Friday, March 22, 2013

EclipseCon 2013 - Last Minute Preview

Now that I know how to find the way from the Logan Intl Airport to the EclipseCon venue at the Seaport WTC in Boston, it's time for some shameless advertising. Since the conference schedule is again packed with deep technical content and all of you have only limited time, I think it's only fair to tell you in advance what you should expect from the sessions that I am giving (of course all of them are highly recommended ;-).

Mon 1:00PM - 4:00PM: Getting Started With Xtend
Monday is Tutorial Day. In the afternoon, Sven and I will give you a jump start with Xtend, a new programming language which makes the day-to-day tasks of us Java developers a real pleasure. We listened to you and prepared some new and entertaining tasks where you get your hands on Active Annotations and some new, interesting puzzlers. To put a long story short: If you want to learn about the hot new programming language that is developed at Eclipse, come and join our tutorial.

Wed 4:15PM - 4:50PM: Null-Safety on Steroids
I'm happy to tell you that the annotation based null-ness analysis of the Eclipse JDT is getting better and better. Since the recent milestones, they do include fields into the analysis and allow to inherit the null specification from super types so the analysis results become much more reliable and easier to adopt. Nevertheless, the JDTs approach is sometimes still based on assumption which I consider ... how shall I put that ... not really pragmatic. In this session, I want to outline the pros and cons of the current state of null analysis in Eclipse. Furthermore, I will talk about other approaches to tackle the occasional NPE that each of us developers is familiar with. I want to discuss the implications of the different solutions and offer advise on how to deal with them.

Thu 11:00AM - 11:35AM: Xtext - More Best Practices
My third session at this year's EclipseCon is a follow-up talk to another one that I gave in Reston, last year. In this years edition of the Xtext - Best Practices, I will focus on other topics, especially on the adoption of the Xbase expressions. If you want to learn more about those, I can also highly recommend Jan's talk on Java DSLs with Xtext on Tuesday.

Anyway, there are still some things to prepare and there is never enough time for polishing. Obviously there are a lot more interesting sessions scheduled than I can list here. I'm really looking forward to a great conference and an intense week packed with interesting discussions. See you in Boston!

Thursday, March 21, 2013

Pimp My Visitors

One of the most noteworthy features of Xtend 2.4 are the Active Annotations. They allow you to participate in the compile process of Xtend code to the Java source code. In fact, you implement some kind of a mini code generator and hook into the Xtend compiler - and all this via a lightweight library. And the IDE is fully aware of all the changes that you do to the generated Java artifacts. The astonishing about it, that this approach allows you to develop the annotation processor and the client code side by side.

Are you facing a repetitive coding problem and want to automate that? Nothing could be simpler. Just annotate your class and implement an Active Annotation and you are good.

Which brings me to design patterns. Those often require a lot of boiler plate code since these patterns describe a blue print on how several objects interact with each other to solve a specific problem. So they are quite useful but also verbose by definition. One of the most tedious examples is the visitor pattern. Since I actually like to use a visitor to handle heterogeneous data structures (you know, decoupling and externalized traversal can be quite convenient) I decided to write a small annotation that creates all the fluff around this pattern on the fly.

In order to implement a visitor, I just have to annotate the root type in the hierarchy and all the accept methods as well as the base class of the visitor implementation are automatically unfolded. You don't even have to define the base class for the visitor itself. The following small example basically expands to the very same, verbose Java code as in the example on Wikipedia .

Especially amazing is the fact, that this allows to define different kinds of visitors easily. Your accept-method has to take additional arguments? Just list them in the prototype method signature. You want to return a result? Nothing's easier than that - simply declare a different return type. The base implementation for all concrete visitors is already known? Just add the method body to the prototype and it will be moved to the visitor itself. Have a look at the complete example to see how beautiful this approach is. If you want to learn more about Active Annotations, you may want to dive into the documentation on and install Xtend to get your hands dirty.

Monday, January 21, 2013

Java Hacks - Changing Final Fields

Changing final fields? Really? Which may sound crazy at a first glance may be helpful in order to implement mocks or fix-up libraries that don't expose the state that you really wanted them to expose. And after all there is not always a fork me button available. But really: Final fields? Yes, indeed. You shall never forget: There is no spoon.

Let's consider the following data class Person that we want to hack.
Once a person was instantiated, it is not possible to change the value of the field name, is it?

Reflection To The Rescue

Fortunately - or unfortunately - Java allows to access fields reflectively, and if (ab)used wisely, it is possible to change their value, too - even for final fields. Key is the method Field#setAccessible. It allows to circumvent visibility rules - which is step one - and interestingly it also implicitly allows change the value of instance fields reflectively, if they were marked as accessible.
Modifying static fields is a little trickier. Even if #setAccessible was invoked, the runtime virtual machine will throw an IllegalAccessException (which I would expect anyway) because one 'Can not set static final my.field.Type field' even though that was perfectly ok for instance fields. And since there is still no spoon - there is a way out. And again it's based on reflection, but this one's a little trickier. If we don't just set the field accessible but also change its modifiers to non-final, it's ok to alter the value of the field.
This hack will allow to change the value of static fields, too. That is, as long as they are not initialized with literals that will be inlined by the compiler. Those include number literals and string literals, which are compiled directly into the call site to save some computation cycles (yes, refering to String constants from other classes does not introduce a runtime dependency to those classes). Despite those cases, other common static field types like loggers or infamous singletons can be easily modified at runtime and even (re)set to null.

The complete code looks like this and as promised, it will print the new name of the person to the console and the changed default name, too. But keep in mind: Don't do this at home!