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Hibernate Persistence

Hibernate is a popular open-source ORM tool that allows transparent persistence of POJOs to a relational database. It minimizes code, doesn't require a container, and retains a natural object model.

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Hibernate Persistence

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  1. Hibernate Persistence

  2. What is Persistence • Persist data to database or other storage. • In OO world, persistence means persist object to external storage. • Relational database • SQL • Using SQL in Java • Persistence object-oriented applications: ORM

  3. What is Hibernate? • Popular Open Source (LGPL) Object/Relational Mapping (ORM) tool • Transparent persistence for POJOs (Plain Old Java Objects) • Core of JBoss CMP 2.0 impl.

  4. Why Hibernate? • Minimizes Code • Does not require a container • Model is not tied to persistence implementation, Retains natural object model (transparent)

  5. JavaObject SQL Table int id; String name; String getName() int getId() void setName(String) void setId(int) id [int] primary key, name [varchar(50)] Magic Happens Here (O/R Mapper – i.e. Hibernate) Object/Relational Mapping

  6. Course Object Model

  7. Persistence Tasks • Class mapping • Class to Table mapping • Object ID to Primary Key • Attribute Mapping • Primary Attribute to Column • User defined data type to embedded type • Inheritance Mapping • Relationship Mapping • One to Many • Many to One

  8. Persistence /Concurrency Pattern Introduced Martin Fowler《企业应用架构模式》

  9. Persistence • What is Persistence • Why Persistence is Importance • History of persistence • Plan text • RDB(OODB?) • XML

  10. Mapping to a Relational Database • Metadata Mapping • Identity Field • Lazy Load • Query Object

  11. Metadata Mapping • Hold details of object-relational mapping in metadata. • Hibernate as sample

  12. Identity Field • Save a database id field in an object to maintain identity between an in-memory object and a database row.

  13. Identity Field • Choosing your Key • meaningful key • meaningless key • simple key • compound key • table-unique key • database-unique key

  14. How to get a new Key • Database counter • it's non-standard and not available in all databases. • Key table • Separate transaction is needed • GUID • Large keys may also lead to performance problems, particularly with indexes

  15. Identity Map • Ensure each object only gets loaded once by keeping every loaded object in a map. Lookup objects using the map when referring to them • A Identity Map keeps a record of all the objects that have been read from the database in a single business transaction. Whenever you want an object, you check the Identity Map first to see if you already have it.

  16. Lazy Load • An object that doesn't contain all of the data you need, but knows how to get it.

  17. Query Object • An object that represents a database query • SQL can be an involved language, and many developers are not particularly familiar with it..

  18. Query Object( cont. )

  19. Concurrency • When Concurrency problem raised? • Why Concurrency is difficult to deal with • It is difficult to enumerate the possible scenarios that can get you into trouble • it is hard to test for

  20. Execution Contexts • Connection • Session • A session is a long running interaction between a client and server • Transaction • Process • Thread

  21. Transactions • The primary tool for handling concurrency in enterprise applications is the transaction. • ATM machine example

  22. Transaction Resources • Most enterprise applications run into transactions in terms of databases. But there are plenty of other things than can be controlled using transactions, such as • message queues, • printers, • As a result technical discussions of transactions use the term 'transactional resource' to mean anything that is transactional: that is uses transactions to control concurrency.

  23. Transaction types • long transaction. • making a transaction span multiple requests is generally known as a long transaction . • Not recommended • request transaction • start a transaction at the beginning of a request and complete it at the end. • late transaction • open a transaction as late as possible ,

  24. Business and System Transactions • System transactions • transactions supported by RDBMS systems and transaction monitors • Business transaction • Transaction that logically defined by a business requirement

  25. Concurrency Patterns • Unit of work • Optimistic Offline Lock • Pessimistic Offline Lock

  26. Unit of Work • Maintains a list of objects that are affected by a business transaction and coordinates the writing out of changes and resolution of concurrency problems.

  27. How it Works • Unit of Work is an object that keeps track of these changed things, such as inserted, updated or deleted.

  28. Unit of Work

  29. Optimistic Offline Lock • Prevent conflicts between concurrent business transactions, by detecting a conflict and rolling back the transaction.

  30. Optimistic Offline Lock

  31. How it Works

  32. Pessimistic Offline Lock • Prevent conflicts between concurrent business transactions by allowing only one business transaction to access data at once

  33. Pessimistic Offline Lock

  34. How it Works • have an exclusive write lock. That is, require only that a business transaction acquire a lock in order to edit session data. This avoids conflict by not allowing two business transactions to simultaneously make changes to the same record.

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