Innocent fun with net send

What happens when you bore a developer to the point when he begins to toy around with the net send tool? You’ll gain a valueable insight about the usefulness of message dialog boxes.

beamed-xmas-treeBecause it is the holiday season and most of us have a straining year of hard work behind us, let me just tell you a little story with more fun than moral in it. The story really happened, but the circumstances and details are altered to protect the innocent.

The setup

Imagine a full day of boring training workshops with a dozen developers in one room, each sitting behind a computer and trying to mimic the tiresome click orgy the instructor presents. Between the developers, there is one web-designer, clearly distinguishable by looks and questions. The workshops drags on and on, until Marvin, the protagonist of our story, loses interest in the clicking and sets out to explore the computer and its possibilities. This all happens a decade ago, when terminal servers were still new and fancy and windows was an open book for those who could read it. The computers were installed by the workshop host and used with a guest login.

The exploration

Marvin notices that the IP address of every computer was written on the computer case. It was just a matter of inconspicuous looks to gather the addresses of the neighbouring machines. The next step was to open a command shell – which was available without any tricks – and try to net send a message to his own machine. Net send was essentially a system service that listened to network messages on a specific port and displayed them as a dialog. So if you’d net send a message to a computer, it would be displayed in a message box in the middle of the screen on top of all active windows with a caption identifying the sender. The user had to acknowledge the dialog by clicking the button to be able to proceed in his original windows. In summary, net send was the perfect remote distraction tool. And it worked: Marvin was able to message himself with net send. The terminal server even disguised the real sender by sending the message with its address instead of the guest machine’s. Now Marvin could anonymously open modal message boxes with a custom message on every computer in the room, given that he knew its address. The workshop promised to be fun again.

The first reaction

After making up some witty messages, Marvin collected all his mental willpower to act indifferent while slowly typing the first message to his neighbour. It just read “Harddisk error” and was only a test drive if he was able to pull this prank without bursting out in laughter or being identified as the source. If he could message his neighbour without him noticing, he could message everybody in the room. After the net send command was complete, Marvin paused a bit and used his little finger to tap enter on the numerical block of the keyboard, to not draw attention to his keyboard pattern. As soon as the command was acknowledged, his neighbour let out a muffled groan and clicked the message box away without even reading it.

The messages

After that, the messages were longer and more sophisticated. After the first few messages, Marvin guessed the pattern in which the IP addresses were located in the room and sent messages to nearly everybody else attending the workshop. Some messages read “Virus found! Need to manually reboot the computer.”, others “Keyboard error. Press Enter to continue.” and the like. The reactions from the developers in front of the machines were always the same: A fretful sound and an acknowledging click without the slightest hesistation. Nobody rebooted or checked the keyboard. The messages were just dismissed and immediately forgotten like a temporary annoyance. Even when the message grew as long as two full sentences, the recipient just clicked it away.

The highlight

During a short recess, Marvin planned the ultimate net send attack: a message on the presenter’s computer, precisely timed to fit the workshop content. He went to the instructor and asked some question while memorizing the IP address of the machine that was connected to the beamer. If he sends a message to this computer, it would be shown on the beamer to the whole audience and the instructor. He used the remaining recess time to formulate the perfect message. The lectures began again, everybody took their seat and concentrated on the topic again. A few minutes into the workshop, Marvin hit enter and the message box appeared on the wall:

Attention! The beamer is overheating. Only a few minutes left before critical temperature level is reached and shutdown is forced to prevent damage.

Everybody stopped and gasped while they finally read a message. Probably only missing the dialog title that clearly stated that the message came from the terminal server, the sole non-developer in the room, the web-designer, asked the one and only legitimate question: “How can the beamer send this message to the computer over the VGA cable?”

Only a split-second later, a developer answered with “there is a standard for that”. Another one chimed in: “your computer also knows which monitor is attached through that cable”. A third suggested a solution: “We might just turn it off a few minutes to cool down. It’ll be okay afterwards.” Clearly out of his comfort zone, the instructor decided: “No, we just had a recess and I’m behind schedule. We’ll see how long this beamer bears with us.”

