Hibernate

Introduction

Hibernate is an open source object/relational mapping tool for Java. Hibernate lets you develop persistent classes following common Java idiom - including association, inheritance, polymorphism, composition and the Java collections framework.

Hibernate makes use of persistent objects commonly called as POJO (POJO = "Plain Old Java Object".) along with XML mapping documents for persisting objects to the database layer. The term POJO refers to a normal Java objects that does not serve any other special role or implement any special interfaces of any of the Java frameworks (EJB, JDBC, DAO, JDO, etc...). Hibernate uses runtime reflection to determine the persistent properties of a class. The objects to be persisted are defined in a mapping document, which serves to describe the persistent fields and associations, as well as any subclasses or proxies of the persistent object. The mapping documents are compiled at application startup time and provide the framework with necessary information for a class. Additionally, they are used in support operations, such as generating the database schema or creating stub Java source files. In order to provide a clean POJO programming model, Hibernate hides itself inside of your POJO by using either its own implementation of the JDK collections or by using a CGLIB proxy to surround an object reference, depending on the type of association it is managing. Since object graphs can be quite large, and in some cases infinite, it is mandatory to draw the line somewhere when loading an object and claim these associations as lazy. This deferral means that at some later point, it may be necessary to fetch the associated objects from the database when this line is crossed.

Configuration and Startup

If there is a file named import.sql e.g. in the root directory of an EJB jar file and hibernate.hbm2ddl.auto in the persistence.xml the file is imported at startup of a JBoss application server.

SessionFactory

A session factory represents a particular logical data-store configuration (s.hibernate.cfg.xml). The JPA aquivalent of the session factory is the entity manager factory, the aquivalent of the session factory configuration is the persistence unit. In most hibernate applications a SessionFactory should be instantiated once during application initialization. There are advanced options to store the factory like JNDI. A simple and pragmatic approach is to use a static global variable and static initialization (s. example uweheuer application server.HibernateUtil).

Session

A session is a particular unit of work, that means a set of data access operations. The aquivalent in JPA is the entity manager.

Startup

A typical example of a Hibernate startup procedure:

SessionFactory sf = new Configuration().configure().buildSessionFactory();

or

uweheuerSessionFactory = new AnnotationConfiguration().configure("uweheuer.hibernate.cfg.xml").buildSessionFactory();

When new configuration() is called Hibernate searches for a file named hibernate.properties in the root of the classpath. If found they are loaded into the configuration object. When configure() is called, Hibernate searches for a file named hibernate.cfg.xml. Bei Web Application ist die Ablage unter \WEB-INF\classes. Its possible to have more than one for multiple databases, one for each database (e.g. project 'uweheuer', files uweheuer.hibernate.cfg.xml and uweswiki.hibernate.cfg.xml).

hibernate.cfg.xml

<hibernate-configuration>
 <session-factory>
  <property name="hbm2ddl.auto">[update|create]</property>

Update scheint die beste Einstellung, Änderungen an der Klasse bewirken beim Initialisieren der Anwendung (Reload for a web application) eine Änderung in der zugehörigen Tabelle. Daten gehen nicht verloren. Allerdings werden z.B. neue java long members in der DB mit NULL initialisiert. Dies führt in der Regel zu einer Exception beim Laden.

Libraries

• ejb3-persistance.jar (javax.persistance.*)
• hibernate3.jar (org.hibernate.*)

Associations

EJB CMP 2.0 also manages associations/relationships (CMR). Associations in CMP are inherently bidirectional, a change to one side is instantly reflected at the other side. Hibernate don't implement managed associations.

Annotations

In addition to the standard JPA annotation set, Hibernate has its own 'vendor' annotations to support additional mapping features and association types that aren’t part of the JPA specificationTo use annotations the java file has to (s. JPA Annotations)

import javax.persistance.*;

and the configuration file has to list all annotated classes like

 <mapping class="<Package>.<Classname>" />

Length

@org.hibernate.validator.Length(min=<Number>, max=<Number>)
<attribute declaration>

for length validation.

NotNull

@org.hibernate.validator.NotNull
<attribute declaration>

for validation.

