Java Persistence Frameworks

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Java Persistence Frameworks

• Patrick W. McMichael
• J2EE Architect, Pillar Technology Group

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Introduction

• Background
• Data Access a necessary evil
• Time to Code
• Time to Test
• Time to Tune
• Purpose and Scope
• Historical Perspective on Various Alternatives
• Primary Focus on Hibernate and EJB 3.0

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Agenda

• Part I A Little History
• JDBC
• Pre-EJB 3.0 CMP Entity Beans
• JDO/Hibernate
• EJB 3.0 Entity Beans
• Part II A Side-By-Side Comparison
• Hibernate vs. EJB 3.0 Entity Beans
• General Approach
• CRUD Operations
• Cross-object Relationships
• Transactions
• Part III Where Do I Go From Here?
• Similarities
• Important Distinctions

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• PART I A Little History

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Howd we get here? -- JDBC

• Call-level DB access via SQL
• This has traditionally been the comfort zone
  for many developers
• Leaves developers fully in control
• BUTrequires manual, relatively tedious work

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Howd we get here? -- JDBC

• Basic Steps
• Obtain a connection to the DB
• Use the connection to create a statement or to
  access a stored procedure
• Execute the Statement
• In the case of queries
• Iterate through each row of data in the result
  set
• For each row, parse the data into the appropriate
  domain objects

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Howd we get here? -- Along Came CMP

• ORM object relational mapping via XML DD
• Container-managed data access vs. manual SQL
• EJB 2.0 added some great features
• Local Interfaces
• CMR container-managed relationships
• EJB QL it took until 2.1 to get a decent subset
  of SQL operations represented

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Howd we get here? -- Along Came CMP

• Pre-EJB 3.0 CMP Challenges
• RMI style access addressed via EJB 2.0 local
  interfaces
• Poor performance -- most often due to abuse of
  Entity Beans
• Massive pool sizes
• Jiffy-Pop syndrome
• Inherent dependency upon the EJB container
• Impacts testability
• Impacts reuse in lightweight alternate
  deployments
• Convoluted class/interface structure
• NOT POJO-centric forced use of Session Facade
• No support for inheritance
• In general, felt like a departure from OO
• Limited query capabilities (EJB 2.1 EJB QL
  enhancements helped)
• Custom finders handled via DD limits ability to
  make query dynamic at request time (e.g. search
  w/ a broad set of possible criteria)

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Howd we get here? JDO/Hibernate

• POJO-Centric Model
• No entity-to-DTO conversion games
• NOT container-dependent
• Facilitates multiple deployment scenarios
• Faciliates out-of-container testing
• JDO
• Standards-based (JSR 12/243 for 1.x/2.0 versions)
• Bytecode processing approach
• Hibernate
• Proprietary (though open-source)
• Uses runtime reflection/CGLIB modification

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Howd we get here? EJB 3.0 Entity Beans

• Plagiarism or Flattery?
• Now POJO-centric
• No longer container-dependent
• Convoluted interface structure done away with
• EJB QL enhancements
• Dynamic query capabilities

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Howd we get here? Summary

• Round One allow SQL access to DB
• Round Two layer of abstraction / ORM
• Round Three POJO-centric response to Entity
  Beans
• Round Four Lessons learned applied
• Seeking to get back to a single core
  specification for Java persistence

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• PART II Side-by-Side Comparison

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Side-By-Side Comparison Cool Mixins
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General Approach -- Hibernate

• POJO-centric model
• Key Interfaces
• SessionFactory like DataSource in JDBC
• Session like Connection in JDBC
• Query means of executing HQL (Hibernate Query
  Language)
• Transaction for programmatic demarcation
• Instance States
• Transient never persisted via a Session
• Persistent associated with an active Session
• Detached previously persisted, but not
  currently associated with an active Session

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General Approach -- Hibernate

• ORM
• Typically handled via XML (.hbm.xml file next to
  POJO)
• XDoclet may be used to generate these
• Support in progress for JDK 1.5 style annotations
• Example CreationDto.hbm.xml
• Referencing mapping files
• Programmatically during Configuration/SessionFacto
  ry setup
• In hibernate.cfg.xml
• Via Spring XML file (my preference!)
• Mapping files can also be used to
• Generate DDL
• Generate the POJO instances

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General Approach EJB 3.0 Entity Beans

• Now a POJO-centric model too
• Key Interfaces
• EntityManagerFactory like SessionFactory in
  Hibernate
• EntityManager like a Session in Hibernate
• Query like its namesake in Hibernate
• EntityTransaction for programmatic (resource
  local) demarcation

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General Approach EJB 3.0 Entity Beans

