Properly set (system) properties in JUnit 5

There is JUnit Pioneer, a "JUnit 5 extension pack". It comes with @ClearSystemProperty and @SetSystemProperty. From the docs:

The @ClearSystemProperty and @SetSystemProperty annotations can be used to clear, respectively, set the values of system properties for a test execution. Both annotations work on the test method and class level, are repeatable as well as combinable. After the annotated method has been executed, the properties mentioned in the annotation will be restored to their original value or will be cleared if they didn't have one before. Other system properties that are changed during the test, are not restored.

Example:

@Test
@ClearSystemProperty(key = "some key")
@SetSystemProperty(key = "another key", value = "new value")
void test() {
    assertNull(System.getProperty("some key"));
    assertEquals("new value", System.getProperty("another key"));
}

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"));
}