In my daily work, I heard many discussions about Test Driven Development (TDD). Different people have different opinions about it. Some believe that testing is a must-have for development, any piece of code should be tested before delivery. Some believe that testing slows down the development progress, it hurts the system (re-)design and the randomly failing tests makes the situation even worst. Some simply disabled the tests when developing…

Face to the confusion about TDD: the theoretical version on the internet and the practical version in real life, I decided to experience TDD in my personal projects and see how it goes. Today, 3 months after, here’s a resume of what I learnt and what works best. In this article, we will talk about:

  • Why using TDD?
  • The importance of clean architecture
  • Using IDE to help you
  • Iteration and regular reviews
  • Write tests during development (not after)
  • Understand the goals of your tests
  • Prefer unit test over other tests
  • Speed up execution
  • Legacy code
  • Limits of TDD and reconsideration

Why Using TDD?

Here’re some of the benefits of using TDD:

  • TDD brings confidence and assurance of the code.
  • TDD reduces bugs in production.
  • TDD avoids manual testing.
  • TDD proves your design

Clean Architecture

TDD approach works best if your application has a clean architecture. In other words, if everything is well designed. It makes your tests easier to write, easier to read, and easier to refactor.

  • Pure functions. Pure functions always run the same output, and have no side effects. Testing pure functions reduces dependencies on I/O, network, logs etc.
  • Design pattern. Using design pattern correctly, can reduce the cost of set-up and tear-down in tests. For example, using the Factory Method pattern makes the set-up easier, because it encapsulates the knowledge of implementation and moves this knowledge out of the framework.
  • Immutable objects. Using immutable objects avoid testing different states of an object. The object itself contains the same values throughout its life.
  • Single responsibility. If the production code has single responsibility, testing it will be easy. The scenario is simple and straightforward. The test will be self explanatory and no needs for comments.
  • Loose coupling. In a loosely coupled system, each of its components has little or no knowledge of the definitions of other separate components. Therefore, mocking or complex test set-up (integration tests or functional tests) can be avoided.

There’re more examples to list, but I think you get my points: having a clean architecture makes the tests minimalist.

Using IDE

Use your IDE to improve your tests. It can:

  • Create tests more easily
  • Perform the tests
  • Measure the test coverage
  • Debug the tests

If you’re using Eclipse IDE, you can see their documentation about Eclipse/Testing. Please let me know if you find a better page.

If you’re using IntelliJ IDEA, you can see their documentation about Testing. Or check their official YouTube video for Unit Testing and Coverage in IntelliJ IDEA.

Iteration and Regular Reviews

My personal experience shows that TDD works best when you do it in iteration.

At the first place, the application is not yet well designed and is not yet be heavily used. At that moment, having tests everywhere slows down the development cycle and requires test re-write when code is being changed. Therefore, it would be good to a basic test suite, where you feel confident about the expected behavior.

Then, the application grows. You’re adding more functionality. New functionality is not yet mature, but the base of your application starts to be stable. You know how to plug a new component into your app without changing existing ones. This is a good opportunity to test more about the base!

Later, application is deployed to a test environment and be used against a small data-set. You have some feedback about application against different scenario. Some bugs are discovered or some functionality need to be changed. This is another good opportunity to add more tests: fix the bugs and shorten the verification cycle via tests.

Application is now running in production. Some components are more frequently used than others. Yet another opportunity to have more tests, and increase the test coverage.

Interaction ensures that the test effort is prioritized and applied to the most important part of the codebase.

Write Tests During Development

… and not after. Some developers think testing is a “constraint” and not a tool to help them. This is completely wrong. Test describes a minimal scenario to assert the behavior of a piece of code. It should help you to develop your application faster. If it’s not the case, maybe the architecture of the application needs to be reviewed, or the test itself is not well written.

Writing tests after the development leads to frustration, and doubt about the necessity of having these tests.

Understand the Goals

Writing tests are not the ultimate goal of testing. In my understanding, testing is a “low-cost” way to assure the application behavior. If we take a deeper look, we can split the goal into smaller ones:

  1. Component Behavior: how the component work itself
  2. Application Acceptance: interactions between components
  3. Security Flaws: prevent security flaws in your application

There’re surely other ways to define goals. But the importance of having goals won’t change: it helps prioritization. It makes the test effort spent on the most mission-critical place.

Prefer Unit Test Over Other Tests

It’s important to distinguish the different types of test: unit test, integration test, functional test, performance test, … Some of them are more difficult to write and maintain than others.

Among all types of test, unit test is by far the closest one to the source code. In a few lines, you can prove the behavior of your source code by giving a simple input, executing the code, and asserting the output. The entire process is simple so that it’s easy to write, easy to understand, and easy to maintain.

Speed Up Execution

When you have many tests, it’s important to ensure the tests run quickly so that it will not slow down the development process. Nobody want to wait for hours for the test results.

When using Maven, you can use option -T <nThreads> to configure the level of parallelism. A typical configuration is to use one thread per logical core (1C), to run the multi-thread build:

mvn <goal> -T 1C

In all my personal projects, build is multi-threaded by default. This is done using Maven configuration file maven.config in folder .mvn. It works great and it forces me to consider thread safety problem at the very first place of development:

cat .mvn/maven.config
-T 1C

However, please notice that some Maven plugins or your source code, might not work properly in a multi-thread build.

Legacy Code

While TDD works with well-designed codebase. It’s much more difficult to start with something existing.

Prefer refactoring over testing. I think it’s more interesting to make the architecture cleaner, easier to understand and easier to change (loose coupling) before adding more tests. Otherwise, the tests will require many factors to setup and will certainly slow down the development process.

Add tests when fixing bugs. It prevents the same case to happen again. Do not force yourself to achieve metric, like 100% coverage.

Add tests when adding new features.

Use continuous code analysis. In Java project, you can use continuous code analysis tool, such as SonarQube, to inspect the code quality and test coverage. You can also see test coverage on the new code, committed recently.

Human factors. Hiring great people can resolve many problems at the first day. Legacy code does not become legacy in one day, many factors can lead to such situation: project prioritization, lack of skills, lack of quality awareness, lack of time, lack of tooling, incorrect road-map, business model changes… It’s very important to consider the human factors, because if you are alone to change the situation, and other factors remain the same, then you will have few chance to success.

Limits and Reconsideration

Each coin has two sides. Here’re some limits and reconsideration about TDD:

  • Writing tests takes time
  • When source code changes, you need to change the tests
  • Maintaining tests is a cost (and they might need refactoring, too)
  • Bad written tests are worse than no tests
  • The test execution time might be significant
  • 100% test coverage != 100% bug free
  • Low test coverage is probably error prone codebase. It’s preferable to rethink about the code inspection than other fields first.
  • Test is not the ultimate goal, it aims to help the development process and the maintainability of your application.
  • Test coverage is not a good metric to measure the quality
  • Different constraint when working with legacy code
  • Testing is a team effort

Human factors:

  • Some people hate tests
  • Some people write verbose tests

In this case, clearly, TDD is not on your side. What you can do is to: setup automatic code quality inspection, testing framework so that testing is at least possible. Try your best to add new tests. Each step forward is a new gain.


In this article, we saw the motivation of doing TDD and different ways to do well in different aspects: architecture, IDE, methodology, type of tests, execution timing, legacy code, and limits of TDD. Hope you enjoy this article, see you the next time!