Hibernate
Hibernate is a powerful, ultra-high performance object/relational persistence and query service for Java. Hibernate lets you develop persistent objects following common Java idiom - including association, inheritance, polymorphism, composition and the Java collections framework. Extremely fine-grained, richly typed object models are possible. The Hibernate Query Language, designed as a \"minimal\" object-oriented extension to SQL, provides an elegant bridge between the object and relational worlds. Hibernate is now the most popular ORM solution for Java.
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License
GNU Library or Lesser General Public License (LGPL)