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JPA Cheatsheet
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| JPA (Java Persistence API) | |
| Transaction Management with an Entity-Mananger: | |
| --- | |
| entityManager.getTransaction().begin(); | |
| entityManager.persist(<some-entity>); | |
| entityManager.getTransaction().commit(); | |
| entityManager.clear(); | |
| SomeEntity entity = entityManager.find(SomeEntity.class, 1); | |
| --- | |
| @OneToOne's fetch type is EAGER by default | |
| Lists + Sets fetch type is LAZY by default | |
| Two types of Lazy Loading implementations | |
| 1. Proxying the object (default in Hibernate) by creating a Subclass of that object at runtime and overwrite the get methods. | |
| This is done by the JavaAssist lib. | |
| 2. ByteCode Enhancement (default in EclipseLink): Add special logic to the get methods inside the Java Bytecode | |
| LazyInitializationExcepiton: | |
| Entity Lifecycle | |
| New ---> em.persist ---> Managed | |
| New ---> em.merge ---> Managed | |
| Managed ---> em.remove ---> Removed | |
| Managed ---> em.find ---> Managed | |
| Managed ---> query.getResultList ---> Managed | |
| Managed ---> query.getSingleResult ---> Managed | |
| Managed ---> em.detach ---> Detached | |
| Managed ---> em.close ---> Detached | |
| Managed ---> em.clear ---> Detached | |
| Detached ---> em.merge ---> Managed | |
| Ein neu angelegtes Entity Object ist im Zustand "New". | |
| Managed - Es gibt einen Entity-Manager, der für dieses Objekt verantwortlich ist: | |
| Vorteile: - Es werden automatisch Änderungen getrackt. | |
| Beim nächsten Transaktions-Commit werden nur die Änderungen in die DB geschrieben. | |
| Lazy Loading funktioniert | |
| Detached - Lazy Loading muss nicht zwangsweise funktionieren |
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