UMLG is a UML to java code generator. From class diagrams, persistent java entities are generated. The entities persist via an embedded graph database. For the graph db Tinkerpop Blueprints is used. The semantics of a Property Graph Model is a natural fit to implement the rich semantics of UML class diagrams in java.
One of the primary objectives of UMLG is to implement the UML2 semantics as accurately as possible.
UML has strong and detailed semantics for specifying structural features. In general it is far easier and quicker to specify the complex structure and relationships of domain entities in UML. UMLG makes that specification much more than just documentation. A large part of the lack of popularity of UML is that it remains only documentation. Many UML tools can generate java, however it is seldom if ever generates persistent entities.
UMLG entities are not POJOs. They are always persistent objects. UMLG entities are not annotated and interceptors are not used in the implementation. Graph databases have very fast startup times that do not increase with the number of entities (vertices and edges). Because of this there is no need to write mock tests nor integration tests. All tests go through the same stack as will execute in a production environment. Many graph databases support an in memory only option. If this is used UMLG's entities will reside in memory only making tests faster.
The basic pattern used is that an entity wraps a vertex and associations between entities are realized as edges between vertices.
Graph databases are very good at traversing relationships. It's their strength after all. This translates to UMLG entities being very efficient and fast at navigating object oriented associations.
UMLG implement most class diagram constructs. This includes: inheritance, interfaces, abstract classes, associations, composition, complex data types, multiplicity, collection types (Set, OrderedSet, Sequence, Bag), qualifiers, constraints.
UMLG has support for OCL (Object Constraint Language). OCL is a powerful constraint and query language that operates directly on the entities. OCL constraints and queries can be specified in the model at design time or executed at runtime.
UMLG can also, optionally, generate a rest interface to the application for performing crud and query operations. A web interface is provided to perform crud operations and execute queries. Queries can be executed in OCL, Gremlin or in the underlying graph databases' native query language.