XStream is nearing a release

XStream is a simple object -> xml -> object serialization tool. There are many other tools that do similar things out there. Here are some of the features that distinguish it from the others:

* Very fast.
* Requires no custom mappings to be created.
* Can serialize objects that have private fields and non-default constructors.
* Handles arbitary objects and collections.
* Produces very clean XML; the kind a human would write.
* Does not duplicate any information in the XML that can be obtained via reflection.
* Decoupled from XML implementations. Use it with DOM, JDOM, DOM4J, or even non-XML streams (such as custom configuration objects, YAML or properties files).
* Open source, BSD license.

Here’s an example chunk of XML produced by XStream:




New Book: Java Open Source Programming

Our new book, (With the snappy title, Java Open Source Programming : with XDoclet, JUnit, WebWork and Hibernate) hits the shelves this months. A joint effort between Pat Lightbody, Ara Abrahamian, Mike Cannon-Brookes and myself.

This book :

* Highlights many of the complexities of J2EE and shows how to leverage best of breed open-source tool to reduce or even eliminate these.
* Introduces you to some of the coolest open-source projects in the history of mankind.
* Demonstrates the test-driven-development to drive design (and even some tests).
* And most importantly of all… shows how to combine these tools and techniques to deliver an end-application.

Go pre-order!

Tutorial: Using mock objects to drive top-down development

Tim Mackinnon and Nat Pryce and Steve Freeman and I are presenting a session on how mock objects can be used to drive code from the top (requirements first, infrastructure last) to produce high quality, clean and decoupled object designs that allow for business change.

Come see us at:
* XPDay Benelux – Fri 21st Nov 2003, Breda, Netherlands
* XPDay London – Tue 2nd Dec 2003, London, UK.
* OT2004 – Tue 30 Mar 2004, Cambridge, UK.


Mock objects are usually regarded as a programming technique that merely supports existing methods of unit testing. But this does not exploit the full potential of mock objects. Fundamentally, mock objects enable an iterative, top-down development process that drives the creation of well designed object-oriented software.

This tutorial will demonstrate the mock object development process in action. We will show how using mock objects to guide your design results in a more effective form of test driven development and more flexible code; how mock objects allow you to concentrate more on end-user requirements than on infrastructure; and how the objects in the resultant code are small and oosely coupled, with well-defined responsibilities.

* Brief introduction to mock objects and the dynamic mock API.
* The mock object design process explained.
* Top down vs. bottom up design.
* What to mock. And what not.


Test Driven Development is not about testing

Dan writes:

bq. “Writing the test before you write the code focuses the mind – and the development process – on delivering only what is absolutely necessary. In the large, this means that the system you develop does exactly what it needs to do and no more. This in turn means that it is easy to modify to make it do more things in the future as they are driven out by more tests. ”

Read the rest here.

Breaking my silence

aslak_hellesoy: i was stoned when you put cheese in my source code

Toywatch: Whiteboard Photo

Mike Brown pointed me to Whiteboard Photo – a simple little tool for converting crappy photos of whiteboards into something usable. It even distorts the image to compensate for the perspective the photo was taken at.


A mere $249 – good value for money.

OpenSymphony.com gets a facelift

OpenSymphony.com is looking nicer these days.

Discuss your maintainability patterns

If you’re anywhere near London on Tuesday 29th July, come along to the eXtreme Tuesday Club (XTC) and we can talk about the coding and design practices that make for maintainable code in the long term.

This is how we’ll do it. Prepare in advance.

Meanwhile, I’ve seen some excellent feedback on what makes for good maintainable code. There is a central overlap of core techniques that developers who have to maintain code in the long term have grown to adopted. And yet, even knowing these things work, they often slip our minds.

Design by contract: testing implementations of interfaces

Here’s a nice way of associating contracts with interfaces and testing the implementations conform.