The moral of the story

Surprisingly, the beamer lasted the whole rest of the day and many days afterwards, without any further hickups. One attendee of the workshop silently laughed for about an hour and the day went by a lot faster. But the most surprising thing was that the only person that grasped the real marvel of the situation was the person with the least technical knowledge in the room. All the seasoned developers missed every clue that there was something fishy with a beamer communicating with the computer over a VGA cable and opening dialog boxes on the computer (and not just in-picture). And nobody reads the text in a dialog box ever. Especially not the title bar!


Marvin wants to apologize to everybody he bothered during this workshop. It was a fun idea originating from boredom, but it turned into a fascinating techno-social experiment. He says he learnt a valueable lesson that day, even if he doesn’t remember any content of the training itself.

Thoughts about TDD

Thoughts and links about test driven development

First a disclaimer: I think tests are a hallmark for professional software development, I like to write tests before the implementation but that’s not always easy or simple (for the difference please refer to Simple made easy). I find it hard to grasp test driven development (TDD) though. The difference between test first and test driven lies in the intention: in both cases tests are written before any implementation code but in TDD the tests drive the design of your implementation.

The problem with opinions of TDD is there are mostly extreme positions: some think “TDD is the (next) holy grail” or the ones which dismissed it. Though reading between the lines there are great discussions about how to do it and what problems arise. Many people (me included) are really trying to get value from TDD. Testing should be fun.
One way in letting the tests drive the way you develop is proposed by Uncle Bob: transformation priority premise. He proposes a list of transformations which introduce new or replace existing constructs like replacing a constant by a variable or adding more logic and gives them a priority. Only if you cannot use a high priority transformation to get the test to pass you look at a transformation with a lower priority.
But how do you determine what you should test next or even which is the first test?
Taking the typical Conway’s game of life kata as an example one thing struck me: I could only get the TDD to work smoothly when I started with the data structure. But why that? Naturally I start with the algorithm (in this case the rules) and write the first test for it. But upon further inspection of the problem and deeper (domain) knowledge it seems the data structure is way more important for solving this kata. So you need to know where the journey goes along beforehand, not every step you will take but the big picture: first the data structure, then the rules in this example. Maybe you should start with the integrations or the functional tests and break them down into units.
What are your experiences using TDD? Do you use or want to use TDD?

An experiment about communication through tests

How effectively communicates our test code? We wanted to know if we were able to recreate a software from its tests alone. The experiment gained us some worthwile insights.

lrg-668-wuerfelRecently, we conducted a little experiment to determine our ability to communicate effectively by only using automatic tests. We wanted to know if the tests we write are sufficient to recreate the entire production code from them and understand the original requirements. We were inspired by a similar experiment performed by the Softwerkskammer Karlsruhe in July 2012.

The rules

We chose a “game master” and two teams of two developers each, named “Team A” and “Team B”. The game master secretly picked two coding exercises with comparable skill and effort and briefed every team to one of them. The other team shouldn’t know the original assignment beforehands, so the briefings were held in isolation. Then, the implementation phase began. The teams were instructed to write extensive tests, be it unit or integration tests, before or after the production code. The teams knew about the further utilization of the tests. After about two hours of implementation time, we stopped development and held a little recreation break. Then, the complete test code of each implementation was transferred to the other team, but all production code was kept back for comparison. So, Team A started with all tests of Team B and had to recreate the complete missing production code to fulfill the assignment of Team B without knowing exactly what it was. Team B had to do the same with the production code and assignment of Team A, using only their test code, too. After the “reengineering phase”, as we called it, we compared the solutions and discussed problems and impressions, essentially performing a retrospective on the experiment.

The assignments

The two coding exercises were taken from the Kata Catalogue and adapted to exhibit slightly different rules:

  • Compare Poker Hands: Given two hands of five poker cards, determine which hand has a higher rank and wins the round.
  • Automatic Yahtzee Player: Given five dice and our local Yahtzee rules, determine a strategy which dice should be rerolled.

