02 hibernateintroduction

Professional Open Source™

Hibernate Introduction
Hello World and the Hibernate APIs

© JBoss, Inc. 2003, 2004. 07/17/04 1

Hello World I

 The Hello World program prints messages
– To demonstrate Hibernate, let’s define a persistent message
– we use a Message persistent class, POJO style

package hello;

public class Message {
private Long id;
private String text;

public String getText() {
return text;
}

public void setText(String text) {
this.text = text;
}

…
}

© JBoss, Inc. 2003, 2004. 2

Hello World II

 Messages don't have to be persistent!
– we can use our persistent classes “outside” of Hibernate
– Hibernate is able to “manage” persistent instances
– but POJO persistent classes don't depend on Hibernate

Message message = new Message("Hello World");
System.out.println( message.getText() );

© JBoss, Inc. 2003, 2004. 3

Hello World VI

 XML mapping metadata:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sf.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>

<class
name="hello.Message"
table="MESSAGES">

<id>...</id>

<property
name="text"
column="MESSAGE_TEXT"/>

</class>

</hibernate-mapping>

© JBoss, Inc. 2003, 2004. 4

Using C3P0 with hibernate.properties

 Let's configure Hibernate to use C3P0 connection pooling
Hibernate automatically loads hibernate.properties from a root
directory of the classpath

hibernate.connection.driver_class = com.mysql.jdbc.Driver
hibernate.connection.url = jdbc:mysql://localhost/helloworld
hibernate.connection.username = root
hibernate.connection.password = root

hibernate.dialect = org.hibernate.dialect.mySQLDialect

hibernate.c3p0.min_size = 5
hibernate.c3p0.max_size = 20
hibernate.c3p0.timeout = 1800
hibernate.c3p0.max_statements = 50
Hibernate.c3p0.validate = true

Don't forget to set the SQL dialect!

© JBoss, Inc. 2003, 2004. 5

Starting Hibernate

 We create a SessionFactory using a Configuration
– download and install JDBC driver in classpath
– copy Hibernate and the required 3rd party libraries
– chose a JDBC connection pool, customize properties
– add mapping files to the Configuration
– build the SessionFactory (immutable!)

SessionFactory sessionFactory = new Configuration()
.addResource("hello/Message.hbm.xml")
.buildSessionFactory();

© JBoss, Inc. 2003, 2004. 6

Configuring hibernate.cfg.xml

 Let's configure Hibernate using hibernate.cfg.xml

<hibernate-configuration>

<session-factory>

<property name="dialect">org.hibernate.dialect.MySQLInnoDBDialect</property>
<property name="show_sql">true</property>
<property name="use_outer_join">false</property>
<property name="connection.username">root</property>
<property name="connection.password">root</property>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="connection.url">jdbc:mysql://localhost/helloworld</property>
<property name="hibernate.hbm2ddl.auto">insert</property>

<property name="cache.provider_class"> org.hibernate.cache.NoCacheProvider</property>

<mapping resource="hello/Message.hbm.xml"/>
</session-factory>

</hibernate-configuration>

© JBoss, Inc. 2003, 2004. 7

Starting Hibernate

 We create a SessionFactory using a Configuration
– download and install JDBC driver in classpath
– copy Hibernate and the required 3rd party libraries
– chose a JDBC connection pool, customize properties
– add mapping files to the Configuration
– build the SessionFactory (immutable!)

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

© JBoss, Inc. 2003, 2004. 8

Configuring Logging SQL

 You have to configure the following hibernate.cfg.xml

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

© JBoss, Inc. 2003, 2004. 9

Hello World III

 Let's persist a message with Session and Transaction:

Session session = getSessionFactory().openSession();
Transaction tx = session.beginTransaction();

Message message = new Message("Hello World");
session.save(message);

tx.commit();
session.close();

 Hibernate executes this SQL:

insert into MESSAGES (MESSAGE_ID, MESSAGE_TEXT)
values (1, 'Hello World')

© JBoss, Inc. 2003, 2004. 10

Hello World IV

 Let's show all persistent messages:

Session newSession = getSessionFactory().openSession();
Transaction newTransaction = newSession.beginTransaction();
Query query== newSession.createQuery("from Message");

List messages =query.list();

