Hibernate.orgCommunity Documentation
Intended for new users, this chapter provides an step-by-step introduction
to Hibernate, starting with a simple application using an in-memory database. The
tutorial is based on an earlier tutorial developed by Michael Gloegl. All
code is contained in the tutorials/web directory of the project
source.
This tutorial expects the user have knowledge of both Java and SQL. If you have a limited knowledge of JAVA or SQL, it is advised that you start with a good introduction to that technology prior to attempting to learn Hibernate.
The distribution contains another example application under
the tutorial/eg project source
directory.
For this example, we will set up a small database application that can store events we want to attend and information about the host(s) of these events.
Although you can use whatever database you feel comfortable using, we will use HSQLDB (an in-memory, Java database) to avoid describing installation/setup of any particular database servers.
The first thing we need to do is to set up the development environment. We
will be using the "standard layout" advocated by alot of build tools such
as Maven. Maven, in particular, has a
good resource describing this layout.
As this tutorial is to be a web application, we will be creating and making
use of src/main/java, src/main/resources
and src/main/webapp directories.
We will be using Maven in this tutorial, taking advantage of its transitive dependency management capabilities as well as the ability of many IDEs to automatically set up a project for us based on the maven descriptor.
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.hibernate.tutorials</groupId>
<artifactId>hibernate-tutorial</artifactId>
<version>1.0.0-SNAPSHOT</version>
<name>First Hibernate Tutorial</name>
<build>
<!-- we dont want the version to be part of the generated war file name -->
<finalName>${artifactId}</finalName>
</build>
<dependencies>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
</dependency>
<!-- Because this is a web app, we also have a dependency on the servlet api. -->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
</dependency>
<!-- Hibernate uses slf4j for logging, for our purposes here use the simple backend -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-simple</artifactId>
</dependency>
<!-- Hibernate gives you a choice of bytecode providers between cglib and javassist -->
<dependency>
<groupId>javassist</groupId>
<artifactId>javassist</artifactId>
</dependency>
</dependencies>
</project>
It is not a requirement to use Maven. If you wish to use something else to
build this tutorial (such as Ant), the layout will remain the same. The only
change is that you will need to manually account for all the needed
dependencies. If you use something like Ivy
providing transitive dependency management you would still use the dependencies
mentioned below. Otherwise, you'd need to grab all
dependencies, both explicit and transitive, and add them to the project's
classpath. If working from the Hibernate distribution bundle, this would mean
hibernate3.jar, all artifacts in the
lib/required directory and all files from either the
lib/bytecode/cglib or lib/bytecode/javassist
directory; additionally you will need both the servlet-api jar and one of the slf4j
logging backends.
Save this file as pom.xml in the project root directory.
Next, we create a class that represents the event we want to store in the database; it is a simple JavaBean class with some properties:
package org.hibernate.tutorial.domain;
import java.util.Date;
public class Event {
private Long id;
private String title;
private Date date;
public Event() {}
public Long getId() {
return id;
}
private void setId(Long id) {
this.id = id;
}
public Date getDate() {
return date;
}
public void setDate(Date date) {
this.date = date;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
}
This class uses standard JavaBean naming conventions for property getter and setter methods, as well as private visibility for the fields. Although this is the recommended design, it is not required. Hibernate can also access fields directly, the benefit of accessor methods is robustness for refactoring.
The id property holds a unique identifier value
for a particular event. All persistent entity classes (there are
less important dependent classes as well) will need such an identifier
property if we want to use the full feature set of Hibernate. In fact,
most applications, especially web applications, need to distinguish
objects by identifier, so you should consider this a feature rather
than a limitation. However, we usually do not manipulate the identity
of an object, hence the setter method should be private. Only Hibernate
will assign identifiers when an object is saved. Hibernate can access
public, private, and protected accessor methods, as well as public,
private and protected fields directly. The choice is up to you and
you can match it to fit your application design.
The no-argument constructor is a requirement for all persistent classes; Hibernate has to create objects for you, using Java Reflection. The constructor can be private, however package or public visibility is required for runtime proxy generation and efficient data retrieval without bytecode instrumentation.
Save this file to the src/main/java/org/hibernate/tutorial/domain
directory.
Hibernate needs to know how to load and store objects of the persistent class. This is where the Hibernate mapping file comes into play. The mapping file tells Hibernate what table in the database it has to access, and what columns in that table it should use.
