Hibernate.orgCommunity Documentation

Hibernate Getting Started Guide

4.2.16.Final

2014-10-30


Table of Contents

Preface
1. Get Involved
2. Tutorial code
1. Obtaining Hibernate
1.1. Release Bundle Downloads
1.2. Maven Repository Artifacts
2. Tutorial Using Native Hibernate APIs and hbm.xml Mappings
2.1. The Hibernate configuration file
2.2. The entity Java class
2.3. The mapping file
2.4. Example code
2.5. Take it further!
3. Tutorial Using Native Hibernate APIs and Annotation Mappings
3.1. The Hibernate configuration file
3.2. The annotated entity Java class
3.3. Example code
3.4. Take it further!
4. Tutorial Using the Java Persistence API (JPA)
4.1. persistence.xml
4.2. The annotated entity Java class
4.3. Example code
4.4. Take it further!
5. Tutorial Using Envers
5.1. persistence.xml
5.2. The annotated entity Java class
5.3. Example code
5.4. Take it further!
6. OSGi Tutorial
6.1. Project Overview
6.2. Project Structure
6.3. TODOs
6.4. Karaf Commands

List of Examples

2.1. The class mapping element
2.2. The id mapping element
2.3. The property mapping element
2.4. Obtaining the org.hibernate.SessionFactory
2.5. Saving entities
2.6. Obtaining a list of entities
3.1. Identifying the class as an entity
3.2. Identifying the identifier property
3.3. Identifying basic properties
4.1. persistence.xml
4.2. Obtaining the javax.persistence.EntityManagerFactory
4.3. Saving (persisting) entities
4.4. Obtaining a list of entities
5.1. Using the org.hibernate.envers.AuditReader

Working with both Object-Oriented software and Relational Databases can be cumbersome and time consuming. Development costs are significantly higher due to a paradigm mismatch between how data is represented in objects versus relational databases. Hibernate is an Object/Relational Mapping solution for Java environments. The term Object/Relational Mapping refers to the technique of mapping data between an object model representation to a relational data model representation. See http://en.wikipedia.org/wiki/Object-relational_mapping for a good high-level discussion.

Note

You do not need a strong background in SQL to use Hibernate, but having a basic understanding of the concepts can help you understand Hibernate more fully and quickly. An understanding of data modeling principles is especially important. You might want to consider these resources as a good starting point:

Hibernate takes care of the mapping from Java classes to database tables, and from Java data types to SQL data types. In addition, it provides data query and retrieval facilities. It can significantly reduce development time otherwise spent with manual data handling in SQL and JDBC. Hibernate’s design goal is to relieve the developer from 95% of common data persistence-related programming tasks by eliminating the need for manual, hand-crafted data processing using SQL and JDBC. However, unlike many other persistence solutions, Hibernate does not hide the power of SQL from you and guarantees that your investment in relational technology and knowledge is as valid as always.

Hibernate may not be the best solution for data-centric applications that only use stored-procedures to implement the business logic in the database, it is most useful with object-oriented domain models and business logic in the Java-based middle-tier. However, Hibernate can certainly help you to remove or encapsulate vendor-specific SQL code and streamlines the common task of translating result sets from a tabular representation to a graph of objects.

The Hibernate team provides release bundles hosted on the SourceForge File Release System, in ZIP and TGZ formats. Each release bundle containsJARs, documentation, source code, and other goodness.

You can download releases of Hibernate, in your chosen format, from the list at http://sourceforge.net/projects/hibernate/files/hibernate4/.

  • The lib/required/ directory contains all the JARs Hibernate requires. All the jars in this directory must also be included in your project's classpath.

  • The /lib/jpa/ directory contains the hibernate-entitymanager jar and its dependencies beyond those in lib/required/. This defines Hibernate support for JPA.

  • The lib/envers directory contains the hibernate-envers jar and its dependencies beyond those in lib/required/

  • The lib/optional directory contains the jars needed for optional features of Hibernate.

Note

The authoritative repository for Hibernate artifacts is the JBoss Maven repository. The team responsible for the JBoss Maven repository maintains a number of Wiki pages that contain important information.

