Entity Beans BMP

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Entity BeansBMP
Celsina Bignoli bignolic_at_smccd.net
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Entity Beans

• Represent persistent objects saved to permanent
  storage
• bank account, employee data, etc
• lifetime is completely independent from the user
  session
• have an identity, used to pass references between
  applications and share entities with other clients

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Entity Beans (2)

• advantages
• compact representation
• can associate methods
• can use implicit middleware services from an
  application server
• can be cached for performance

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Persistence

• Object-relational mapping
• save an object to a relational database table
• use JDBC to map the object data into a relational
  database
• read data from the database and load it into the
  fields of a object instance
• more sophisticated than Java serialization
• can take advantage of SQL query capabilities
• mapping can be handcrafted or facilitated using
  a object-relational mapping tool like Oracle
  TopLink or Hibernate

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Entity Bean Features

• survive failures
• they are just representation of data in a
  fault-tolerant storage
• have a lifetime much longer than a client session
• they are a view into a database
• they should not be seen as a separate version of
  the data in the database
• the EJB container maintains the in memory data
  and the data in the database in sync using the
  ejbLoad() and ejbStore() methods implemented by
  the entity bean class

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Entity Bean Features(2)

• multiple instances represent same data
• one instance servicing multiple requests would
  require multi-threading but this would lead to
  complex, error-prone implementations
• one instance servicing one request at a time
  would create bottlenecks
• several in-cache instances represent same
  underlying database data must solve possible data
  corruption
• container must synchronize instances periodically
  using ejbLoad() and ejbStore() methods
• the frequency of the synchronization is dictated
  by transactions

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Entity Bean Features(3)

• instances can be pooled
• entity bean objects are recyclable
• the container may pool and reuse instances to
  represent different entries in the underlying
  data storage
• more efficient
• must acquire and release resources no longer in
  use using ejbActivate() and ejbPassivate()
  methods
• ejbPassivate entails a state save (ejbSave())
• ejbActivate entails a state load (ejbLoad())

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How to Persist Entity Beans

• bean-managed persistence
• developer must write code to load, save and find
  data from data storage
• must use persistence API such as JDBC
• container-managed persistence
• strip the bean from any persistence logic
• inform the container how youd like the data to
  be persisted using container tools
• container generates automatically the persistence
  code

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Creating an Entity Bean
EJB Container
1 create()
Home Object
Client Code
2ejbCreate()
6 return EJB object
Entity Bean Instance
5 create EJB object
4return Primary Key
EJB Object
3 create database data
Database
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Destroying an Entity Bean
EJB Container
Home Object
2ejbRemove()
1 remove()
Entity Bean Instance
Client Code
EJB Object
1 remove()
2ejbRemove()
3 remove database data
Database
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Finding Entity Beans

• since entity beans are persisted to data storage
  they can be found (unlike session beans, which
  only live for the duration of a client session)
• home interface exposes finder methods to find
  entity beans
• session beans do not have finder methods

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Bean-managed Persistent (BMP) Entity Beans

• developer must write code to map the entity bean
  to and from storage
• Can use
• a database API such as JDBC
• a object/relation mapping framework (TopLink or
  Hibernate)
• usually, preferred to Container-managed
  persistent (CMP) entity beans ONLY when
  performance is an issue

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BMP Entity Bean Coding Basics

• must implement the javax.ejb.EntityBean interface

javax.ejbEnterpriseBean
javax.ejb.EntityBean void
setEntityContext(javax.ejb.EntityContext) void
unsetEntityContext() void ejbRemove() void
ejbActivate() void ejbPassivate() void
ejbLoad() void ejbStore()
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BMP Entity Bean Coding Basics (2)

• must define finder methods ejbFindltgt() in the
  local and remote home interfaces
• must implement finder methods in the bean
  implementation to find an existing entity bean in
  storage
• Must define at least one finder method
  ejbFindByPrimaryKey()

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Entity Bean Files

• remote/local interface
• the enterprise bean class
• maps to an entity in a database (table). An
  instance maps to a row in a table
• exposes callback methods used by the container to
  manage the bean
• exposes methods to update fields
• primary key class
• uniquely identifies each bean
• must be serializable
• deployment descriptor

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BMP Life cycle
does not exist
1 newInstance() 2 setEntityContext()
1 unsetEntityContext() 2 JVM will garbage
collect and call finalize()
pooled
ejbHome()
ejbFind()
1 ejbActivate() 2 ejbLoad()
1 ejbCreate() 2 ejbPostCreate()
1 ejbStore() 2 ejbPassivate()
ejbRemove()
ready
ejbLoad()
ejbStore()
Business Method