible for updating the code.

Repeat after me

HotSwap and Hot deploy is not the same thing!

What is HotSwap?

HotSwap (тм) is the technology in HotSpot JVM that is tailored at updating class definitions at runtime. Most importantly, "HotSwap adds functionality to the Java Platform Debugger Architecture (JPDA) to allow a class to be updated while under the control of a debugger”.

So when it comes to IDEs, once an application is started in a debug mode, the IDE can trigger class redefinition in a running JVM by utilizing JPDA.

If you are interested in the intimate details of HotSwap, read the “Safe class and data evolution in large and long-lived java applications” paper by M. Dmitriev.

It is important to understand the limitations of HotSwap: it is limited only to updating statements of code inside methods. Can’t change method signatures, can’t add new methods, fields, etc. Some JVM implementations are able to do a bit more. For instance, with IBM J9 JVM it is possible to add new methods to an existing class. Nevertheless, HotSwap capabilities are minimal. A JEP for enhanced class redefinition has been submitted long ago, and even a research project was sponsored by Oracle, but no further progress was made.

The bottom line here is that HotSwap is not a feature of an IDE, it is the ability of a JVM that you use.

What is hot deploy?

Hot deploy is the ability of application container to automatically deploy (web) applications at the startup. Obviously, the same feature can be applied to re-deploy the applications without restarting the JVM process.

Hot deploy is not a feature in any of the IDEs either. IDEs can only trigger (re)deployment of an application by either copying the artefacts to a correct location, or by using hooks if provided by the application server. So this is totally application server specific - this is what server adapters are for! It requires the IDE to be aware of the application server specifics, hence some people affiliate this functionality to their IDE.

Redeploying the application drops its state. Sometimes, application server can serialize/deserialize HTTP session state, but that's about it, not more. It can't preserve the state of the structures inside the application; internal caches have to be warmed up; framework internals have to be reinitialized, etc. The process is time consuming.

Application servers rely on class loader magic to redeploy applications. You can read about it in details in ZeroTurnaround’s blog.


Make sure you use the terms correctly -- 'HotSwap' and 'hot deploy' is not the same thing! You may other terms, like 'hot update' -- then make sure to ask, what does the person actually means by this, because the devil is in the details.

Thursday, December 10, 2015

Another great Java interview question: Singleton

I'm not a big fan to ask to write code at the interviews. But I still find it useful to do some coding exercises at the whiteboard. One of my favourites is the Singleton pattern. Because Singleton is so simple, you can use it as a starter for so many interesting discussions.

it often comes down to the discussions about the Singleton being lazy or eager. And while it leads to the discussion about Java Memory Model, it's not the most interesting one. No one understands Java Memory Model anyway :)

BTW, did you know that a single-element enum type is the best way to implement a Singleton?

Yes! And you can't imagine how many people do fail with this. If you deploy 2 web applications with the same Singleton class, will there be two instances of the same Singleton or one? Of course, there isn't one true answer for this question - you have to ask the details. The the answer depends much on how the class is loaded. If the class is packaged within the WARs, then you get 2 instances of the Singleton.

This is why Singleton is such a great interview question - it opens a lot of topics for further discussion!

Saturday, November 21, 2015


I have been interviewing candidates for Java developer jobs for a full decade at this point. I have tried various approaches for the interviews: various tests about language and the APIs, whiteboard programming, bug hunting, homework assessments, etc. There is no best approach for the interviews - it merely depends on the expectations, candidate background, position, day of the week, weather, whatever else.

Despite all the details, I’ve found one interview question that works like a charm. It is almost the best question to start with. And it is quite efficient in filtering the candidates early enough if have to screen a lot of candidates.

Here’ it is:

What is the different between final, finally & finalize?