There was no obligation to complete the exercise, only to develop from a reasonable starting point in a comprehensible direction. The code should be correct and compileable virtually all the time. The test coverage should be near to 100%, even if test driven development or test first wasn’t explicitely required. The emphasis of effort should be on the test code, not on the production code.

The implementation

Both teams understood the assignment immediately and had their “natural” way to develop the code. Programming language of choice was Java for both teams. The game master oscillated between the teams to answer minor questions and gather impressions. After about two hours, we decided to end the phase and stop coding with the next passing test. No team completed their assignment, but the resulting code was very similar in size and other key figures:

  • Team A: 217 lines production code, 198 lines test code. 5 production classes, 17 tests. Test coverage of 94,1%
  • Team B: 199 lines production code, 166 lines test code. 7 production classes, 17 tests. Test coverage of 94,1%

In summary, each team produced half a dozen production classes with a total of ~200 lines of code. 17 tests with a total of ~180 lines of code covered more than 90% of the production code.

The reengineering

After a short break, the teams started with all the test code of the other team, but no production code. The first step was to let the IDE create the missing classes and methods to get the tests to compile. Then, the teams chose basic unit tests to build up the initial production code base. This succeeded very quickly and turned a lot of tests to green. Both teams struggled later on when the tests (and production code) increased in complexity. Both teams introduced new classes to the codebase even when the tests didn’t suggest to do so. Both teams justified their decision with a “better code design” and “ease of implementation”. After about 90 minutes (and nearly simultaneous), both teams had implemented enough production code to turn all tests to green. Both teams were confident to understand the initial assignment and to have implemented a solution equal to the original production code base.

The examination

We gathered for the examination and found that both teams met their requirements: The recreated code bases were correct in terms of the original solution and the assignment. We have shown that communication through only test code is possible for us. But that wasn’t the deepest insight we got from the experiment. Here are a few insights we gathered during the retrospective:

  • Both teams had trouble to effectively distinguish between requirements from the assignment and implementation decisions made by the other team. The tests didn’t transport this aspect good enough. See an example below.
  • The recreated production code turned out to be slightly more precise and concise than the original code. This surprised us a bit and is a huge hint that test driven development, if applied with the “right state of mind”, might improve code quality (at least for this problem domain and these developers).
  • The classes that were introduced during the reengineering phase were present in the original code, too. They just didn’t explicitely show up in the test code.
  • The test code alone wasn’t really helpful in several cases, like:
    • Deciding if a class was/should be an Enum or a normal class
    • Figuring out the meaning of arguments with primitive values. A language with named parameter support would alleviate this problem. In Java, you might consider to use Code Squiggles if you want to prepare for this scenario.
  • The original team would greatly benefit from watching the reengineering team during their coding. The reengineering team would not benefit from interference by the original team. For a solution to this problem, see below.

The revelation

One revelation we can directly apply to our test code was how to help with the distinction between a requirement (“has to be this way”) and implementator’s choice (“incidentally is this way”). Let’s look at an example:

In the poker hands coding exercise, every card is represented by two characters, like “2D” for a two of diamonds or “AS” for an ace of spades. The encoding is straight-forward, except for the 10, it is represented by a “T” and not a “10”: “TH” is a ten of hearts. This is a requirement, the implementator cannot choose another encoding. The test for the encoding looks like this:

public void parseValueForSymbol() {
  assertEquals(Value._2, Value.forSymbol("2"));
  assertEquals(Value._10, Value.forSymbol("T"));
  assertEquals(Value.ACE, Value.forSymbol("A"));

If you write the test like this, there is a clear definition of the encoding, but not of the underlying decision for it. Let’s rewrite the test to communicate that the “T” for ten isn’t an arbitrary choice:

public void parseValueForSymbol() {
  assertEquals(Value._2, Value.forSymbol("2"));
  assertEquals(Value.ACE, Value.forSymbol("A"));

public void tenIsRequiredToBeRepresentedByT() {
  assertEquals(Value._10, Value.forSymbol("T"));

Just by extracting this encoding to a special test case, you emphasize that you are aware of the “inconsistency”. By the test name, you state that it wasn’t your choice to encode it this way.

