JUnit 4 Rule to run individual tests with a different default locale

DefaultLocaleRule.java

import java.util.Locale;
import org.junit.rules.TestWatcher;
import org.junit.runner.Description;

public class DefaultLocaleRule extends TestWatcher {

    private Locale originalDefault;
    private Locale currentDefault;
    
    public DefaultLocaleRule() {
        this( null );
    }
    
    public DefaultLocaleRule( Locale defaultForTests ) {
        currentDefault = defaultForTests;
    }
    
    @Override
    protected void starting( Description description ) {
        originalDefault = Locale.getDefault();
        
        if( null != currentDefault ) {
            Locale.setDefault( currentDefault );
        }
    }
    
    @Override
    protected void finished( Description description ) {
        Locale.setDefault( originalDefault );
    }
    
    public void setDefault( Locale locale ) {
        if( null == locale ) {
            locale = originalDefault;
        }
        
        Locale.setDefault( locale );
    }
    
    public static DefaultLocaleRule en() {
        return new DefaultLocaleRule( Locale.ENGLISH );
    }
    
    public static DefaultLocaleRule de() {
        return new DefaultLocaleRule( Locale.GERMAN );
    }
    
    public static DefaultLocaleRule fr() {
        return new DefaultLocaleRule( Locale.FRENCH );
    }
}

Docs

To use the rule, add this to your JUnit test class:

    // make all tests run with the English locale
    @Rule
    public DefaultLocaleRule defaultLocaleRule = DefaultLocaleRule.en();

If a test has special demands, you can switch:

    @Test
    public void testDE() {

        defaultLocaleRule.setDefault( Locale.GERMAN );

        ...
    }

I found the reason. It seems like @BeforeClass block are run first, then @rule block is run. So the solute I found is to replace the @rule annotation with @ClassRule that is run before @BeforeClass block and the rule variable should be static:

@ClassRule
    public static DefaultLocaleRule defaultLocaleRule = DefaultLocaleRule.fr();

    @BeforeClass
    public static void setUp() {
        ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
        validator = factory.getValidator();
    }