System.out.println( messages.size() + " message(s) found:" );

for ( Iterator iter = messages.iterator(); iter.hasNext(); ) {
Message message = (Message) iter.next();
System.out.println( message.getText() );
}

newTransaction.commit();
newSession.close();

select m.MESSAGE_ID, m.MESSAGE_TEXT from MESSAGES m

© JBoss, Inc. 2003, 2004. 11

Hello World V

 Let's update a message:

Session session = getSessionFactory().openSession();
Transaction tx = session.beginTransaction();

// 1 is the generated id of the first message
Message message = session.load( Message.class, new Long(1) );

message.setText("Greetings Earthling");

tx.commit();
session.close();

select m.MESSAGE_TEXT from MESSAGES m where m.MESSAGE_ID = 1
update MESSAGES set MESSAGE_TEXT = ‘Greetings Earthling'

Notice that we did not explicitly call any update() method - automatic dirty
checking gives us more flexibility when organizing data access code!
© JBoss, Inc. 2003, 2004. 12

Other configuration options

 Instead of using a hibernate.properties file, we may also
– pass an instance of Properties to Configuration programmatically
– set System properties using java -Dproperty=value
– use a hibernate.cfg.xml file in the classpath

The XML-based configuration is almost equivalent to the
properties, it has some more features (cache tuning). The XML file
overrides the hibernate.properties options.

We usually prefer the XML configuration file,
especially in managed environments.

© JBoss, Inc. 2003, 2004. 13

Hibernate Architecture

Transaction Query Application

Session

Session Factory

Configuration

Hibernate.cfg.xml Hibernate.properties Mapping files

© JBoss, Inc. 2003, 2004. 14

SessionFactory

SessionFactory (org.hibernate.SessionFactory)

A thread safe (immutable) cache of compiled
mappings for a single database. A factory for
Session and a client of Connection Provider.
Might hold an optional (second-level) cache of
data that is reusable between transactions, at a
process- or cluster-level.

© JBoss, Inc. 2003, 2004. 18

Session

Session (org.hibernate.Session)

A single-threaded, short-lived object representing
a conversation between the application and the
persistent store

Wraps a JDBC connection. Factory for
Transaction. Holds a mandatory (first-level)
cache of persistent objects, used when navigating
the object graph or looking up objects by
identifier.
© JBoss, Inc. 2003, 2004. 19

Persistent Objects and Collections

Persistent objects and collections

Short-lived, single threaded objects containing
persistent state and business function. These
might be ordinary JavaBeans/POJOs, the only
special thing about them is that they are currently
associated with (exactly one) Session. As soon as
the Session is closed, they will be detached and
free to use in any application layer (e.g. directly
as data transfer objects to and from presentation).

© JBoss, Inc. 2003, 2004. 20

Transient and detached objects and collections

Transient and detached objects and collections

Instances of persistent classes that are not
currently associated with a Session. They may
have been instantiated by the application and not
(yet) persisted or they may have been instantiated
by a closed Session.
.

© JBoss, Inc. 2003, 2004. 21

Transaction

Transaction (org.hibernate.Transaction)

(Optional) A single-threaded, short-lived object
used by the application to specify atomic units of
work. Abstracts application from underlying
JDBC, JTA or CORBA transaction. A Session
might span several Transactions in some cases.
However, transaction demarcation, either using
the underlying API or Transaction, is never
optional!
.

© JBoss, Inc. 2003, 2004. 22

ConnectionProvider

ConnectionProvider

(org.hibernate.connection.ConnectionProvider)
(Optional) A factory for (and pool of) JDBC
connections. Abstracts application from
underlying Datasource or DriverManager. Not
exposed to application, but can be
extended/implemented by the developer.

© JBoss, Inc. 2003, 2004. 23

TransactionFactory

TransactionFactory

(org.hibernate.TransactionFactory)
(Optional) A factory for Transaction instances.
Not exposed to the application, but can be
extended/implemented by the developer.

© JBoss, Inc. 2003, 2004. 24

Extension Interfaces

Extension Interfaces
Hibernate offers many optional extension interfaces
you can implement to customize the behavior of your
persistence layer.(Ex. Primary key generation,SQL
Dialet , Caching,JDBC connection, Proxy
creationetc.,

Given a "lite" architecture, the application bypasses
the Transaction/TransactionFactory and/or
ConnectionProvider APIs to talk to JTA or JDBC
directly.