The improvement

We definitely want to repeat this experiment again in the future, but with some improvements. One would be that the reengineering phases should be recorded with a screencast software to be able to watch the steps in detail and listen to the discussions without the possibility to interact or influence. Both original teams had great interest in the details of the recreation process and the problems with their tests. The other improvement might be an easing on the time axis, as with the recorded implementation phases, there would be no need for a direct observation by a game master or even a concurrent performance. The tasks could be bigger and a bit more relaxed.

In short: It was fun, challenging, informative and reaffirming. A great experience!

Web apps: Security is more than you think

Security in web apps is an ever increasing important topic: in this post we take a look at injection attacks especially SQL injection, the number one OWASP security problem.

Security in web apps is an ever increasing important topic besides securing the machine or your web/application containers on which your apps run you need to deal with some security related issues in your own apps. In this article we take a look at the number one (according to OWASP)risk in web apps:

Injection attacks

Every web app takes some kind of user input (usually through web forms) and works with it. If the web app does not properly handle the user input malicious entries can lead to severe problems like stealing or losing of data. But how do you identify problems in your code? Take a look at a naive but not uncommon implementation of a SQL query:

query("select * from user_data where username='" + username + "'")

Using the input of the user directly in a query like this is devastating, examples include dropping tables or changing data. Even if your library prevents you from using more than one statement in a query you can change this query to return other users’ data.
Blacklisting special characters is not a solution since you need some of them in your input or there are methods to circumvent your blacklists.
The solution here is to proper escape your input using your libraries mechanisms (e.g. with Groovy SpringJDBC):

query("select * from user_data where username=:username", [username: username])

But even when you escape everything you need to take care what you inject in your query. In this example all data is stored with a key of

query("select * from user_data where key like :username '.%' ", [username: username])

In this case everything will be escaped correctly but what happens when your user names himself % ? He gets the data of all users.

Is SQL the only vulnerable part of your app? No, every part which interprets your input and executes it is vulnerable. Examples include shell commands or JavaScript which we will look at in a future blog post.

As the last query showed: besides using proper escaping, setting your mind for security problems is the first and foremost step to a secure app.

A small test saves the day

You think a method is too trivial to write a test for it? Think again if the method is mission-critical!

Just recently, I had to write a connection between an existing application and a new hardware unit. This is a fairly common job for our company, even considering the circumstances that I’d never even seen the hardware, let alone being able to connect to it. The hardware unit itself was rather big and it was installed in a security sensitive area with restricted access. So, I only got a specification of the protocol to use and a description of the hardware’s features.

Our common procedure to include hardware dependent modules into an application is to write two implementations of the module: One implementation is the real deal and interacts with the hardware over ethernet, USB, serial port or whatever proprietary communication device is used. This version of the module can only work as intended if the hardware is present. The other implementation acts as an emulation of the hardware, without any dependencies. If you are familiar with unit tests, think of a big test mock. The emulation version is used during development to test and run the application without requirements about the hardware. There are a lot of subtle pitfalls to consider and avoid, but on a bird-view level of abstraction, these interchangeable implementations of a module enable us to develop software with hardware dependencies without need for the actual hardware.

The first piece of code that’s used of a module is a factory/builder class that chooses between the available implementations, based on some configuration entry (or hardware availability, etc.). A typical implementation of the responsible method might look like this:

public HardwareModule createFor(ModuleConfiguration configuration) {
  if (configuration.isHardwarePresent()) {
    new RealHardwareModule();
  return new EmulatedHardwareModule();

If the configuration object says that the hardware is present, the real implementation is used, subsequentially opening a connection to the hardware and talking the client side of the given protocol. Otherwise, the emulation is created and returned, maybe opening a debug GUI window to display certain internal states and values and providing controls to mess with the application during development.

