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Example – Java Persistence API as implemented by two application servers

Java EE describes the Java Persistence API (JPA). It is an object relation mapper (ORM) API, a layer between Java objects and relational databases (often SQL databases, such as Oracle database, Oracle MySQL, PostgreSQL, and so on). With a few lines of code, the content of JVM objects can be written to the database or vice versa: read from the database and put in the object.

Oracle's own reference implementation of Java EE is an open source application server called GlassFish. GlassFish bundles the existing EclipseLink open source project as the implementer of the JPA standard. Meanwhile, Red Hat's WildFly, a different open source Java EE application server, bundles Red Hat's own, more popular, Hibernate ORM open source project, which also implements the JPA standard.

If developers only use the features documented in the JPA standard, then it should not matter to them which implementation is used, but problems arise once features are used that are unique to a specific implementation.

If you do not agree with the choices made by the vendor of your application server, it is often possible to switch implementations of a particular standard. Yes, JVM developers really love having choices!