Jpa & hibernate
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This document discusses JPA and Hibernate. It begins with an agenda that includes motivation, descriptions of JPA and Hibernate, features missing in JPA 2.0, best practices, and questions. It then explains that JPA is a specification for ORM APIs while Hibernate is a JPA implementation. Features like @BatchSize and @Fetch(FetchMode.SUBSELECT) that are missing from JPA 2.0 are described. Best practices recommend using JPA annotations where possible and Hibernate annotations if there is no JPA equivalent. The summary states that Hibernate annotations will continue to be used and some may be replaced by JPA 2.1 annotations, but Hibernate will likely
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Jpa & hibernate
- 1. www.querity.cz JPA & HIBERNATE Ing. Jiří Kiml, 27/11/2014, Bern What is missing?
- 2. www.querity.cz Agenda ►Motivation ►JPA && Hibernate ►Where the problem is? ►Summary ►Questions
- 3. www.querity.cz Motivation One man’s crappy software is another man’s full time job. ~Jessica Gaston ►I was asked to talk about it by Jakob ►I like performance analysis ►…...
- 4. www.querity.cz Agenda ►Motivation ►JPA && Hibernate ►What is missing in JPA? ►Best practices ►Summary ►Questions
- 5. www.querity.cz JPA && Hibernate ►JPA is the dance, Hibernate is the dancer. ►JPA is the Art, Hibernate is the artist. ►JPA is a specification to standardize ORM-APIs ►Hibernate is a vendor of a JPA implementation “ Rules of Optimization: Rule 1: Don't do it. Rule 2 (for experts only): Don't do it yet. ” ~ Michael A. Jackson
- 6. www.querity.cz Agenda ►Motivation ►JPA && Hibernate ►What is missing in JPA? ►Best practices ►Summary ►Questions
- 7. www.querity.cz What is missing in JPA 2.0?
- 8. www.querity.cz Missing features in JPA 2.0 ►@BatchSize ►@Fetch(FetchMode.SUBSELECT) ►@ForeignKey ►@Index ►Data generation Before software can be reusable it first has to be usable. ~Ralph Johnson
- 9. www.querity.cz Lazy or Eager fetch? ►LAZY = fetch when needed ►EAGER = fetch immediately LAZY = fetch when TO is created!!!
- 10. www.querity.cz @BatchSize ►Using batch fetching, Hibernate can load several uninitialized proxies if one proxy is accessed. Batch fetching is an optimization of the lazy select fetching strategy ►//FIXME HUP Find a solution for jpa 2.0 // @BatchSize(size = 100)
- 11. www.querity.cz @Fetch(FetchMode.SUBSELECT) ►If one lazy collection or single-valued proxy has to be fetched, Hibernate will load all of them, re-running the original query in a subselect. Don’t worry if it doesn’t work right. If everything did, you’d be out of a job. ~Mosher’s Law of Software Engineering
- 12. www.querity.cz N+1 query problem *----------------* *-----------------* | pet | | owner | |----------------| * 1 |-----------------| | - id |-----------------| - id | | - name | | - pet_id | *----------------* | - name | *-----------------* -- get all of the pets first select * from pet -- get the owner for each pet returned select * from owner where pet_id = 1 select * from owner where pet_id = 2 select * from owner where pet_id = ...... select * from owner where pet_id = N
- 13. www.querity.cz @BatchSize(size=N) -- get all of the pets first select * from pet -- get all owners in a single select select * from owner where pet_id in (1, 2, 3, …..N) *----------------* *-----------------* | pet | | owner | |----------------| * 1 |-----------------| | - id |-----------------| - id | | - name | | - pet_id | *----------------* | - name | *-----------------*
- 14. www.querity.cz @Fetch(FetchMode.SUBSELECT) select * from pet select * from owner where pet_id in (select id from pet) This is very similar to the previous examples, but all of the burden is now put on the database; and the batch size is effectively infinity. *----------------* *-----------------* | pet | | owner | |----------------| * 1 |-----------------| | - id |-----------------| - id | | - name | | - pet_id | *----------------* | - name | *-----------------*
- 15. www.querity.cz @ForeignKey ►org.hibernate.annotations.ForeignKey ►java.persistence.ForeignKey (JPA 2.1/J2EE 7) ►Used to specify the handling of foreign key constraints when schema generation is in effect. hibernate3:hbm2dd
- 16. www.querity.cz @org.hibernate.annotations.Index ►You can define an index on a particular column using the @Index annotation on a one column property ►Composite index as part of @Table annotation @org.hibernate.annotations.Table(appliesTo = “tableName”, indexes = { @Index(name = “forestidx”, columnNames = { “indexedcolumn” }) }) ►Part of jpa 2.1/ J2EE 7
- 17. www.querity.cz @UniqueConstraint ►Part of jpa 1.0 ►One column: @Column(unique=true) ►Multiple columns: @Entity @Table( name="EMPLOYEE", uniqueConstraints= @UniqueConstraint(columnNames={"EMP_ID", "EMP_NAME"}) ) public class Employee { ... }
- 18. www.querity.cz Agenda ►Motivation ►JPA && Hibernate ►What is missing in JPA? ►Best practices ►Summary ►Questions
- 19. www.querity.cz Best Practices ►Use JPA annotations if possible ►Use hibernate annotations if there is no equivalent in JPA ►Do NOT use // FIXME HUP “The question of whether computers can think is like the question of whether submarines can swim.” ~Edsger W. Dijkstra
- 20. www.querity.cz Agenda ►Motivation ►Swiss knife ►What do we use? ►Best practices ►Summary ►Questions
- 21. www.querity.cz Summary – what we use? ►We can usese hibernate annotations ►We will replace (some of) them by JPA 2.1 annotations ►We will probably NEVER replace hibernate “I’ve finally learned what ‘upward compatible’ means. It means we get to keep all our old mistakes.” ~Dennie van Tassel
- 23. www.querity.cz Agenda ►Motivation ►Swiss knife ►What do we use? ►Best practices ►Summary ►Questions
- 24. www.querity.cz Questions … … and maybe answers
- 25. www.querity.cz Thank you Ing. Jiří Kiml, 27/11/2014, Bern