Testing code that uses Java System Properties

Sometimes your code uses any Java System Property to configure itself. Usually these classes are configuration classes that can get properties from different sources and one of valid one is using Java System Properties.

The problem is how to write tests for this code? Obviously to maintain your tests isolated you need to set and unset them for each test, restoring old value if for example you are dealing with a global property which is already set before executing tests, and of course in case of an error do not forget to unset/restore the value. So arrived at this point the test might look like:

Notice that this structure should be repeated for each test method that requires an specific system property, so this structure becomes a boilerplate code.

To avoid having to repeat this code over and over again, you can use System Rules project which is a collection of JUnit rules for testing code that uses java.lang.System 

The first thing you need to do is add System Rules dependency as test scope in your build tool, which in this case is com.github.stefanbirkner:system-rules:1.16.0.

Then you need to register the JUnit Rule into your test class.

In this case, you can freely set a System property in your test, the JUnit rule will take care of saving and restoring the System property.

That's it, really easy, no boilerplate code and help you maintaining your tests clean.

Just one notice, these kind of tests are not thread safe, so you need to be really cautious when you use for example surefire plugin with forks. One possible way to avoiding this is by using net.jcip.annotations.NotThreadSafe.class capabilities.

We keep learning,

Alex

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