The basic structure of a mapping file looks like this:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="org.hibernate.tutorial.domain">
[...]
</hibernate-mapping>
Hibernate DTD is sophisticated. You can use it for auto-completion
of XML mapping elements and attributes in your editor or IDE.
Opening up the DTD file in your text editor is the easiest way to
get an overview of all elements and attributes, and to view the
defaults, as well as some comments. Hibernate will not load the
DTD file from the web, but first look it up from the classpath of
the application. The DTD file is included in
hibernate-core.jar (it is also included in the
hibernate3.jar, if using the distribution bundle).
We will omit the DTD declaration in future examples to shorten the code. It is, of course, not optional.
Between the two hibernate-mapping tags, include a
class element. All persistent entity classes (again, there
might be dependent classes later on, which are not first-class entities) need
a mapping to a table in the SQL database:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
</class>
</hibernate-mapping>
So far we have told Hibernate how to persist and load object of
class Event to the table
EVENTS. Each instance is now represented by a
row in that table. Now we can continue by mapping the unique
identifier property to the tables primary key. As we do not want
to care about handling this identifier, we configure Hibernate's
identifier generation strategy for a surrogate primary key column:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
</id>
</class>
</hibernate-mapping>
The id element is the declaration of the
identifier property. The name="id" mapping
attribute declares the name of the JavaBean property and tells
Hibernate to use the getId() and
setId() methods to access the property. The
column attribute tells Hibernate which column of the
EVENTS table holds the primary key value.
The nested generator element specifies the
identifier generation strategy (aka how are identifier values
generated?). In this case we choose native,
which offers a level of portability depending on the configured
database dialect. Hibernate supports database generated, globally
unique, as well as application assigned, identifiers. Identifier
value generation is also one of Hibernate's many extension points
and you can plugin in your own strategy.
native is no longer consider the best strategy in terms of portability. for further
discussion, see Section 28.4, “Identifier generation”
Lastly, we need to tell Hibernate about the remaining entity class properties. By default, no properties of the class are considered persistent:
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Event" table="EVENTS">
<id name="id" column="EVENT_ID">
<generator class="native"/>
</id>
<property name="date" type="timestamp" column="EVENT_DATE"/>
<property name="title"/>
</class>
</hibernate-mapping>
Similar to the id element, the
name attribute of the
property element tells Hibernate which getter
and setter methods to use. In this case, Hibernate will search
for getDate(), setDate(),
getTitle() and setTitle()
methods.
Why does the date property mapping include the
column attribute, but the title
does not? Without the column attribute, Hibernate
by default uses the property name as the column name. This works for
title, however, date is a reserved
keyword in most databases so you will need to map it to a different name.
The title mapping also lacks a type attribute. The
types declared and used in the mapping files are not Java data types; they are not SQL
database types either. These types are called Hibernate mapping types,
converters which can translate from Java to SQL data types and vice versa. Again,
Hibernate will try to determine the correct conversion and mapping type itself if
the type attribute is not present in the mapping. In some cases this
automatic detection using Reflection on the Java class might not have the default you
expect or need. This is the case with the date property. Hibernate cannot
know if the property, which is of java.util.Date, should map to a
SQL date, timestamp, or time column.
Full date and time information is preserved by mapping the property with a
timestamp converter.
Hibernate makes this mapping type determination using reflection when the mapping files are processed. This can take time and resources, so if startup performance is important you should consider explicitly defining the type to use.
Save this mapping file as
src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml.
At this point, you should have the persistent class and its mapping file in place. It is now time to configure Hibernate. First let's set up HSQLDB to run in "server mode"
We do this do that the data remains between runs.
We will utilize the Maven exec plugin to launch the HSQLDB server
by running:
mvn exec:java -Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0 file:target/data/tutorial"
You will see it start up and bind to a TCP/IP socket; this is where
our application will connect later. If you want to start
with a fresh database during this tutorial, shutdown HSQLDB, delete
all files in the target/data directory,
and start HSQLDB again.
Hibernate will be connecting to the database on behalf of your application, so it needs to know
how to obtain connections. For this tutorial we will be using a standalone connection
pool (as opposed to a javax.sql.DataSource). Hibernate comes with
support for two third-party open source JDBC connection pools:
c3p0 and
proxool. However, we will be using the
Hibernate built-in connection pool for this tutorial.
