| Object-relational mapping |
Which helps you in converting the relational databases to object oriented programming languages,which help a developer to keep the persistence of the domain object.Basically ORM tools will provide the bridge between the database and the application by storing application objects in the database for the developer, rather than requiring the developer to write and maintain mountains of code to store and retrieve objects.
In other words we can say, Object/relational mapping (ORM) is the process of persisting objects in a relational database. ORM bridges the gap between object and relational schemas, allowing your application to persist objects directly without requiring you to convert objects to and from a relational format.There are List of ORM softwares for Java. |
| All among the list of Software’s Hibernate, requires a small amount of metadata for each persistent object. Hibernate operates independently of application architecture, allowing it to be used in various applications. It provides full object/relational mapping,meaning that it supports all the available object-oriented features that relational databases lack. |
| Hibernate - Free GPL,ORM Library |
The primary feature’s are,
- Mapping - Hibernate maps the java classes a.k.a domain objects to db tables through xml configuration. This xml configuration tells the Hibernate how to persist these objects.
- Persistence - Hibernate provides transparent persistence for Plain Old Java Objects (POJOs). The only strict requirement for a persistent class is a no-argument constructor, not necessarily public
- Hibernate Query Language (HQL) - Hibernate provides a SQL inspired language called Hibernate Query Language (HQL) which allows SQL-like queries to be written against Hibernate’s data objects. Criteria Queries are provided as an object-oriented alternative to QL.
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| Mapping XML |
| Mapping definitions, also called mapping documents, are used to provide Hibernate with information to persist objects to a relational database.The mapping files also provide support features, such as creating the database schema from a collection of mapping files.The mapping file tells Hibernate what table in the database it has to access,and what columns in that table it should use. |
<?xml version="1.0"?>
<hibernate-mapping package="com.techmaddy.domain">
[...]
</hibernate-mapping>
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| Now we’ll try to add the domain object Book, which holds Author, Edition. By this we are telling the Hibernate to persist this object and map from RDBMS to java code.So the above mentioned XML will be changed to the following. |
<?xml version="1.0"?>
<hibernate-mapping package="com.techmaddy.domain">
<class name="com.techmaddy.domain.Book" table="HBD_BOOKS">
<property name="authorName" type="string" column="HBD_AUTHNAME" not-null="true" />
<property name="edition" type="string" column="HBD_EDITION" not-null="true" />
</class>
</hibernate-mapping>
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| From the above mentioned config file, we’ll end up generating the domain class, with two member. The name attribute tells the hibernate to generate the getter and setter for that property. So in our exmaple it’ll be like getAuthorName() and getEdition(). And the same for setters also. Now we’ll see one more scenario, every domain object we add may have the primary key associated. Most of the time this key will be autogenerated, the property tag doesn’t have any special attribute to specify this. So we are going to make use of the id tag, which tells the hibernate, this column is the primary key and it’s going to be generated automatically. So i’m going to add some more tags in the above xml. |
<?xml version="1.0"?>
<hibernate-mapping package="com.techmaddy.domain">
<class name="com.techmaddy.domain.Book" table="HBD_BOOKS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
</id>
<property name="authorName" type="string" column="HBD_AUTHNAME" not-null="true" />
<property name="edition" type="string" column="HBD_EDITION" not-null="true" />
</class>
</hibernate-mapping>
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| The generator creates the primary key value for the persistent class.Native generators provide portability for mapping documents since the framework can determine the generator method supported by the database. Generators using the native class will use identity or sequence columns depending on available database support. If neither method is supported, the native generator falls back to a high/low generator method to create unique primary key values. |
| The other generators are Assigned and Select. The Assign lets you generate and assign the object ID.Select generator, which retrieves the primary key value by selecting a value from a database trigger. The generator type you choose determines its behavior based on the underlying database. |
| Hibernate Components & Configuration |
| Hibernate can be used with Web Application and with an Standalone java applications also. The below image will give you the basic components of Hibernate. |
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| hibernate.cfg.xml & hibernate.properties - These are used to configure the hibernate service. Both these files does the samething.When these two are present in the classpath, then hibernate.cfg.xml will overrides the hibernate.properties file. Basically these files are used to basic connection info, whcih we are going to use it in our application. |
| Session Factory - SessionFactory allows application to create the Hibernate Sesssion by reading the configuration from hibernate.cfg.xml file.It implements a design pattern, that ensures that only one instance of the session is used per thread. |
| Session - represents a single-threaded unit of work.Session instances are your primary interface to the Hibernate persistence service. |
| By using the Hibernate we are going to connect to DB, So the Hibernate should be instructed to pick the drivers and get the connection.The following XML, will gives us the skeleton of the hibernate.cfg.xml |
<hibernate-configuration>
<session-factory>
[...Properties for JDBC Connection]
....
[...]
[..Mapping resources]
....
[...]
</session-factory>
</hibernate-configuration>
|
| For the above hibernate configuration, we’ll try to add the properties for HSQL database. Then sessionfactory tag will becomes the following |
<session-factory>
<property name="connection.driver_class">org.hsqldb.jdbcDriver</property>
<property name="connection.url">jdbc:hsqldb:hsql://localhost</property>
<property name="connection.username">sa</property>
<property name="connection.password"></property>
<property name="connection.pool_size">1</property>
<property name="dialect">org.hibernate.dialect.HSQLDialect</property>
</session-factory>
|
| This configures the connection properties, along with the SQL Dialect. The SQL dialect tells the hibernate that we are using a specific JDBC drivers. In this example we are going to make use of HSQL DB.The List of dialects are available here. |
| Now we’ll add the domain object mappings to hibernate.cfg.xml. The following syntax shows how to add an mapping xml for an domain object. |
<mapping resource="com/techmaddy/domain/Book.hbm.xml"/>
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| Adding this completes the hibernate basic configuration, along with this we may need to add the libraries for the hibernate.Creating the Startups for creating the session factory and helpers we’ll be dicuss in the next article. |
, I read some wher if you keep your file in deploy folder, then jBoss will treat it as a Service/Application, get back here after a week, Will let you know.