Java Persistence Programming with Entities

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Java Persistence Programming with Entities
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What Well Cover

• Overview
• ORM
• ORM Example
• What is an Entity?
• Entities vs Session Beans
• Entity classes
• Accessing Entities in the Persistence Context
• persistence.xml file
• EntityManager API
• Entity Life Cycle
• Entity Life Cycle Diagram
• Life-cycle call backs
• Entity Synchronization
• Transaction Isolation
• Tuning for concurrent access
• Dirty Read
• Unrepeatable Read
• Entity Lookup and Query API

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Overview

• Java Persistence provides
• Standard OR mappings
• Not tied to the J2EE container
• Defines an SPI so different service providers can
  be used

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ORM

• Simplest Java Serialization objects written
  to files
• Store Java Objects in RDBMS (JDBC maps your
  object data to DB)
• ORM- Object Relational Mapping
• Can have arbitrary queries for information
• Can visually inspect data
• Can handcraft, or use tools like Hibernate,
  TopLink, etc.

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ORM Example
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What is an Entity?

• Two different kinds of objects application
  logic and persistent data
• Persistent data can be saved to storage by
  persistence mechanisms
• Called entities in Java Persistence Specification
• Store data as fields and have getters/setters
• Can group related data into coherent objects and
  factor out common attributes in inheritance
  hierarchy

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Entities vs Session Beans

• Entities have a client-visible persistent
  identity
• Entities have a visible state
• Entities are not remotely acessible
• Entities lifetimes are independent of
  applications lifetime
• Entities are translated back and forth by the
  persistence provider

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Entity classes

• Entity is a POJO (can implement Serializable so
  it can be used as a data record)
• Entity maps to a definition in an RDBMS
• Must have a primary key
• Access via direct field state or setter/getter
• Entity can expose business methods

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Accessing Entities in the Persistence Context

• Clients must retrieve entity from the persistence
  context or create one
• Persistence context connection between in
  memory instances and the database EntityManager
  API
• Persist() method puts an entity into the managed
  state
• Transaction scoped context Persistence Context
  ends with enclosing txn
• Extended persistence context ends when
  enclosing stateful bean is removed by the
  container (managed across several invocations
• Detached state entities have no connection with
  a entity manager and are not synchronized with
  the DB
• Detached entities have to be retrieved by id
  (reattached or merged into new context).

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persistence.xml file

• Must have persistence-unit element with name
  attribute
• Example
• ltpersistencegt
• ltpersistence-unit name"manager1"gt
• ltdescriptiongtps unit onelt/descriptiongt
• ltjta-data-sourcegtjava/DefaultDSlt/jta-data-sourcegt
  
• ltmapping-filegtormap2.xmllt/mapping-filegt
• ltexclude-unlisted-classes/gt
• ltjar-filegt../MyApp.jarlt/jar-filegt
• ltclassgtorg.acme.Employeelt/classgt
• ltpropertiesgt
• ltproperty name"hibernate.dialect"
  value"org.hibernate.dialect.HSQLDialect"/gt
• lt/propertiesgt
• lt/persistence-unitgt
• lt/persistencegt

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EntityManager API

• Container-managed injected via
  _at_PersistenceContext
• Application-managed created via
  EntityManagerFactory
• Three types of operations
• Life-cycle management
• DB synchronization
• Entity lookup/queries

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Entity Life Cycle

• new entity instance created in memory, not
  associated with persistence identity or context
  (right after creation)
• managed has a persistent identity, changes are
  synched w/ DB (after persist())
• detached does have a persistent id but not
  associated with a persistence context
• removed associated with a context, but is
  scheduled for removal from the DB (after remove()
  illegal on a detached entity)

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Life-cycle call backs

• _at_PrePersist
• _at_PostPersist
• _at_PreRemove
• _at_PostRemove
• _at_PreUpdate
• _at_PostUpdate
• _at_PostLoad
• Can also define separate listener classes
• _at_EntityListeners(MyListener.class)
• Callback methods for listeners takes an entity
  argument

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Entity Synchronization

• SetFlushMode
• COMMIT default synchronize entities in
  context at commit
• AUTO at commit and before query execution
• Flush() synchs the state of all entities in the
  context, but does not refresh from DB

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Transaction Isolation

• Protected by transaction isolation
• Container-managed txns use isolation levels
  configured by persistence provider or containers
  txn service
• SERIALIZABLE max txn isolation, may degrade
  performance
• READ UNCOMMITTED may lead to inconsistent
  data/bad behavior

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Tuning for concurrent access

• Optimistic locking using _at_Version
• Not done by default
• Checks for version of entity before updates
• Globally set stricter txn isolation
• Application level locks
• manager.lock(account LockMode.WRITE)
• READ prevents dirty/non-repeatable reads
• WRITE READ plus forces an increase in the
  version number (if any)

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Dirty Read
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Unrepeatable Read
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Entity Lookup and Query API

• Find by primary key with the EntityManager.find()
• Create a query
• Get a javax.persistence.Query from the EM
• Customize (if needed) query params/result set
  limits
• Execute
• EM.createQuery EJB QL or createNativeQuery
  standard SQL
• Named query
• _at_NamedQuery (namename queryStringqueryString

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What We Covered

• Overview
• ORM
• ORM Example
• What is an Entity?
• Entities vs Session Beans
• Entity classes
• Accessing Entities in the Persistence Context
• persistence.xml file
• EntityManager API
• Entity Life Cycle
• Entity Life Cycle Diagram
• Life-cycle call backs
• Entity Synchronization
• Transaction Isolation
• Tuning for concurrent access
• Dirty Read
• Unrepeatable Read
• Entity Lookup and Query API