# Tutorial

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.

## Part 1 - The first Hibernate Application

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.

### Setup

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.

### The first class

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.

### The mapping file

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

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`.

### Hibernate configuration

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.internal.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 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.

### Building with Maven

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] ------------------------------------------------------------------------

### Startup and helpers

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.

### Loading and storing objects

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 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 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"`

## Part 2 - Mapping associations

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.

### Mapping the Person class

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.

### A unidirectional Set-based association

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   |
                                                      |_____________|

### Working the association

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.

### Collection of values

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.

### Bi-directional associations

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.

### Working bi-directional links

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.

## Part 3 - The EventManager web application

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.

### Writing the basic servlet

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.

### Processing and rendering

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.

### Deploying and testing

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 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.

## Summary

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.