Properly set (system) properties in JUnit 5

Question

We are using an approach similar to System Rules to handle (system) properties in our JUnit 4 tests. The main reason for this is to clean up the environment after each test, so that other tests do not inadvertently depend on possible side effects.

Since JUnit 5 is released, I wonder if there is a "JUnit 5 way" of doing this?

Answer 1

You can use the extension API. You could create an annotation which defines your extension to a test method.

import org.junit.jupiter.api.extension.ExtendWith;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@ExtendWith(SystemPropertyExtension.class)
public @interface SystemProperty {

    String key();

    String value();
}

Then, you can create the extension class:

import org.junit.jupiter.api.extension.AfterEachCallback;
import org.junit.jupiter.api.extension.BeforeEachCallback;
import org.junit.jupiter.api.extension.ExtensionContext;

public class SystemPropertyExtension implements AfterEachCallback, BeforeEachCallback {

    @Override
    public void afterEach(ExtensionContext extensionContext) throws Exception {
        SystemProperty annotation = extensionContext.getTestMethod().get().getAnnotation(SystemProperty.class);
        System.clearProperty(annotation.key());
    }

    @Override
    public void beforeEach(ExtensionContext extensionContext) throws Exception {
        SystemProperty annotation = extensionContext.getTestMethod().get().getAnnotation(SystemProperty.class);
        System.setProperty(annotation.key(), annotation.value());
    }
}

Finally, you can annotate your test with properties:

@Test
@SystemProperty(key = "key", value = "value")
void testPropertey() {
    System.out.println(System.getProperty("key"));
}

This solution supports only one system property for each test. If you want to support multiple test, you could use a nested annotation and the extension could handle this as well:

@Test
@SystemProperties({
    @SystemProperty(key = "key1", value = "value"),
    @SystemProperty(key = "key2", value = "value")
})
void testPropertey() {
    System.out.println(System.getProperty("key1"));
    System.out.println(System.getProperty("key2"));
}