Patterns

Ordererd unidirectional OneToMany Association

Class A has many Bs, B has one A, but doesn't know about it:

class A
{
  @OneToMany([<fetching>|<cascading>])
  @IndexColumn(name = <name>) // list order
  private List<B> bs = new ArrayList<B>;

Merkwürdigerweise liegt dies eine zusätzliche Mapping-Tabelle namens A_B an, die man eigentlich vermeiden könnte, aber es funktioniert. Es muss immer das Objekt der Klasse A gespeichert werden. Auf der Seite von B steht nich

Non Ordered bedirectional OneToMany Association

Class A has many Bs, B has one A:

class A
{
  @OneToMany(mappedBy = "<MemberOfB>")
  private <Collection><B> bs = new <ConcreteCollection>();
  
  public void addB(B b)
  {
    b.setA(this); // neccessary, that the relationship can be handled only by the other end
    bs.add(b);
  }

mappedBy signals that the associations is managed by the member of B (foreign key of table of B).

Ordered biderectional OneToMany Association

in the one class:

@OneToMany(cascade = {CascadeType.ALL})	
@JoinColumn(name = "menu_id", nullable = false)
@IndexColumn(name = "menu_list_position")	// list order      
private List<Urlx> urls = new ArrayList<Urlx>()

in the many class:

@ManyToOne
@JoinColumn(name = "menu_id", nullable = false, updatable = false, insertable = false)
private Menux  menu;

s. also compare http://forum.hibernate.org/viewtopic.php?t=974127

Ordered biderectional ZeroToMany Association

in the zero or one class:

@OneToMany(cascade = {CascadeType.ALL})	
@JoinColumn(name = "parent_id", nullable = true) // nullable is true
@IndexColumn(name = "parent_list_position")	 // list order            
private List<Menux> menus = new ArrayList<Menux>();

in the many class:

@ManyToOne
@JoinColumn(name = "parent_id", nullable = true, updatable = false, insertable = false)
private Menux  parent;

Development Process

1. copy the following libs to the application lib directory (hibernate core, hibernate annotations, hibernate validation):
   ant-antlr-1.6.5.jar
   asm-attrs.jar
   asm.jar
   cglib-2.1.3.jar
   commons-collections-2.1.1.jar
   commons-logging-1.0.4.jar
   dom4j-1.6.1.jar
   ejb3-persistence.jar
   hibernate-annotations.jar
   hibernate-commons-annotations.jar
   hibernate-validator.jar
   hibernate3.jar
   jta.jar
   log4j-1.2.11.jar
   <JDBCDriver>.jar
   
2. create the annotated class e.g. Action.java
3. create the Hibernate configuration file hibernate.cfg.xml
4. use the persistance classes

Building an Application

• add libs
• reengineer db to java classes or create java classes
• create Util class for session e.g. HibernateUtil.java in project 'ballaballa_hiber'

Designing the domain model

Entitity Types vs. Value Types

Hibernate supports a fine-grained domain model (more classes than tables). An object of entity type has its own identity (primary key value). An object reference to an entity ist persisted as a reference in the database (a foreign key value). An entity has its own life cylce, it may exist independently of other entities. Criteria for an entity is:

• shared references
• lifecycle dependencies
• identity

An object of value type has no database identity, it belongs to an entity instance and its persistent state ist embedded in the table row of its owning entity.

Associations

To specifiy the multiplicity it is neccessary to ask:

• can there be more than one object of class A for a particular object of class B
• can there be more than one object of class B for a particular object of class A

For a many-end association end the propertie must be of an interface type like java.util.set or java.util.List.

Reverse Engineer a Database

see Eclipse

Adding persistent class members

• add it to the class
• rebuild and reload application
• the hibernate config file manages the db, if set correspondingly

Storing new or detached objects

org.hibernate.Session session = server.HibernateUtil.getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
session.save(vocable); // update() for detached objects
tx.commit();
session.close();

Debugging Concepts

The best way is to enabling logging via log4j:

Log4j

In JBoss Logging configuration is done here.

special entries in log4j.properties:

### log the sql inclusive values
log4j.logger.org.hibernate.SQL=debug
log4j.logger.org.hibernate.type=debug

ConfigurationFiles

Add the following entries:

<property name="show_sql">true</property>
<property name="format_sql">true</property>
<property name="use_sql_comments">true</property>

then all SQL statements are written to stdout.

Literatur

1. Java Persistance with Hibernate
2. Online-Buch zu Annotations