Unified Modeling Language (UML) specification

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1 Metamodel

A model is an example of a metamodel . A metamodel is a model .

This naturally leads to meta models （ element 2 Model ）、 element 3 Hierarchy of models, etc .

2 UML The specification uses a four level meta model ：M0、M1、M2 and M3

You can think of an object graph as an instance of a class graph 0 Level model (M0).

You can think of class diagrams as instantiations 2 Level meta model 1 Level meta model (M1) .

2 Level meta model (M2) yes UML standard .M2 Instantiation OMG Of Metaobject facilities (MOF), This is a 3 Level element 3 Model (M3).