The built-in Hibernate connection pool is in no way intended for production use. It lacks several features found on any decent connection pool.
For Hibernate's configuration, we can use a simple hibernate.properties file, a
more sophisticated hibernate.cfg.xml file, or even complete
programmatic setup. Most users prefer the XML configuration file:
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- Database connection settings -->
<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>
<!-- JDBC connection pool (use the built-in) -->
<property name="connection.pool_size">1</property>
<!-- SQL dialect -->
<property name="dialect">org.hibernate.dialect.HSQLDialect</property>
<!-- Enable Hibernate's automatic session context management -->
<property name="current_session_context_class">thread</property>
<!-- Disable the second-level cache -->
<property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
<!-- Echo all executed SQL to stdout -->
<property name="show_sql">true</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">update</property>
<mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>
</session-factory>
</hibernate-configuration>
Notice that this configuration file specifies a different DTD
You configure Hibernate's SessionFactory. SessionFactory is a global
factory responsible for a particular database. If you have several databases, for easier
startup you should use several <session-factory> configurations in
several configuration files.
The first four property elements contain the necessary
configuration for the JDBC connection. The dialect property
element specifies the particular SQL variant Hibernate generates.
In most cases, Hibernate is able to properly determine which dialect to use. See Section 28.3, “Dialect resolution” for more information.
Hibernate's automatic session management for persistence contexts is particularly useful
in this context. The hbm2ddl.auto option turns on automatic generation of
database schemas directly into the database. This can also be turned
off by removing the configuration option, or redirected to a file with the help of
the SchemaExport Ant task. Finally, add the mapping file(s)
for persistent classes to the configuration.
Save this file as hibernate.cfg.xml into the
src/main/resources directory.
We will now build the tutorial with Maven. You will need to
have Maven installed; it is available from the
Maven download page.
Maven will read the /pom.xml file we created
earlier and know how to perform some basic project tasks. First,
lets run the compile goal to make sure we can compile
everything so far:
[hibernateTutorial]$ mvn compile [INFO] Scanning for projects... [INFO] ------------------------------------------------------------------------ [INFO] Building First Hibernate Tutorial [INFO] task-segment: [compile] [INFO] ------------------------------------------------------------------------ [INFO] [resources:resources] [INFO] Using default encoding to copy filtered resources. [INFO] [compiler:compile] [INFO] Compiling 1 source file to /home/steve/projects/sandbox/hibernateTutorial/target/classes [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESSFUL [INFO] ------------------------------------------------------------------------ [INFO] Total time: 2 seconds [INFO] Finished at: Tue Jun 09 12:25:25 CDT 2009 [INFO] Final Memory: 5M/547M [INFO] ------------------------------------------------------------------------
It is time to load and store some Event
objects, but first you have to complete the setup with some
infrastructure code. You have to startup Hibernate by building
a global org.hibernate.SessionFactory
object and storing it somewhere for easy access in application code. A
org.hibernate.SessionFactory is used to
obtain org.hibernate.Session instances.
A org.hibernate.Session represents a
single-threaded unit of work. The
org.hibernate.SessionFactory is a
thread-safe global object that is instantiated once.
We will create a HibernateUtil helper class that
takes care of startup and makes accessing the
org.hibernate.SessionFactory more convenient.
package org.hibernate.tutorial.util;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
public class HibernateUtil {
private static final SessionFactory sessionFactory = buildSessionFactory();
private static SessionFactory buildSessionFactory() {
try {
// Create the SessionFactory from hibernate.cfg.xml
return new Configuration().configure().buildSessionFactory();
}
catch (Throwable ex) {
// Make sure you log the exception, as it might be swallowed
System.err.println("Initial SessionFactory creation failed." + ex);
throw new ExceptionInInitializerError(ex);
}
}
public static SessionFactory getSessionFactory() {
return sessionFactory;
}
}
Save this code as
src/main/java/org/hibernate/tutorial/util/HibernateUtil.java
This class not only produces the global
org.hibernate.SessionFactory reference in
its static initializer; it also hides the fact that it uses a
static singleton. We might just as well have looked up the
org.hibernate.SessionFactory reference from
JNDI in an application server or any other location for that matter.
If you give the org.hibernate.SessionFactory
a name in your configuration, Hibernate will try to bind it to
JNDI under that name after it has been built. Another, better option is to
use a JMX deployment and let the JMX-capable container instantiate and bind
a HibernateService to JNDI. Such advanced options are
discussed later.
