Martin: Created page with "* Containers support caching and coherence across containers, this leads to a scalable solution with little additional work * Performance problems are much reduced from the ea..."

2021-09-14T21:28:41Z

Created page with "* Containers support caching and coherence across containers, this leads to a scalable solution with little additional work * Performance problems are much reduced from the ea..."

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* Containers support caching and coherence across containers, this leads to a scalable solution with little additional work
* Performance problems are much reduced from the early CMP
** still need to be careful about linking objects and activating too many

= Hibernate =
* Used to require a XML mapping file
* Supports annotations much like JPA
* Tends to have additional features and push them into the JPA
* Supports one persistence unit per file -
* Entity classes must be defined - no auto discovery
* Includes Lucene search - full text
* Indexed collections
* Filters - similar to views but support parameters and dynamic application
* Shorthand HQL
* Manual flush + validator - user transactions

= JPA =
* requires a persistence unit descriptor META-INF/persistence.xml
** specifying the datasource
** JPA specific configuration such as provider and it's options
* one persistence unit per datasource
* annotated classes automatically discovered
* xml overrides annotations
* supports multiple persistence units in persistence.xml file

== Property/Field ==
* public attributes are persisted using field based access i.e. direct access
* private attributes are persisted property based access i.e. using getter methods
* Can not mix field and property based access - can mark which by putting annotation against field or getter method
* Getters/setters can not be final
== Embeded ==
Object that doesn't exist on it's own outside of the object referring to it
* can not be persisted separately

== Persistence Manager Factory ==
* Scans the classes for annotations
* Creates persistence manager for managing an entity instance

== Persistence Manager ==
* Not thread safe - Nature of session beans this isn't a problem
** stateful beans tied to one thread
** stateless beans only server one thread per call => context per method
# manages entity - CRUD, findById, query. Manages state
## transient - created but not commited
## persisted - managed context
## detached - context closed or deleted
## removed - deleted
# persistence context - manage cache - queued sql, optimisations
# automated dirty checking - batched dirty cache flushing
# optimistic locking for long lived sessions
#* writes/locking delayed to the commit and locking/consistency checks performed then.
# can be configured to batch fetch when an iterator is accessed
# for a instance of the persistence manager and a persisted object multiple lookups guaranteed the same object instance

* Allows traversing of relations - performs the lazy loading on demand
* Caches and manages updates until flushed then cascades any changes required
* Designed to serve a use case - stateful bean may (probably will want to) keep open across calls [[#Extended context]]
** stateful bean configured with @PersistenceContext(type = PersistenceContextType.EXTENDED)

= JDO =
Java Data Objects
* JDO2 more coverage than JPA
* Separate persistence manager for transactions
* Intro http://www.ibm.com/developerworks/java/library/j-pj2ee4.html
* Comparison http://db.apache.org/jdo/jdo_v_jpa_orm.html

= Entity =
<pre>
import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class Foo implements Serializable {
@Id
private int id;

public void setId(int id) {
this.id = id;
}

public int getId...
}
</pre>
* relationships: @OneToOne @OneToMany @ManyToMany
** (mappedBy = "attributeName", fetch = EAGER/LAZY, optional = true/false, cascade=true/false, targetEntity=ClassRequired.clazz)
*** mappedBy when the relationship points only one way - specify owner of relationship
**** not on ManyToOne - must be owner - no mappedBy
*** optional if related object is required
**** not available on OneToMany or ManyToMany
* mapping to tables: @Table @Column @JoinColumn
* not persisted @Transient

http://db.apache.org/jdo/jdo_v_jpa.html

Persisted types:
* primitive types, wrappers of primitives
* String, BigInteger, BigDecimal, java.util.Date, java.util.Calendar
* java.sql.Date, java.sql.Time, java.sql.Timestamp
* anything Serializable
* arrays (of things that can be persisted)
* Enums
* Collections (of things that can be persisted)

= Entity Manger =
<pre>
@PersistenceContext(unitName="ComponetEntityManager")
private EntityManager em;

em.persist(foo);
</pre>

= JPQL =
<pre>
import javax.persistence.EntityManager;

@PersistenceContext(unitName="ComponetEntityManager")
private EntityManager entityManager;

Query q = entityManager.createQuery("select t from Type t where t.foo = :foo");
q.setParameter("foo", bar);
Type type = (Type)q.getSingleResult( );
</pre>

* Definition BNF
** http://java.sun.com/javaee/5/docs/tutorial/backup/update3/doc/QueryLanguage5.html
* Examples on previous page:
** http://java.sun.com/javaee/5/docs/tutorial/backup/update3/doc/QueryLanguage4.html