© JBoss, Inc. 2003, 2004. 25

Instance States

Instance states
An instance of a persistent classes may be in one
of three different states, which are defined with
respect to a persistence context.

© JBoss, Inc. 2003, 2004. 26

Instance States

Instance states
An instance of a persistent classes may be in one
of three different states, which are defined with
respect to a persistence context.

© JBoss, Inc. 2003, 2004. 27

Instance States

Transient

The instance is not, and has never been associated
with any persistence context. It has no persistent
identity (primary key value).

© JBoss, Inc. 2003, 2004. 28

Instance States

Persistent

The instance is currently associated with a
persistence context. It has a persistent identity
(primary key value) and, perhaps, a corresponding
row in the database. For a particular persistence
context, Hibernate guarantees that persistent identity
is equivalent to Java identity (in-memory location of
the object).

© JBoss, Inc. 2003, 2004. 29

Instance States

Detached

The instance was once associated with a persistence
context, but that context was closed, or the instance
was serialized to another process. It has a persistent
identity and, perhaps, a corresponding row in the
database.

For detached instances, Hibernate makes no
guarantees about the relationship between persistent
identity and Java identity.

© JBoss, Inc. 2003, 2004. 30

Managed environments

 Hibernate can be used in an application server:

Managed environment

Application Hibernate
Transaction
EJB Session Manager

EJB Transaction
Resource
EJB Query Manager

Each database has it's own SessionFactory!

© JBoss, Inc. 2003, 2004. 31

Configuration: “non-managed” environments

 In a “non-managed” environment (eg. Tomcat), we need a JDBC
connection pool:
C3P0, Proxool, custom ConnectionProvider

Non-managed environment

Application Hibernate

JSP Session
Connection
Servlet Transaction Pool

main() Query

© JBoss, Inc. 2003, 2004. 32

Managed environment with XML configuration file

<hibernate-configuration>
<session-factory name="java:hibernate/SessionFactory">

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

<property name="connection.datasource">
java:/comp/env/jdbc/HelloDB
</property>

<property name="transaction.factory_class">
net.sf.hibernate.transaction.JTATransactionFactory
</property>

<property name="transaction.manager_lookup_class">
net.sf.hibernate.transaction.JBossTransactionManagerLookup
</property>

<property name="dialect">
net.sf.hibernate.dialect.PostgreSQLDialect
</property>

<mapping resource="hello/Message.hbm.xml"/>

</session-factory>
</hibernate-configuration>

© JBoss, Inc. 2003, 2004. 33

Session Cache

 To improve the performance within the Hibernate service, as well as
your application, is to cache objects.

 By caching objects in memory, Hibernate avoids the overhead of
retrieving them from the database each time.

 Other than saving overhead when retrieving objects, the Session
cache also impacts saving and updating objects.

 The session interface supports a simple instance cache for each
object that is loaded or saved during the lifetime of a given
 Session.

© JBoss, Inc. 2003, 2004. 34

Session Cache

 Code watch :

 Session session = factory.openSession();
 Event e = (Event) session.load(Event.class, myEventId);
 e.setName(“Hibernate training”);
 session.saveOrUpdate(e);

 // later, with the same Session instance
 Event e = (Event) session.load(Event.class, myEventId);
 e.setDuration(180);
 session.saveOrUpdate(e);
 session.flush();

© JBoss, Inc. 2003, 2004. 35

The Session Cache – Common problem

 Don’t associate two instances of the same object with the same Session
instance, resulting in a NonUniqueObjectException.

 Session session = factory.openSession();
 Event firstEvent = (Event) session.load(Event.class, myEventId);
 //peform some operation on firstEvent

 Event secondEvent = new Event();
 secondEvent.setId(myEventId);
 Session.save(secondEvent);

 This code opens the Session instance, loads an Event instance with a given
ID, creates a second Event instance with the same Id, and then attempts to
save the second Event instance, resulting in the
 NonUniqueObjectException.

© JBoss, Inc. 2003, 2004. 36

The Session Cache – Object Information

 To see whether an object is contained in the cache
 Session.contains()

 Objects can be evicted from the cache by calling
 Session.evict()

 To clear all the objects from Session cache
 Session.clear()

© JBoss, Inc. 2003, 2004. 37

02 hibernateintroduction

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