The method itself looks very innocent and meager. There is not much going on, so what could possibly go wrong?

I’m not the most eager test-driven developer in the world, I have to admit. But I see the value of tests (and unit tests in particular) and adhere to the A-TRIP rules defined by Andy Hunt and (pragmatic) Dave Thomas:

  • Automatic
  • Thorough
  • Repeatable
  • Independent
  • Professional

For a complete definition of the rules, read the linked blog entry or, even better, buy the book. It’s small and cheap, but contains a lot of profound basic knowledge about unit testing.

The “Thorough” rule is more of a rule of thumb than a hard scientific formula for good unit tests: Always write a test if you’ve found a bug or if the code you’re writing is mission-critical. This was when my gut feeling told me that while the method above might seem trivial, it is definitely essential for the hardware module. So I wrote a test:

  public void providesEmulationIfUnspecified() {
    HardwareModuleFactory factory = new HardwareModuleFactory();
    HardwareModule hardware = factory.createFor(configuration(""));
    assertEquals("not the hardware emulation", EmulatedHardwareModule.class, hardware.getClass());

  public void providesEmulationIfHardwareAbsent() {
    HardwareModuleFactory factory = new HardwareModuleFactory();
    HardwareModule hardware = factory.createFor(configuration("hardware.present=false"));
    assertEquals("not the hardware emulation", EmulatedHardwareModule.class, hardware.getClass());

  public void providesRealImplementationIfHardwarePresent() {
    HardwareModuleFactory factory = new HardwareModuleFactory();
    HardwareModule hardware = factory.createFor(configuration("hardware.present=true"));
    assertEquals("not the real hardware implementation", RealHardwareModule.class, hardware.getClass());

To my surprise, the test immediately went red for the third test method. After double-checking the test code, I was certain that the test was correct. The test discovered a bug in the production code. And being a mostly independent unit test, it pointed to the problematic lines right away: the method implementation above. The helper method named configuration() spared in the code sample was very unlikely to contain a bug.

After a short moment of reading the code again, I corrected it (note the added return statement in line 3):

public HardwareModule createFor(ModuleConfiguration configuration) {
  if (configuration.isHardwarePresent()) {
    return new RealHardwareModule();
  return new EmulatedHardwareModule();

This might not seem like the most disastrous bug ever, but it would have made for a nasty start when I finally would have tried the application with the real hardware. There is nothing more valueable than to be able to keep your cool “in the wild” and work on the real problems like faulty protocol specifications or unexpected/undocumented hardware behaviour. So, my gut feeling (and the Thorough rule) were right and my brain, telling me “skip this petty test” longer than I like to admit, was wrong. A small test for a small method paid off immediately and saved the day, at least for me.

Testing on .NET: Choosing NUnit over MSTest

We sometimes do smaller .NET projects for our clients even though we are mostly a Java/JVM shop. Our key infrastructure stays the same for all projects – regardless of the platform. That means the .NET projects get integrated into our existing continuous integration (CI) infrastructure based on Jenkins. This works suprisingly well even though you need a windows slave and the MSBuild plugin.

One point you should think about is which testing framework to use. MSTest is part of Visual Studio and provides nice integration into the IDE. Using it in conjunction with Jenkins is possible since there is a MSTest plugin for our favorite CI server. One downside is that you need either Visual Studio itself or the Windows SDK (500MB download, 300MB install) installed on the build server in addition to .NET. Another one is that it does not work with the “Express” editions of Visual Studio. Usually that is not a problem for companies but it raises the entry barrier for open source or other non-profit projects by requiring relatively expensive Visual Studio licences.

In our scenarios NUnit proved much lighter and friendlier in installation and usage. You can easily bundle it with your sources to improve self-containment of the project and lessen the burden on the system and tools. If you plug the NUnit tool into the external tools-section of Visual Studio (which also works with Express) the integration is acceptable, too.


