}
        return null;
    }

    @Nonnull
    public static Path findMandatoryRoot(Path topDirectory) {
        requireNonNull(topDirectory, "topDirectory");
        return getCanonicalPath(Optional.ofNullable(
                        ServiceLoader.load(RootLocator.class).iterator().next().findMandatoryRoot(topDirectory))
                .orElseThrow());
    }

            + "Create a .mvn directory in the root directory or add the root=\"true\""
            + " attribute on the root project's model to identify it.";

    @Nonnull
    default Path findMandatoryRoot(Path basedir) {
        Path rootDirectory = findRoot(basedir);
        if (rootDirectory == null) {
            throw new IllegalStateException(getNoRootMessage());
        }
        return rootDirectory;
    }

                if ("project.rootDirectory".equals(expression)) {
                    Path base = basedir != null ? basedir.toPath() : null;
                    Path root = rootLocator.findMandatoryRoot(base);
                    return root.toFile().getAbsolutePath();
                }
                return null;
            }
        });