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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 | |
| */ | |
| // Use a database sequence on id field | |
| @Entity | |
| @Table(name = "ADDRESS") | |
| public class Address { | |
| @Id | |
| @SequenceGenerator( | |
| name = "address_seq", | |
| sequenceName = "address_seq", | |
| allocationSize = 1 | |
| ) | |
| @GeneratedValue( | |
| strategy = GenerationType.SEQUENCE, | |
| generator = "address_seq" | |
| ) | |
| private long id; | |
| } | |
| // Delete dependent children, when the parent is going to be deleted (child-entites are orphans (=Waisen) then) | |
| @OneToMany(mappedBy="foo", orphanRemoval=true) | |
| private List<Bikes> bikes; | |
| // Model a m:n relationship where the corresponding relationship record would be deleted when a entity record is deleted | |
| @Entity | |
| public class Team { | |
| @ManyToMany(cascade = { CascadeType.MERGE, CascadeType.PERSIST }, mappedBy="teams") | |
| private List<Match> matches; | |
| } | |
| @Entity | |
| public class Match { | |
| @ManyToMany(cascade = { CascadeType.MERGE, CascadeType.PERSIST }) | |
| @JoinTable( | |
| name="MATCH_TEAM", | |
| joinColumns={@JoinColumn(name="MATCH_ID", referencedColumnName="ID")}, | |
| inverseJoinColumns={@JoinColumn(name="TEAM_ID", referencedColumnName="ID")} | |
| ) | |
| private List<Team> teams; | |
| } | |
| // Remove Child Records, when the child record is set to null in the parents collection of children | |
| // by setting "orphanRemoval = true" | |
| @OneToOne(cascade = CascadeType.ALL, fetch=FetchType.LAZY, orphanRemoval = true) | |
| private AvatarImage avatarImage; | |
| // Mark children elements as "CascadeType.ALL" to refresh/delete/... them if the parent refreshes/deletes/... | |
| // CascadeType.ALL contains PERSIST, REMOVE, REFRESH, MERGE, DETACH | |
| // Several ways to delete records in the db | |
| // http://www.objectdb.com/java/jpa/persistence/delete#Orphan_Removal | |
| TODO: how to implemwnt equals and hashcode: https://vladmihalcea.com/2016/10/20/the-best-way-to-implement-equals-hashcode-and-tostring-with-jpa-and-hibernate/ | |
| Query Data | |
| * JDBC - Plain SQL statements | |
| * JPQL - Query language based on Enitity names (not on table names) | |
| * Criteria Query - Build a query with a Java based API | |
| * Native Query - Plain SQL statements | |
| // Initialize DB schema in a Spring Boot App | |
| resources/data.sql | |
| // Generic Rowmapper that maps all the columns to a Java Pojo | |
| List<Person> persons = jdbcTemplate.query("select * from person", new BeanPropertyRowMapper<Person>(Person.class)) |
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