If you are not completely on the full Microsoft stack for you project infrastructure using Visual Studio, TeamCity, Sourcesafe et al. it is worth considering choosing NUnit over MSTest because of its leaner size and looser coupling to the Mircosoft stack.

Antipatterns: Convenience Constructors

Lately I stumble a lot upon code I wrote 4 or more years ago. In the light of introducing new features the code gets tested for its quality. One antipattern I’ve found which I had used in the past but which is really hard to extend is convenience constructors.

Lately I stumble a lot upon code I wrote 4 or more years ago. In the light of introducing new features the code gets tested for its quality. One antipattern I’ve found which I had used in the past but which is really hard to extend is convenience constructors. Take a constructor for a command object for example:

    public SetProperty(String filename, String key, String value) {
        this(filename, key, value, null);

    public SetProperty(String filename,
            String key, String value, String comment) {
        this(filename, ReferenceTo.key(key), value, comment);

    public SetProperty(String filename,
            String sectionType, String sectionName,
            String key, String value) {
        this(filename, sectionType, sectionName, key, value, null);

    public SetProperty(String filename,
            String sectionType, String sectionName,
            String key, String value, String comment) {
        this(filename, ReferenceTo.sectionAndKey(sectionType, sectionName, key), value, comment);

    public SetProperty(String filename,
            AdvancedPropertyReference propertyReference,
            String value, String comment) {
        this(filename, propertyReference, value, comment);

    public SetProperty(String filename,
            AdvancedPropertyReference propertyReference,
            String value, String comment) {
        this.propertyReference = propertyReference;
        this.value = value;
        this.comment = comment;

We need to add a new feature which enables us to append properties not just set and replace them. One way could be to extend the class. But this is overkill. Just adding a new parameter flag should suffice. But this would blow up the number of constructors because you need to include a version with and without the new parameter for each (used) constructor. Here an old friend comes to the rescue: design patterns. Looking at the GoF book shows a good solution to the problem: the builder pattern.

public class SetPropertyBuilder {
    private final String filename;
    private String sectionType;
    private String sectionName;
    private String referenceKey;
    private String value;
    private String comment;
    private boolean append;

    public SetPropertyBuilder(String filename) {
        this.filename = filename;

    public SetPropertyBuilder set(String key, String newValue) {
        this.referenceKey = key;
        this.value = newValue;
        return this;

    public SetPropertyBuilder append(String key, String additionalValue) {
        set(key, additionalValue);
        this.append = true;
        return this;

    public SetPropertyBuilder inSection(String type, String name) {
        this.sectionType = type;
        this.sectionName = name;
        return this;

    public SetProperty build() {
        AdvancedPropertyReference reference = ReferenceTo.key(this.referenceKey);
        if (this.sectionType != null && this.sectionName != null) {
            reference = ReferenceTo.sectionAndKey(this.sectionType, this.sectionName, this.referenceKey);
        return new SetProperty(this.filename, reference, this.value, this.comment, this.append);

Now we can eleminate all but one constructor from the SetProperty command. Adding a new property now yields one new method in the builder.

Summary of the Schneide Dev Brunch at 2012-10-14

If you couldn’t attend the Schneide Dev Brunch in October 2012, here are the main topics we discussed neatly summarized.

Two weeks ago, we held another Schneide Dev Brunch. The Dev Brunch is a regular brunch on a sunday, only that all attendees want to talk about software development and various other topics. If you bring a software-related topic along with your food, everyone has something to share. The brunch was so well attended that we had trouble to all find a chair and fit at the table. We had to stay inside as the weather was rainy and too cold for prolonged outdoor sessions. Let’s have a look at the main topics we discussed:

Work hard, play hard

The first topic was a summary of the contents of the documentary movie “work hard play hard” about our modern work places. The documentary is a recommended watch for everyone thinking about joining this side of the industry. It’s beautiful sometimes and very painful to watch most times. You might cherish some of the rougher edges on your workplace afterwards. The DVD is out now.

Dual Monitoring

A short discussion about the efficiency increase that happens just by adding another monitor to your desk. There was no dispute: If you don’t at least try it, you waste money. That’s what I meant when I blogged about the second monitor being an profitable investment. Just one downfall, it shouldn’t end like this.