A sample interface:

interface CheeseMaker {
int getCheeseCount();
void addCheese(Cheese cheese);

With the contracts:

* there should be zero cheeses on creation.
* adding a cheese should increment the count.
* unless the cheese is a duplicate.
* when adding a cheese, the cheese cannot be null.

You can create an abstract unit test for this interface that tests these contracts. The only thing it doesn’t do is provide an implementation – instead it has an abstract factory method.

public abstract class CheeseMakerTest extends TestCase {

// abstract factory method
protected abstract CheeseMaker createCheeseMaker();

public void testZeroCheesesOnCreation() {
CheeseMaker cheeseMaker = createCheeseMaker();
assertEquals(0, cheeseMaker.getCheeseCount());

public void testAddingACheeseIncrementsCount() {
CheeseMaker cheeseMaker = createCheeseMaker();
cheeseMaker.addCheese(new Cheese("Cheddar"));
cheeseMaker.addCheese(new Cheese("Wensleydale"));
assertEquals(2, cheeseMaker.getCheeseCount());

public void testDuplicateCheesesDoNotIncrementCount() {
CheeseMaker cheeseMaker = createCheeseMaker();
cheeseMaker.addCheese(new Cheese("Cheddar"));
cheeseMaker.addCheese(new Cheese("Cheddar"));
assertEquals(1, cheeseMaker.getCheeseCount());

public void testNullCheeseCausesIllegalArgumentException() {
CheeseMaker cheeseMaker = createCheeseMaker();
try {
fail("expected exception");
} catch (IllegalArgumentException e) {} // good


Now, every time you create an implementation of CheeseMaker, the test should extend CheeseMakerTest and you inherit the contract tests for free.

For example:

public class BigCheeseMaker implements CheeseMaker {
// ... ommited for sanity

public class BigCheeseMakerTest extends CheeseMakerTest {

// factory method implementation
protected CheeseMaker createCheeseMaker() {
return new BigCheeseMaker();

// ... any additional tests go here


It’s important to note that this tests the contract but does not enforce them. It’s very flexible and there are very few contracts (if any at all) that couldn’t be expressed in a unit-test.

This helps defensive development with the added safety of unit-tests.

As a bonus, you can use TestDox to generate documentation for interfaces (much more useful than implementations), like so:

h4. CheeseMaker

* Zero cheeses on creation.
* Adding a cheese increments count.
* Duplicate cheeses do not increment count.
* Null cheese causes illegal argument exception.

Maintainability patterns

I love tech. Look at all the fun stuff that’s happening in this development wonderland at the moment.

We have libraries to do everything under the sun. Persistence, web-apps, security, code generation, presentation, remoting, messaging, concurrency, XML processing, testing, containers, you name it. In Java-land, opensource seems to be dominating the market and we’re even starting to see books dedicated to the subject (end of shameless plug).

It’s easy to get seduced by the sparkle of how these tools can make magic happen quickly. With these tools, a system can be delivered in record time. Win!

But delivery is really just a short term win. After delivery comes maintenance and this is where most development teams get stung in the long term. It becomes harder and harder to add or refine features as time goes on. Chris Matts pointed out that over the life of a project, the cost spent on development is insignificant compared to the cost of maintenance.

It’s impossible for a sexy library alone to make your code maintainable. Using JUnit or an AOP library will not solve the problems. It’s the *techniques* that complement these that make applications more maintainable. The tools are just the icing on the cake and you really don’t have to use them.

The techniques are unfortunately less seductive than the tools, yet far more important. I often find myself answerless when faced with explaining the benefits of separating the interface from the implementation, using mock objects, inversion of control, aspect oriented programming or avoiding statics.

In most cases, I found there is no benefit of using a technique in isolation. Instead, they benefit each other. It’s the relationship between the techniques that are important and they all support one thing – maintainability. The relationships are patterns.

So, I have a theme for upcoming posts on the blog: *maintainability patterns*. What techniques we can employ to make code more maintainable and how they relate to other techniques.