Maven Repository Wiki Pages

Hibernate produces a number of artifacts (all under the org.hibernate groupId):

Hibernate Artifacts under groupId org.hibernate

hibernate-core

The main artifact, needed to build applications using the native Hibernate APIs including defining metadata in both annotations as well as Hibernate's own hbm.xml format.

hibernate-entitymanager

Represents Hibernate's implementation of JPA, as specified at http://jcp.org/en/jsr/detail?id=317.

This artifact depends on hibernate-core

hibernate-envers

An optional module that provides historical auditing of changes to your entities.

This artifact depends on both hibernate-core and hibernate-entitymanager.

hibernate-c3p0

Provides integration between Hibernate and the C3P0 connection pool library. See http://sourceforge.net/projects/c3p0/ for information about C3P0.

This artifact depends on hibernate-core, but is generally included in a project as a runtime dependency. It pulls in the C3P0 dependencies automatically.

hibernate-proxool

Provides integration between Hibernate and the Proxool connection pool library. See http://proxool.sourceforge.net/ for more information about this library.

This artifact depends on hibernate-core, but is generally included in a project as a runtime dependency. It pulls in the Proxool dependencies automatically..

hibernate-ehcache

Privides integration between Hibernate and EhCache, as a second-level cache. See http://ehcache.sourceforge.net/ for more information aboutEhCache.

This artifact depends on hibernate-core, but is generally included in a project as a runtime dependency. It pulls in the Ehcache dependencies automatically.

hibernate-infinispan

Provides integration between Hibernate and Infinispan, as a second-level cache. See http://jboss.org/infinispan for more information about Infinispan.

This artifact depends on hibernate-core, but is generally included in a project as a runtime dependency. It pulls in the Infinispan dependencies automatically.

This tutorial is located within the download bundle under basic/.

Objectives

  • using Hibernate mapping files (hbm.xml) to provide mapping information

  • using the native Hibernate APIs

The hbm.xml mapping file for this tutorial is the classpath resource org/hibernate/tutorial/hbm/Event.hbm.xml as we saw in Section 2.1, “The Hibernate configuration file”

Hibernate uses the mapping metadata to determine how to load and store objects of the persistent class. The Hibernate mapping file is one choice for providing Hibernate with this metadata.


Functions of the class mapping element

  1. The name attribute (combined here with the package attribute from the containing hibernate-mapping element) names the FQN of the class to be defined as an entity.

  2. The table attribute names the database table which contains the data for this entity.

Instances of the Event class are now mapped to rows in the EVENTS table.


Hibernate uses the property named by the id element to uniquely identify rows in the table.

Important

It is not required for the id element to map to the table's actual primary key column(s), but it is the normal convention. Tables mapped in Hibernate do not even need to define primary keys. However, it is strongly recommend that all schemas define proper referential integrity. Therefore id and primary key are used interchangeably throughout Hibernate documentation.

The id element here identifies the EVENT_ID column as the primary key of the EVENTS table. It also identifies the id property of the Event class as the property containing the identifier value.

The generator element nested inside the id element informs Hibernate about which strategy is used to generated primary key values for this entity. This example uses a simple incrementing count.


The two property elements declare the remaining two properties of the Event class: date and title. The date property mapping includes the column attribute, but the title does not. In the absence of a column attribute, Hibernate uses the property name as the column name. This is appropriate for title, but since date is a reserved keyword in most databases, you need to specify a non-reserved word for the column name.

The title mapping also lacks a type attribute. The types declared and used in the mapping files are neither Java data types nor SQL database types. Instead, they are Hibernate mapping types. Hibernate mapping types are converters which translate between Java and SQL data types. Hibernate attempts to determine the correct conversion and mapping type autonomously if the type attribute is not present in the mapping, by using Java reflection to determine the Java type of the declared property and using a default mapping type for that Java type.

In some cases this automatic detection might not chose the default you expect or need, as seen with the date property. Hibernate cannot know if the property, which is of type java.util.Date, should map to a SQL DATE, TIME, or TIMESTAMP datatype. Full date and time information is preserved by mapping the property to a timestamp converter, which identifies an instance of the class org.hibernate.type.TimestampType.