You now need to configure a logging
system. Hibernate uses commons logging and provides two choices: Log4j and
JDK 1.4 logging. Most developers prefer Log4j: copy log4j.properties
from the Hibernate distribution in the etc/ directory to
your src directory, next to hibernate.cfg.xml.
If you prefer to have
more verbose output than that provided in the example configuration, you can change the settings. By default, only the Hibernate startup message is shown on stdout.
The tutorial infrastructure is complete and you are now ready to do some real work with Hibernate.
We are now ready to start doing some real work with Hibernate.
Let's start by writing an EventManager class
with a main() method:
package org.hibernate.tutorial;
import org.hibernate.Session;
import java.util.*;
import org.hibernate.tutorial.domain.Event;
import org.hibernate.tutorial.util.HibernateUtil;
public class EventManager {
public static void main(String[] args) {
EventManager mgr = new EventManager();
if (args[0].equals("store")) {
mgr.createAndStoreEvent("My Event", new Date());
}
HibernateUtil.getSessionFactory().close();
}
private void createAndStoreEvent(String title, Date theDate) {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Event theEvent = new Event();
theEvent.setTitle(title);
theEvent.setDate(theDate);
session.save(theEvent);
session.getTransaction().commit();
}
}
In createAndStoreEvent() we created a new
Event object and handed it over to Hibernate.
At that point, Hibernate takes care of the SQL and executes an
INSERT on the database.
A org.hibernate.Session is designed to
represent a single unit of work (a single atomic piece of work
to be performed). For now we will keep things simple and assume
a one-to-one granularity between a Hibernate
org.hibernate.Session and a database
transaction. To shield our code from the actual underlying
transaction system we use the Hibernate
org.hibernate.Transaction API.
In this particular case we are using JDBC-based transactional
semantics, but it could also run with JTA.
What does sessionFactory.getCurrentSession() do?
First, you can call it as many times and anywhere you like
once you get hold of your
org.hibernate.SessionFactory.
The getCurrentSession() method always returns
the "current" unit of work. Remember that we switched
the configuration option for this mechanism to "thread" in our
src/main/resources/hibernate.cfg.xml?
Due to that setting, the context of a current unit of work is bound
to the current Java thread that executes the application.
Hibernate offers three methods of current session tracking. The "thread" based method is not intended for production use; it is merely useful for prototyping and tutorials such as this one. Current session tracking is discussed in more detail later on.
A org.hibernate.Session begins when the
first call to getCurrentSession() is made for
the current thread. It is then bound by Hibernate to the current
thread. When the transaction ends, either through commit or
rollback, Hibernate automatically unbinds the
org.hibernate.Session from the thread
and closes it for you. If you call
getCurrentSession() again, you get a new
org.hibernate.Session and can start a
new unit of work.
Related to the unit of work scope, should the Hibernate org.hibernate.Session be used to execute one or several database operations? The above example uses one org.hibernate.Session for one operation. However this is pure coincidence; the example is just not complex enough to show any other approach. The scope of a Hibernate org.hibernate.Session is flexible but you should never design your application to use a new Hibernate org.hibernate.Session for every database operation. Even though it is used in the following examples, consider session-per-operation an anti-pattern. A real web application is shown later in the tutorial which will help illustrate this.
See Chapter 13, Transactions and Concurrency for more information about transaction handling and demarcation. The previous example also skipped any error handling and rollback.
To run this, we will make use of the Maven exec plugin to call our class
with the necessary classpath setup:
mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="store"
You may need to perform mvn compile first.
You should see Hibernate starting up and, depending on your configuration, lots of log output. Towards the end, the following line will be displayed:
[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values (?, ?, ?)
This is the INSERT executed by Hibernate.
To list stored events an option is added to the main method:
if (args[0].equals("store")) {
mgr.createAndStoreEvent("My Event", new Date());
}
else if (args[0].equals("list")) {
List events = mgr.listEvents();
for (int i = 0; i < events.size(); i++) {
Event theEvent = (Event) events.get(i);
System.out.println(
"Event: " + theEvent.getTitle() + " Time: " + theEvent.getDate()
);
}
}
A new listEvents() method is also added:
private List listEvents() {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
List result = session.createQuery("from Event").list();
session.getTransaction().commit();
return result;
}
Here, we are using a Hibernate Query Language (HQL) query to load all existing
Event objects from the database. Hibernate will generate the
appropriate SQL, send it to the database and populate Event objects
with the data. You can create more complex queries with HQL. See Chapter 16, HQL: The Hibernate Query Language
for more information.