Management by Directive

Another discussion about the management of large departments. The “directive issuer” manager style is a common sight in this environment. I won’t repeat the discussion itself, but rather add an amusing story about an ex-military commander running a software development company. Enjoy!

Review of the Sneak Preview “Quality Assurance Best Practices in Karlsruhe”

There was a “sneak preview” organised by the VKSI, a local association of software engineers a few weeks ago. The topic of the whole event was “Quality Assurance Best Practices in Karlsruhe“. The event was divided into three independent presentations with different topics:

  • Non-Functional Software Tests” by Gebhard Ebeling: The talk was about realistic load- and performance testing of complex applications (and websites). While the presentation omitted tools and code completely, there were some take-aways even for developers that had never performed these types of tests before. This was arguably the best presentation of the event.
  • Contracts im Software Engineering” by Ben Romberg and Stefan Schürle: This talk was about the benefits of software contracts (think about checked method or class invariants) and the presentation of a particular implementation in Java, namely C4J. The perceived problem with this solution was the rather clumsy source code necessary to define the contracts.
  • MoDisco Software Modernization & Analysis” by Benjamin Klatt: MoDisco builds a model out of source code that is detailled enough to apply meaningful transformations to it and have the exact same source code (plus transformed code) as output. The idea looked very promising, but the presentation lacked actual source code examples. Nonetheless, MoDisco proves that there is a future for modell-driven analysis.

We had a lengthy discussion about software contracts and Design By Contract (DBC) in general. One tool that got mentioned several times was “CoFoJa” from (at least initially) Google.

Book review: Java Application Architecture: Modularity Patterns with Examples Using OSGI

In the rather new book of the Robert C. Martin signature series, Kirk Knoernschild tackles the hard task to teach software architecture through a book. One participant read the book and is very happy about the experience and insight he got from it. The book itself is repetitive at times, but that adds to the accessibility of the topic at hand when you jump right into a chapter. Additionally to the modularity and architecture aspects, you’ll learn OSGI through the code examples. This books gets a recommendation.

Book review: ATDD by Example

Another new book is from Markus Gärtner, of the Kent Beck signature series this time. It takes the reader by the hand and shows a way to use Cucumber, FitNesse and of course Behavior-Driven Development as a tool-and-process framework to implement (Acceptance-) Test Driven Development. None of our participants read the book fully yet, but it’s already a promising start. If you are looking for a new book about testing (after having read the great GOOS book), don’t hesitate. Another recommendation to read.

Visitor design pattern breaks modularization

One participant brought up the problem that he wanted absolute modularization in his application layout, but used a visitor design pattern at some central place. This breaks modularization, as the type information is exposed too much. We discussed the problem with some diagrams and sketches and came up with several alternatives, each with their own advantages and drawbacks. That was a great code design session among seasoned professionals.

Why are services included into Grails?

Another discussion was about the Grails web framework and the necessity for a service layer or service classes explicitly. We sketched out the fundamental architecture of a Grails application and discussed different possible alternatives to a dedicated service layer. There are some nice features about Grails services (like injection by convention, transaction and scope), but nothing really too sophisticated to distinguish them from POGOs. The discussion was open-ended, as usual with complex topics.

Review of a workshop on agile software-engineering

Lately, a participant visited a workshop on agile software-engineering, focussing a lot on SCRUM and XP. The workshop ran for several days and included lots of hands-on exercises. The workshop itself provided not much new content for seasoned agile developers, but served as an accurate and thorough introduction for younger developers. A major part of the workshop were social aspects of agile environments. Concepts like team empowerement are usually not taught in technical workshops. Important additional topics comprised of agile planning and estimation and proper retrospectives. The workshop itself was more of a entry-level introduction to agile development, but very effective in that regard.