* updates entityManager.flush()
** entityManager.merge(type) issues:
*** loads another copy for comparison - extra read
*** may overwrite other changes - if detached
*** may hit non uniqueness issues and uninitialised associations

= Intefaces =
Much simpler than EJB 2.0 - No longer have to implement any interfaces - though some still exist - see stateful

Interface now only needs to include annotation @Remote or @Local

* @Remote
* remote business interface

= Local/Remote interface =
<pre>
import javax.ejb.Remote;

@Remote
public interface FooBeanRemote {
...
}
</pre>

= Entity Beans =
* Just POJOs
* Needs to implement java.io.Serializable or Externalizable to be accessed remotely
* May throw javax.ejb.EJBException - extends java.lang.RuntimeException
* May declare exception behaviour using @javax.ejb.ApplicationException or <application-exception>
* May reference resources using annotations @Resource(name="xxx") rather than jndi lookup though InitialContext in code
* Much of the deployment descriptor is now available as annotations. Deployment descriptor can still be used to argument or override
** Note if full deployment descriptor used to define local, remote and session-type, bean could be devoid of any references to javax.ejb

== Statelesss ==
<pre>
import javax.ejb.Stateless;
@Stateless
public class FooBean implements FooBeanRemote, FooBeanLocal {
@Resource(mappedName="fooDB")
DataSource ds;
...
}
</pre>
May also define local and remote interfaces with annotations
This prevents compile time checking of the interfaces though
<pre>
@Stateless
@Local(FooBeanRemote.class)
@Remote(FooBeanLocal.class)
public class FooBean {
</pre>
== Stateful ==
* @Remove - annotates methods - releases bean from client when called

=== Extended context ===
<pre>
import static javax.persistence.PersistenceContextType.EXTENDED;
...
@PersistenceContext(unitName="ComponetEntityManager", type=EXTENDED)
private EntityManager entityManager;

...
public void updateFoo(String high) {
foo.setBar(high);
}
</pre>
* Entity manager continues to track changes to entity
* Updates to entity automatically get commited when the method completes - no need to call merge() or flush()
* Normal behaviour is to only track within method
==== Nesting ====
* When one stateful bean in nested in another the create and remove are cascaded to the nested bean automatically
* They can share the same persistence context if
** both are local
** both extended
** both use the same named context

== Resources ==
* @Resource may also be used for any Enterprise Naming Context resource such as deployment descriptor references

<pre>
<ejb-jar
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/ejb-jar_3_0.xsd"
version="3.0">
<enterprise-beans>
<session>
<ejb-name>FooBean</ejb-name>
<env-entry>
<env-entry-name>bar</env-entry-name>
<env-entry-type>java.lang.String</env-entry-type>
<env-entry-value>ENC example</env-entry-value>
</env-entry>
</session>
</enterprise-beans>
</ejb-jar>
</pre>

= EJB core classes =
== javax.ejb.SessionContext ==
[http://java.sun.com/javaee/5/docs/api/index.html?javax/ejb/SessionContext.html javax.ejb.SessionContext]
Now accessed with
<pre>
@Resource
javax.ejb.SessionContext ctx
</pre>

* getEJBObject() and getEJBLocalObject() - now obsolete and throw an exception
* <T> getBusinessObect(Class<T> businessInterface) - now returns the reference to current EJB to pass as a callback
** businessInterface - must be local or remote interface
** can not pass this reference to another bean

* Class getInvolkedBusinessInterface() - returns local,remote or web service interface depending on which interface the current call was made

== javax.ejb.EJBContext ==
SessionContext extends EJBContext
* Object lookup(String name) - Convince method
* TimerService getTimerService() - Gets a timerService to set up an event at a specific time/interval - also available as @Resource
* [http://java.sun.com/j2se/1.4.2/docs/api/java/security/Principal.html java.security.Principal] getCallerPrincipal() - can be used to get the name of the caller
* UserTransaction getUserTransaction()

== javax.transaction.UserTransaction
* void begin()
* void commit()
* void rollback() - when complete no transaction is associated
* void setRollbackOnly() - marks transaction for rollback
* void setTransactionTimeout(int seconds)
* int getStatus()

= Other technologies =
Lower level:
* JdbcTemplate http://www.vogella.de/articles/SpringJDBC/article.html
* iBATIS http://ibatis.apache.org/

EJB 3 - Revision history

Martin: Created page with "* Containers support caching and coherence across containers, this leads to a scalable solution with little additional work * Performance problems are much reduced from the ea..."