Now we can call our new functionality, again using the Maven exec plugin:
mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"
So far we have mapped a single persistent entity class to a table in isolation. Let's expand on that a bit and add some class associations. We will add people to the application and store a list of events in which they participate.
The first cut of the Person class looks like this:
package org.hibernate.tutorial.domain;
public class Person {
private Long id;
private int age;
private String firstname;
private String lastname;
public Person() {}
// Accessor methods for all properties, private setter for 'id'
}
Save this to a file named
src/main/java/org/hibernate/tutorial/domain/Person.java
Next, create the new mapping file as
src/main/resources/org/hibernate/tutorial/domain/Person.hbm.xml
<hibernate-mapping package="org.hibernate.tutorial.domain">
<class name="Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
</class>
</hibernate-mapping>
Finally, add the new mapping to Hibernate's configuration:
<mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>
<mapping resource="org/hibernate/tutorial/domain/Person.hbm.xml"/>
Create an association between these two entities. Persons can participate in events, and events have participants. The design questions you have to deal with are: directionality, multiplicity, and collection behavior.
By adding a collection of events to the Person
class, you can easily navigate to the events for a particular person,
without executing an explicit query - by calling
Person#getEvents. Multi-valued associations
are represented in Hibernate by one of the Java Collection Framework
contracts; here we choose a java.util.Set
because the collection will not contain duplicate elements and the ordering
is not relevant to our examples:
public class Person {
private Set events = new HashSet();
public Set getEvents() {
return events;
}
public void setEvents(Set events) {
this.events = events;
}
}
Before mapping this association, let's consider the other side.
We could just keep this unidirectional or create another
collection on the Event, if we wanted to be
able to navigate it from both directions. This is not necessary,
from a functional perspective. You can always execute an explicit
query to retrieve the participants for a particular event. This
is a design choice left to you, but what is clear from this
discussion is the multiplicity of the association: "many" valued
on both sides is called a many-to-many
association. Hence, we use Hibernate's many-to-many mapping:
<class name="Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
<set name="events" table="PERSON_EVENT">
<key column="PERSON_ID"/>
<many-to-many column="EVENT_ID" class="Event"/>
</set>
</class>
Hibernate supports a broad range of collection mappings, a
set being most common. For a many-to-many
association, or n:m entity relationship, an
association table is required. Each row in this table represents
a link between a person and an event. The table name is
decalred using the table attribute of the
set element. The identifier column name in
the association, for the person side, is defined with the
key element, the column name for the event's
side with the column attribute of the
many-to-many. You also have to tell Hibernate
the class of the objects in your collection (the class on the
other side of the collection of references).
The database schema for this mapping is therefore:
_____________ __________________
| | | | _____________
| EVENTS | | PERSON_EVENT | | |
|_____________| |__________________| | PERSON |
| | | | |_____________|
| *EVENT_ID | <--> | *EVENT_ID | | |
| EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID |
| TITLE | |__________________| | AGE |
|_____________| | FIRSTNAME |
| LASTNAME |
|_____________|
Now we will bring some people and events together in a new method in EventManager:
private void addPersonToEvent(Long personId, Long eventId) {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session.load(Person.class, personId);
Event anEvent = (Event) session.load(Event.class, eventId);
aPerson.getEvents().add(anEvent);
session.getTransaction().commit();
}
After loading a Person and an
Event, simply modify the collection using the
normal collection methods. There is no explicit call to
update() or save();
Hibernate automatically detects that the collection has been modified
and needs to be updated. This is called
automatic dirty checking. You can also try
it by modifying the name or the date property of any of your
objects. As long as they are in persistent
state, that is, bound to a particular Hibernate
org.hibernate.Session, Hibernate
monitors any changes and executes SQL in a write-behind fashion.
The process of synchronizing the memory state with the database,
usually only at the end of a unit of work, is called
flushing. In our code, the unit of work
ends with a commit, or rollback, of the database transaction.
