Model:
	refs+=Ref*
;

Ref:
	"ref" ref=FQN
;
FQN: ID ("." ID)*;

@ImplementedBy(ITypeValidationHelper.NullImpl.class)
public interface ITypeValidationHelper {
	
	boolean validate(URI uri, String name);
	
	public static class NullImpl implements ITypeValidationHelper {

		@Override
		public boolean validate(URI uri, String name) {
			return true;
		}
		
	}

}

public class MyDslJavaValidator extends AbstractMyDslJavaValidator {
	
	@Inject
	ITypeValidationHelper tvh;

	@Check
	public void checkType(Ref ref) {
		if (!tvh.validate(ref.eResource().getURI(), ref.getRef())) {
			warning("type does not exist", MyDslPackage.Literals.REF__REF);
		}
	}

}

public class JdtTypeValidationHelper implements ITypeValidationHelper {
	
	@Inject
	IWorkspace workspace;

	@Override
	public boolean validate(URI uri, String name) {
		IProject project = workspace.getRoot().getFile(new Path(uri.toPlatformString(true))).getProject();
		IJavaProject javaProject = JavaCore.create(project);
		try {
			IType type = javaProject.findType(name);
			return type != null;
		} catch (JavaModelException e) {
			return false;
		}
	}

}

public class MyDslUiModule extends AbstractMyDslUiModule {
	public MyDslUiModule(AbstractUIPlugin plugin) {
		super(plugin);
	}
	
	public Class<? extends ITypeValidationHelper> bindITypeValidationHelper() {
		return JdtTypeValidationHelper.class;
	}
}

final URI uri = javaType.eResource().getURI();
final String qualifiedName = javaType.getQualifiedName();
if (qualifiedName != null && uri != null) {
    final IJavaProject javaProject = JavaCore.create(workspace.getRoot().getFile(new Path(uri.toPlatformString(true))).getProject());
    final IType type = javaProject.findType(qualifiedName);
    if (type == null) {
        error(...);
    } else {
        if (Flags.isFinal(type.getFlags())) {
            error(...);
        }
    }
}