Tip

Hibernate determines the mapping type using reflection when the mapping files are processed. This process adds overhead in terms of time and resources. If startup performance is important, consider explicitly defining the type to use.

The org.hibernate.tutorial.hbm.NativeApiIllustrationTest class illustrates using the Hibernate native API.

Note

The examples in these tutorials are presented as JUnit tests, for ease of use. One benefit of this approach is that setUp and tearDown roughly illustrate how a org.hibernate.SessionFactory is created at the start-up of an application and closed at the end of the application lifecycle.


Procedure 2.1. Tutorial Workflow

  1. The configuration is loaded.

    The org.hibernate.cfg.Configuration class is the first thing to notice. In this tutorial, all configuration details are located in the hibernate.cfg.xml file discussed in Section 2.1, “The Hibernate configuration file”.

  2. The org.hibernate.SessionFactory is created.

    The org.hibernate.cfg.Configuration then creates the org.hibernate.SessionFactory which is a thread-safe object that is instantiated once to serve the entire application.

  3. SessionFactory creates Session instances.

    The org.hibernate.SessionFactory acts as a factory for org.hibernate.Session instances as can be seen in the testBasicUsage method.

  4. Sessions perform work.

    A org.hibernate.Session should be thought of as a corollary to a "unit of work".


testBasicUsage first creates some new Event objects and hands them over to Hibernate for management, using the save method. Hibernate now takes responsibility to perform an INSERT on the database.


testBasicUsage illustrates use of the Hibernate Query Language (HQL) to load all existing Event objects from the database and generate the appropriate SELECT SQL, send it to the database and populate Event objects with the result set data.

This tutorial is located within the download bundle under annotations.

Objectives

  • Use annotations to provide mapping information

  • Use the native Hibernate APIs

The contents are identical to Section 2.1, “The Hibernate configuration file”, with one important difference. The mapping element at the very end naming the annotated entity class using the class attribute.

The entity class in this tutorial is org.hibernate.tutorial.annotations.Event which follows JavaBean conventions. In fact the class itself is identical to the one in Section 2.2, “The entity Java class”, except that annotations are used to provide the metadata, rather than a separate hbm.xml file.


The @javax.persistence.Entity annotation is used to mark a class as an entity. It functions the same as the class mapping element discussed in Section 2.3, “The mapping file”. Additionally the @javax.persistence.Table annotation explicitly specifies the table name. Without this specification, the default table name would be EVENT).


@javax.persistence.Id marks the property which defines the entity's identifier. @javax.persistence.GeneratedValue and @org.hibernate.annotations.GenericGenerator work in tandem to indicate that Hibernate should use Hibernate's increment generation strategy for this entity's identifier values.


As in Section 2.3, “The mapping file”, the date property needs special handling to account for its special naming and its SQL type.

This tutorial is located within the download bundle under entitymanager.

Objectives

  • Use annotations to provide mapping information.

  • Use JPA.

The previous tutorials used the Hibernate APIs. This tutorial uses the JPA APIs.


Notice again that the persistence unit name is org.hibernate.tutorial.jpa, which matches Example 4.1, “persistence.xml


The code is similar to Example 2.5, “Saving entities”. An javax.persistence.EntityManager interface is used instead of a org.hibernate.Session interface. JPA calls this operation persist instead of save.


Again, the code is pretty similar to Example 2.6, “Obtaining a list of entities”.

This tutorial is located within the download bundle under envers.

Objectives

  • Configure Envers.

  • Use the Envers APIs to view and analyze historical data.

Again, the entity is largely the same as in Section 4.2, “The annotated entity Java class”. The major difference is the addition of the @org.hibernate.envers.Audited annotation, which tells Envers to automatically track changes to this entity.

Hibernate targets the OSGi 4.3 spec or later and supports three types of configurations.

For more details about OSGi, the three configurations, hibernate-osgi, extensions points, and caveats, please see the OSGi chapter of the Developer's Guide!