You can load person and event in different units of work. Or
you can modify an object outside of a
org.hibernate.Session, when it
is not in persistent state (if it was persistent before, this
state is called detached). You can even
modify a collection when it is detached:
private void addPersonToEvent(Long personId, Long eventId) {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session
.createQuery("select p from Person p left join fetch p.events where p.id = :pid")
.setParameter("pid", personId)
.uniqueResult(); // Eager fetch the collection so we can use it detached
Event anEvent = (Event) session.load(Event.class, eventId);
session.getTransaction().commit();
// End of first unit of work
aPerson.getEvents().add(anEvent); // aPerson (and its collection) is detached
// Begin second unit of work
Session session2 = HibernateUtil.getSessionFactory().getCurrentSession();
session2.beginTransaction();
session2.update(aPerson); // Reattachment of aPerson
session2.getTransaction().commit();
}
The call to update makes a detached object
persistent again by binding it to a new unit of work, so any
modifications you made to it while detached can be saved to
the database. This includes any modifications
(additions/deletions) you made to a collection of that entity
object.
This is not much use in our example, but it is an important concept you can
incorporate into your own application. Complete this exercise by adding a new action
to the main method of the EventManager and call it from the command line. If
you need the identifiers of a person and an event - the save() method
returns it (you might have to modify some of the previous methods to return that identifier):
else if (args[0].equals("addpersontoevent")) {
Long eventId = mgr.createAndStoreEvent("My Event", new Date());
Long personId = mgr.createAndStorePerson("Foo", "Bar");
mgr.addPersonToEvent(personId, eventId);
System.out.println("Added person " + personId + " to event " + eventId);
}
This is an example of an association between two equally important
classes : two entities. As mentioned earlier, there are other
classes and types in a typical model, usually "less important".
Some you have already seen, like an int or a
java.lang.String. We call these classes
value types, and their instances
depend on a particular entity. Instances of
these types do not have their own identity, nor are they shared
between entities. Two persons do not reference the same
firstname object, even if they have the same
first name. Value types cannot only be found in the JDK , but
you can also write dependent classes yourself
such as an Address or
MonetaryAmount class. In fact, in a Hibernate
application all JDK classes are considered value types.
You can also design a collection of value types. This is conceptually different from a collection of references to other entities, but looks almost the same in Java.
Let's add a collection of email addresses to the
Person entity. This will be represented as a
java.util.Set of
java.lang.String instances:
private Set emailAddresses = new HashSet();
public Set getEmailAddresses() {
return emailAddresses;
}
public void setEmailAddresses(Set emailAddresses) {
this.emailAddresses = emailAddresses;
}
The mapping of this Set is as follows:
<set name="emailAddresses" table="PERSON_EMAIL_ADDR">
<key column="PERSON_ID"/>
<element type="string" column="EMAIL_ADDR"/>
</set>
The difference compared with the earlier mapping is the use of
the element part which tells Hibernate that the
collection does not contain references to another entity, but is
rather a collection whose elements are values types, here specifically
of type string. The lowercase name tells you
it is a Hibernate mapping type/converter. Again the
table attribute of the set
element determines the table name for the collection. The
key element defines the foreign-key column
name in the collection table. The column
attribute in the element element defines the
column name where the email address values will actually
be stored.
Here is the updated schema:
_____________ __________________
| | | | _____________
| EVENTS | | PERSON_EVENT | | | ___________________
|_____________| |__________________| | PERSON | | |
| | | | |_____________| | PERSON_EMAIL_ADDR |
| *EVENT_ID | <--> | *EVENT_ID | | | |___________________|
| EVENT_DATE | | *PERSON_ID | <--> | *PERSON_ID | <--> | *PERSON_ID |
| TITLE | |__________________| | AGE | | *EMAIL_ADDR |
|_____________| | FIRSTNAME | |___________________|
| LASTNAME |
|_____________|
You can see that the primary key of the collection table is in fact a composite key that uses both columns. This also implies that there cannot be duplicate email addresses per person, which is exactly the semantics we need for a set in Java.
You can now try to add elements to this collection, just like we did before by linking persons and events. It is the same code in Java:
private void addEmailToPerson(Long personId, String emailAddress) {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
Person aPerson = (Person) session.load(Person.class, personId);
// adding to the emailAddress collection might trigger a lazy load of the collection
aPerson.getEmailAddresses().add(emailAddress);
session.getTransaction().commit();
}
This time we did not use a fetch query to initialize the collection. Monitor the SQL log and try to optimize this with an eager fetch.
