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.

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

To use annotations the java file has to (s. EJB 3.0)

import javax.persistance.*;

and the hibernate.cfg.xml has to list all annotated classes like

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

The only neccessary annotation for a class ist

@entity

than all attributes are taken as persistent.

This annotations

@id

also specifies, which access method is taken. If it is placed directly at the attribute the access is taken directly, else via getter and setter.

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

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.

Hibernate Infrastructure

SessionFactory

In most hibernate applications SessionFactory should be instantiated once during application initialization (s. example uweheuer application server.HibernateUtil).

Patterns

One_to_Many List

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;

Zero_or_One_to_Many List

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;

Debugging Concepts

The best way is to enabling logging via log4j:

Log4j

special entries in log4j.properties:

hibernate.cfg.xml

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.

Hibernate Configuration

hibernate.cfg.xml

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

update scheint die beste Einstellung, Änderungen an der Klasse bewirken eine Änderung in der zugehörigen Tabelle. Ob Daten verloren gehen muss noch geklärt werden.