As usual, the Dev Brunch contained a lot more chatter and talk than listed here. The high number of attendees makes for an unique experience every time. We are looking forward to the next Dev Brunch at the Softwareschneiderei. And as always, we are open for guests and future regulars. Just drop us a notice and we’ll invite you over next time.

Checking preconditions in advance vs. on demand vs. exceptions

Usually, it is good practice to check certain preconditions before applying operations to input data. This is often referred to as defensive programming. Many people are used to lines like:

public void preformOn(String foo) {
  if (!myMap.containsKey(foo)) {
    // handle it correctly
  // do something with the entry

While there is nothing wrong with such kind of “in advance checking” it may have performance implications – especially when IO is involved.

We had a problem some time ago when working with some thousand wrappers for File objects. The wrappers checked if the given File object actually is a file using the innocent isFile()-method in the constructor which caused hard disk access each time. So building our collection of wrapped files took quite some time (dozens of seconds) and our client complained (rightfully so!) about the performance. Once the collection was built the operations were fast because no checking was needed anymore.

Our first optimization step was deferring the check to the point where the file was actually used. This sped up the creation of the wrappers so it was barely noticeable but processing a bunch of elements took longer because of additional disk accesses. Even though this approach may work for a plethora of situations for our typical use cases the effect of this optimization was not enough.

So we looked at our problem from another perspective: The vast majority of file handles were actually existing and readable files and directories and foreign/unknown files were the exception. Because of this fact we chose to simply leave out any kind of checks and handle the exceptions! Exception handling is often referred to as slow but if exceptions are rare it can make a difference in some orders of magnitude. Our speed up using this approach was enourmous and the client was happy about sub-second responsiveness for his typical operations. In addition we think that the code now expresses more cleary that irregular files really are the exception and not the rule for this particular code.


There are different approaches to handling of parameters and input data. Depending on the cost of the check and the frequency of special input different strategies may prove beneficial both in expressing your intent and the perceived performance of your application.

Solutions to common Java enum problems

More readable solutions to using enums with attributes for categorization or representation.

Say, you have an enum representing a state:

enum State {
  A, B, C, D;

And you want to know if a state is a final state. In our example C and D should be final.
An initial attempt might be to use a simple method:

public boolean isFinal() {
	return State.C == this || State.D == this;

When there are two states this might seem reasonable but adding more states to this condition makes it unreadable pretty fast.
So why not use the enum hierarchy?

A(false), B(false), C(true), D(true);

private boolean isFinal;

private State(boolean isFinal) {
  this.isFinal = isFinal;

public boolean isFinal() {
  return isFinal;

This was and is in some cases a good approach but also gets cumbersome if you have more than one attribute in your constructor.
Another attempt I’ve seen:

public boolean isFinal() {
        for (State finalState : State.getFinalStates()) {
            if (this == finalState) {
                return true;
        return false;

    public static List<State> getFinalStates() {
        List<State> finalStates = new ArrayList<State>();
        return finalStates;

This code gets one thing right: the separation of the final attribute from the states. But it can be written in a clearer way:

List<State> FINAL_STATES = Arrays.asList(C, D)

public boolean isFinal() {
	return FINAL_STATES.contains(this);

Another common problem with enums is constructing them via an external representation, e.g. a text.
The classic dispatch looks like this:

    public static State createFrom(String text) {
        if ("A".equals(text) || "FIRST".equals(text)) {
            return State.A;
        } else if ("B".equals(text)) {
            return State.B;
        } else if ("C".equals(text)) {
            return State.C;
        } else if ("D".equals(text) || "LAST".equals(text)) {
            return State.D;
        } else {
            throw new IllegalArgumentException("Invalid state: " + text);

Readers of refactoring sense a code smell here and promptly want to refactor to a dispatch using the hierarchy.

A("A", "FIRST"),
D("D", "LAST");

private List<String> representations;

private State(String... representations) {
  this.representations = Arrays.asList(representations);

public static State createFrom(String text) {
  for (State state : values()) {
    if (state.representations.contains(text)) {
      return state;
  throw new IllegalArgumentException("Invalid state: " + text);

Much better.