How is this even a question, you would ask? Asking about the difference of the things that cannot be compared!? Well, apparently, a lot of developers can't make a clear difference. Those who don’t - you just don’t have to interview them further :)

OK, you asked this and candidate answered this brilliantly, now what? Well, I did tell you that it is a very good question to start with, didn’t I? Next, you can take it to any direction of your choice:

  • final - you may take the discussion to Reflection API, for instance. Or you can discuss how the final keyword helps with concurrent programming in Java.
  • finally - talk more about the exceptions in Java and discuss some puzzles. Like the one below. What does it print?
  • finalize - the discussion about finalize() method is only useful to validate the nerd level of the candidate. Usually you’d check why one shouldn't use finalize() in first place. Maybe some rare candidate can tell about legitimate uses of finalize(). This most likely shows that he or she remembers what is written in Item 7 from Effective Java.

I hope you get my point now, why this strange question is a very good one for the Java interviews. Have fun!

Thursday, July 23, 2015

I will be speaking at JavaOne 2015

I'll be speaking at JavaOne this year again! This time I have 2 talks accepted:

CON3597 - Having Fun with Javassist. This is merely a live coding session where I demonstrate various uses of the Javassist library for Java bytecode manipulation. I've delivered this talk multiple times and every time it is different as it turns out quite interactive and attendees usually ask questions right in the middle of the talk so I have to adjust the content as I go. Usually it's quite fun, so I enjoy presenting this talk.

CON6699 - What's the Best IDE for Java EE? I'm not sure how this one turns out - it's so much to talk about and so little time. I'll be presenting this talk along with Max Rydahl Andersen and Adam Bien. This time we're focusing solely on Java EE. Basically - it's and overview of what's available for Java EE users in Eclipse, NetBeans IDE, and IntelliJ IDEA.

Both the talks can be found in the content catalogue for JavaOne.

Friday, July 10, 2015

GeekOut 2015: CompletableFuture

The talk by Tomasz Nurkiewicz about CompletableFuture was rated the highest at GeekOut this year. This is really interesting API that appeared in Java 8

A Future that may be explicitly completed (setting its value and status), and may be used as a CompletionStage, supporting dependent functions and actions that trigger upon its completion.

Tomasz Nurkiewicz - CompletableFuture in Java 8, asynchronous processing done right. from Official ZeroTurnaround Account on Vimeo.

Some time ago Tomasz published a really nice series of articles at his blog - worth reading!

Java 8: Definitive guide to CompletableFuture
Java 8: CompletableFuture in action

And there's more!

Wednesday, April 8, 2015

XRebel 2.0 with Application Profiling

From the very start, our users requested profiler capabilities in XRebel. As of 2.0, it is possible to get the performance overview for every request and identify the slowest methods.

The profiler view shows the time distribution in the call tree by assigning the percentages to the individual nodes that represent method invocations. The slowest methods are also accompanied with an extra percentage figure that indicates the method own contribution time.

JSP tag mapping is one neat little feature, new in XRebel 2.0. Instead of a cryptic runtime name XRebel displays the real JSP tag.

In 2.0, there are some more notable improvements to the existing features. The session component is now able to handle very large HTTP session snapshots. And of course, there's a ton of little UI improvements -- all to make the profiler more pleasant to use.

Links for XRebel:

Sunday, April 5, 2015

Grails 3 Released. Setting up -javaagent

Grails 3 was released just recently and with all the new stuff it looks really-really-really awesome release! (Really hope that Grails will find the new home now). The two key changes for me are 1) moving to Gradle instead of Gant, and 2) building on top of Spring Boot. NOw it looks like it's basically the Gradle project with custom conventions that are derived from Grails 2.x.

For the first time, it feels like Grails is not a toy framework any more :)

What's not that cool (my own very subjective opinion), is the introduction of application.yml. It's almost impossible to modify it without reading the documentation. Even XML version of it (yes!) would have been more practical.

There are many other nice things added - go look for yourself.

Setting up a -javaagent argument for Grails 3

My personal interest with any new framework or server is usually related to the projects I'm working with. Thus, the first thing I wanted to check is how could I set up a -javaagent for Grails 3 application. Turns out, it's not as simple as you would expect.

Thanks to @bsideup, here's the snippet that you'd have to add to build.gradle file to setup a -javaagent argument, given that the agent JAR is located somewhere in file system:

In the example above, xrebel.jar is the agent package that is located somewhere in my file system. One can use the absolute path just fine in there.

Here's the another snippet, with DSL-style:

With this, I can confirm, that XRebel works with Grails 3 :)

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