Next you will map a bi-directional association. You will make the association between person and event work from both sides in Java. The database schema does not change, so you will still have many-to-many multiplicity.
A relational database is more flexible than a network programming language, in that it does not need a navigation direction; data can be viewed and retrieved in any possible way.
First, add a collection of participants to the
Event class:
private Set participants = new HashSet();
public Set getParticipants() {
return participants;
}
public void setParticipants(Set participants) {
this.participants = participants;
}
Now map this side of the association in Event.hbm.xml.
<set name="participants" table="PERSON_EVENT" inverse="true">
<key column="EVENT_ID"/>
<many-to-many column="PERSON_ID" class="Person"/>
</set>
These are normal set mappings in both mapping documents.
Notice that the column names in key and many-to-many
swap in both mapping documents. The most important addition here is the
inverse="true" attribute in the set element of the
Event's collection mapping.
What this means is that Hibernate should take the other side, the Person class,
when it needs to find out information about the link between the two. This will be a lot easier to
understand once you see how the bi-directional link between our two entities is created.
First, keep in mind that Hibernate does not affect normal Java semantics. How did we create a
link between a Person and an Event in the unidirectional
example? You add an instance of Event to the collection of event references,
of an instance of Person. If you want to make this link
bi-directional, you have to do the same on the other side by adding a Person
reference to the collection in an Event. This process of "setting the link on both sides"
is absolutely necessary with bi-directional links.
Many developers program defensively and create link management methods to
correctly set both sides (for example, in Person):
protected Set getEvents() {
return events;
}
protected void setEvents(Set events) {
this.events = events;
}
public void addToEvent(Event event) {
this.getEvents().add(event);
event.getParticipants().add(this);
}
public void removeFromEvent(Event event) {
this.getEvents().remove(event);
event.getParticipants().remove(this);
}
The get and set methods for the collection are now protected. This allows classes in the same package and subclasses to still access the methods, but prevents everybody else from altering the collections directly. Repeat the steps for the collection on the other side.
What about the inverse mapping attribute? For you, and for Java, a bi-directional
link is simply a matter of setting the references on both sides correctly. Hibernate, however, does not
have enough information to correctly arrange SQL INSERT and UPDATE
statements (to avoid constraint violations). Making one side of the association inverse tells Hibernate to consider it a mirror of the other side. That is all that is necessary
for Hibernate to resolve any issues that arise when transforming a directional navigation model to
a SQL database schema. The rules are straightforward: all bi-directional associations
need one side as inverse. In a one-to-many association it has to be the many-side,
and in many-to-many association you can select either side.
A Hibernate web application uses Session and Transaction
almost like a standalone application. However, some common patterns are useful. You can now write
an EventManagerServlet. This servlet can list all events stored in the
database, and it provides an HTML form to enter new events.
First we need create our basic processing servlet. Since our
servlet only handles HTTP GET requests, we
will only implement the doGet() method:
package org.hibernate.tutorial.web;
// Imports
public class EventManagerServlet extends HttpServlet {
protected void doGet(
HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
SimpleDateFormat dateFormatter = new SimpleDateFormat( "dd.MM.yyyy" );
try {
// Begin unit of work
HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction();
// Process request and render page...
// End unit of work
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().commit();
}
catch (Exception ex) {
HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rollback();
if ( ServletException.class.isInstance( ex ) ) {
throw ( ServletException ) ex;
}
else {
throw new ServletException( ex );
}
}
}
}
Save this servlet as
src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java
The pattern applied here is called session-per-request.
When a request hits the servlet, a new Hibernate Session is
opened through the first call to getCurrentSession() on the
SessionFactory. A database transaction is then started. All
data access occurs inside a transaction irrespective of whether the data is read or written.
Do not use the auto-commit mode in applications.
Do not use a new Hibernate Session for
every database operation. Use one Hibernate Session that is
scoped to the whole request. Use getCurrentSession(), so that
it is automatically bound to the current Java thread.
Next, the possible actions of the request are processed and the response HTML is rendered. We will get to that part soon.
Finally, the unit of work ends when processing and rendering are complete. If any
problems occurred during processing or rendering, an exception will be thrown
and the database transaction rolled back. This completes the
session-per-request pattern. Instead of the transaction
demarcation code in every servlet, you could also write a servlet filter.
