return Collections.unmodifiableMap(new PropertiesAsMap(getMavenSession().getSystemProperties()));
    }

    @Nonnull
    @Override
    public Map<String, String> getEffectiveProperties(@Nullable Project project) {
        HashMap<String, String> result = new HashMap<>(getSystemProperties());
        if (project != null) {
            result.putAll(project.getModel().getProperties());
        }

        return Map.of();
    }

    @Override
    public Map<String, String> getSystemProperties() {
        return systemProperties;
    }

    @Override
    public Map<String, String> getEffectiveProperties(Project project) {
        HashMap<String, String> result = new HashMap<>(getSystemProperties());
        if (project != null) {
            result.putAll(project.getModel().getProperties());
        }

     * needs to be passed) or not.
     *
     * @param project {@link Project} or {@code null}.
     * @return the effective properties, never {@code null}
     */
    @Nonnull
    Map<String, String> getEffectiveProperties(@Nullable Project project);

    /**
     * Returns the current maven version.
     *
     * @return the maven version, never {@code null}
     */
    @Nonnull
    Version getMavenVersion();