public static MappingDirection valueOf(String mappingDirection) {
        if (BI_DIRECTIONAL_VALUE.equals(mappingDirection)) {
            return BI_DIRECTIONAL;
        } else if (ONE_WAY_VALUE.equals(mappingDirection)) {
            return ONE_WAY;
        } else if (StringUtils.isEmpty(mappingDirection)) {
            return null;
        }

        throw new IllegalStateException("type should be bi-directional or one-way. " + mappingDirection);
    }
Unidirectional mapping when a to b to a' then a != a'
core/src/main/java/com/github/dozermapper/core/classmap/MappingDirection.java