See the Hibernate website and Wiki for more information about this pattern
called Open Session in View. You will need it as soon
as you consider rendering your view in JSP, not in a servlet.
Now you can implement the processing of the request and the rendering of the page.
// Write HTML header
PrintWriter out = response.getWriter();
out.println("<html><head><title>Event Manager</title></head><body>");
// Handle actions
if ( "store".equals(request.getParameter("action")) ) {
String eventTitle = request.getParameter("eventTitle");
String eventDate = request.getParameter("eventDate");
if ( "".equals(eventTitle) || "".equals(eventDate) ) {
out.println("<b><i>Please enter event title and date.</i></b>");
}
else {
createAndStoreEvent(eventTitle, dateFormatter.parse(eventDate));
out.println("<b><i>Added event.</i></b>");
}
}
// Print page
printEventForm(out);
listEvents(out, dateFormatter);
// Write HTML footer
out.println("</body></html>");
out.flush();
out.close();
This coding style, with a mix of Java and HTML, would not scale in a more complex application-keep in mind that we are only illustrating basic Hibernate concepts in this tutorial. The code prints an HTML header and a footer. Inside this page, an HTML form for event entry and a list of all events in the database are printed. The first method is trivial and only outputs HTML:
private void printEventForm(PrintWriter out) {
out.println("<h2>Add new event:</h2>");
out.println("<form>");
out.println("Title: <input name='eventTitle' length='50'/><br/>");
out.println("Date (e.g. 24.12.2009): <input name='eventDate' length='10'/><br/>");
out.println("<input type='submit' name='action' value='store'/>");
out.println("</form>");
}
The listEvents() method uses the Hibernate
Session bound to the current thread to execute
a query:
private void listEvents(PrintWriter out, SimpleDateFormat dateFormatter) {
List result = HibernateUtil.getSessionFactory()
.getCurrentSession().createCriteria(Event.class).list();
if (result.size() > 0) {
out.println("<h2>Events in database:</h2>");
out.println("<table border='1'>");
out.println("<tr>");
out.println("<th>Event title</th>");
out.println("<th>Event date</th>");
out.println("</tr>");
Iterator it = result.iterator();
while (it.hasNext()) {
Event event = (Event) it.next();
out.println("<tr>");
out.println("<td>" + event.getTitle() + "</td>");
out.println("<td>" + dateFormatter.format(event.getDate()) + "</td>");
out.println("</tr>");
}
out.println("</table>");
}
}
Finally, the store action is dispatched to the
createAndStoreEvent() method, which also uses
the Session of the current thread:
protected void createAndStoreEvent(String title, Date theDate) {
Event theEvent = new Event();
theEvent.setTitle(title);
theEvent.setDate(theDate);
HibernateUtil.getSessionFactory()
.getCurrentSession().save(theEvent);
}
The servlet is now complete. A request to the servlet will be processed
in a single Session and Transaction. As
earlier in the standalone application, Hibernate can automatically bind these
objects to the current thread of execution. This gives you the freedom to layer
your code and access the SessionFactory in any way you like.
Usually you would use a more sophisticated design and move the data access code
into data access objects (the DAO pattern). See the Hibernate Wiki for more
examples.
To deploy this application for testing we must create a
Web ARchive (WAR). First we must define the WAR descriptor
as src/main/webapp/WEB-INF/web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
<servlet-name>Event Manager</servlet-name>
<servlet-class>org.hibernate.tutorial.web.EventManagerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Event Manager</servlet-name>
<url-pattern>/eventmanager</url-pattern>
</servlet-mapping>
</web-app>
To build and deploy call mvn package in your
project directory and copy the hibernate-tutorial.war
file into your Tomcat webapps directory.
If you do not have Tomcat installed, download it from http://tomcat.apache.org/ and follow the installation instructions. Our application requires no changes to the standard Tomcat configuration.
Once deployed and Tomcat is running, access the application at
http://localhost:8080/hibernate-tutorial/eventmanager. Make
sure you watch the Tomcat log to see Hibernate initialize when the first
request hits your servlet (the static initializer in HibernateUtil
is called) and to get the detailed output if any exceptions occurs.
This tutorial covered the basics of writing a simple standalone Hibernate application and a small web application. More tutorials are available from the Hibernate website.
Copyright © 2004 Red Hat, Inc.