Easily handle Java system properties with JUnit 5 extensions

SystemProperties.java

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

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

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

	SystemProperty[] value();

}

SystemProperty.java

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

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

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

	String key();

	String value() default "";

}

SystemPropertyExtension.java

import java.util.Properties;

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

public class SystemPropertyExtension implements BeforeEachCallback, AfterEachCallback {

	private static final String BACKUP_STORE_KEY = "backup";

	@Override
	public void beforeEach( final ExtensionContext context ) throws Exception {
		final Properties backup = new Properties();
		backup.putAll( System.getProperties() );
		final Store store = context.getStore( Namespace.create( getClass(), context.getRequiredTestMethod() ) );
		store.put( BACKUP_STORE_KEY, backup );

		final Method testMethod = context.getTestMethod().get();
		if ( testMethod.isAnnotationPresent( SystemProperty.class ) ) {
			final SystemProperty prop = testMethod.getAnnotation( SystemProperty.class );
			prepareSystemProperty( prop );
		} else {
			final SystemProperties props = testMethod.getAnnotation( SystemProperties.class );
			Arrays.stream( props.value() ).forEach( this::prepareSystemProperty );
		}
	}

	private void prepareSystemProperty( final SystemProperty prop ) {
		if ( prop.value().isEmpty() ) {
			System.clearProperty( prop.key() );
		} else {
			System.setProperty( prop.key(), prop.value() );
		}
	}

	@Override
	public void afterEach( final ExtensionContext context ) throws Exception {
		final Store store = context.getStore( Namespace.create( getClass(), context.getRequiredTestMethod() ) );
		final Properties backup = store.get( BACKUP_STORE_KEY, Properties.class );
		System.setProperties( backup );
	}

}

SystemPropertyExtensionTest.java

import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;

class SystemPropertyExtensionTest {

	@BeforeAll
	static void setUpOnce() {
		System.setProperty( "some property", "old value" );
	}

	@SystemProperty( key = "some property" )
	@Test
	void extension_should_set_property_to_null() {
		Assertions.assertNull( System.getProperty( "some property" ) );
	}

	@SystemProperty( key = "some property", value = "new value" )
	@Test
	void extension_should_set_property_to_value() {
		Assertions.assertEquals( System.getProperty( "some property" ), "new value" );
	}

	@SystemProperty( key = "some property" )
	@SystemProperty( key = "another property", value = "new value" )
	@Test
	void extension_should_be_repeatable() {
		Assertions.assertNull( System.getProperty( "some property" ) );
		Assertions.assertEquals( System.getProperty( "another property" ), "new value" );
	}

	@AfterAll
	static void tearDownOnce() {
		Assertions.assertEquals( System.getProperty( "some property" ), "old value" );
	}

}

For those who are interested: I have proposed this feature to be part of JUnit Pioneer (see junit-pioneer/junit-pioneer#129).

Now part of JUnit Pioneer.