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Category: Testing

JUnit and static fields

Published on December 18, 2025

I’m sure you’ve seen this pattern of using static fields to maintain application state:

public class AppState {

    static MovieRepository movieRepository;
    static StarRepository starRepository;

    static {
        AppState.movieRepository = new MovieRepository();
        AppState.starRepository = new StarRepository();
    }
}

One reason this is a bad idea is it can cause inconsistent test results. If one test modifies the application state and then a second test runs, the second test will use the first test’s state.

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Testing System.exit()

Published on October 30, 2022

Using JUnit for testing System.exit() calls from application code can be tricky. This is because System.exit() terminates the JVM running it. If you’re running JUnit, this is the JUnit runner. If JUnit invokes System.exit() in application code it will end your test without deciding a success / fail status and will also terminate the test run.

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Testing Spring reactive WebClient

Published on January 15, 2022

Spring WebClient is the reactive replacement for the legacy RestTemplate. It has a more modern fluent API and first class support for Reactive Streams. This means it supports non-blocking, asynchronous responses.

However, the reactive WebClient does not yet have the mature test support that RestTemplate now has. There is not yet a standard recipe to test Spring WebClient applications. No doubt support will be improved in future versions but for now, here’s what works for me.

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Unit test time based logic

Published on October 23, 2018

A standard unit testing problem is how to unit test code that has a dependency on dates or times. For example a method that returns a greeting according to the time of day:

public String timeOfDayGreeting() {
    LocalTime now = LocalTime.now();
    if (now.isBefore(LocalTime.NOON)) {
        return "Good morning";
    } else if (now.isBefore(LocalTime.of(18, 00))) {
        return "Good afternoon";
    } else {
        return "Good evening";
    }
}

If we were to call this method from a test fixture (say JUnit), it would return different values depending on when the test was run. This is not ideal. Unit tests should pass or fail consistently.

Here’s a simple solution for testing time based code.

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Test First React part 1: setup and first tests

Published on August 25, 2018

React is a great choice for writing test first client side Javascript. The test ecosystem is mature enough to enable test first development of complex components. This article shows how to build a React component test first and introduces supporting test libraries Jest and Enzyme. In the next article we’ll look at more advanced testing including API testing and module mocking.

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Test System.out with JUnit

Published on May 12, 2017

Edit: see also the follow up article on how to Test log4j with JUnit if you’re interested in specifically testing log output.

Just occasionally, it can be useful to verify output to System.out in a unit test. For example, if you’re testing application logging or if you’re using log output to sense some other behaviour. It can be tricky to properly mock behaviour of System.out but fortunately, it is possible to test System.out with JUnit.

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