• Persistence Context
• Lifecycle in container determined by JTA
  transaction in place
• EXCEPT when coordinated with Stateful Session
  Beans, where its lifecycle may span JTA
  transactions (extended P.C.)
• Outside the container, it is tightly bound to the
  lifecycle of the EntityManager
• Instance States
• New like Transient in Hibernate
• Managed like Persistent in Hibernate
• Detached previously persisted, but not
  currently associated with an active Session
• Removed Currently associated with a persistence
  context, but scheduled for removal from the DB

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General Approach EJB 3.0 Entity Beans

• ORM
• Typically handled via JDK 1.5 style annotations
• May be specified via XML files
• Example CreationDto.java
• Obtaining an EntityManager
• JNDI lookups
• Dependency Injection (J2EE)
• _at_PersistenceContext
• EntityManager em
• getEntityManager on EntityManagerFactory (J2SE)
• EntityManagerFactory emf
• Persistence.createEntityManagerFactory()
• EntityManager em emf.createEntityManager()

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CRUD Operation Examples -- Hibernate

• createCreation()
• session.save()
• findCreationsByIceCreamFlavor()
• Using Criteria instance
• very object-centric approach
• findCreationsByMixin()
• Example of named query
• Using HQL

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CRUD Operation Examples EJB 3.0

• createCreation()
• em.persist()
• findCreationsByIceCreamFlavor()
• Using EJB QL
• findCreationsByMixin()
• Example of named query
• Using EJB QL

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Cross-Object Relationships -- Hibernate

• Example mixins property of Creation object
• many-to-many relationship
• Associative table CREATION_MIXIN
• key-column CREATION_ID
• Column for id of related entity MIXIN_ID
• ltbag name"mixins" table"CREATION_MIXIN"
  cascade"none"gt
• ltkey column"CREATION_ID"/gt
• ltmany-to-many column"MIXIN_ID"
• class"com.coolmixins.hibernate.dto.Mi
  xinDto"/gt
• lt/baggt

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Cross-Object Relationships EJB 3.0

• Annotation Examples for CreationDto.java
• Example _at_ManyToOne properties
• Inventor
• Ice Cream
• Example _at_ManyToMany property
• _at_JoinTable usage
• _at_JoinColumn name/referenced column name

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Transactions -- Hibernate

• In-container
• Seamlessly ties into CMT
• Using just the open and close calls on the
  session
• Out-of-container
• Session.beginTransaction()
• Transaction interface (commit/rollback)

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Transactions EJB 3.0 Entity Beans

• In-container
• Seamlessly ties into CMT
• e.g. Session Façade with JTA transaction in place
• Out-of-container
• EntityManager.getTransaction()
• EntityTransaction interface (begin/commit/rollback
  )

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• PART III Where do I go from here?

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Where do I go from here?

• Hibernate (pre-3.1) / EJB 3.0 Entity Bean
  similarities
• Both meet todays key persistence framework
  demands
• In/Out of container usage with transactional
  support
• POJO-centric, OO friendly model
• Automation of data storage/retrieval
• HQL and EJB QL in EJB 3.0 both provide rich query
  capabilities

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Where do I go from here?

• Important Points of Distinction (aka Wait One
  Slide)
• Hibernate is proprietary
• EJB 3.0 Entity Beans are based on a JCP-driven
  spec
• Hibernate is available NOW
• EJB 3.0 Entity Beans are still a work in progress
  (though beta/preview versions are available)
• Hibernate (pre-3.1) has been largely focused on
  the external XML approach to ORM
• EJB 3.0 Entity Beans focus on annotations as the
  main approach
• ? What happens when the same resource needs to
  be configured in different ways in different
  contexts (e.g. legacy and enterprise mappings of
  the same POJO)
• Hibernate is offering Annotations and EJB 3.0
  style methods
• EJB 3.0 is slated to allow XML ORM as an
  alternative
• This should boil down to CHOICE for developers

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Where do I go from here?

• A convergence in the force
• Finallythis is really welcome after the whole
  JDO vs. EJB 3.0 fiasco
• Hibernate 3.0.x started adding EJB 3.0 style
  methods (e.g. persist and merge)
• Preview releases (JBoss, built on top of
  Hibernate), Oracle (built upon TopLink)
• Hibernate 3.1/Annotations used by JBoss, for
  example
• JBoss PAR files can use EJB 3.0 annotatations
• They can also tie in to .hbm.xml ORM files
• The EJB 3.0 Hibernate Fallacy --
  http//www.jroller.com/comments/mkeith/Weblog/the_
  ejb_3_0_hibernate
• BEA Acquisition of SolarMetric (Kodo JDO / EJB
  3.0)
• Slated for WLS 9.5