Simple Hibernate Eager Loading Example

[Christopher Rygaard]

Eager Loading Example

This is the simplest way to illustrate eager loading that I could cook up.

These persistent classes are nearly identical to those in the lazy loading example I posted earlier. The only difference is the annotation on Container._children.

The bit that illustrates the eager loading is in HibernateExample.main().

Notice: This is provided with no warranty whatsoever. Use at your own risk.

This is the contained class:

@javax.persistence.Entity
public class Contained
{
   @javax.persistence.Id
   @javax.persistence.GeneratedValue( generator = "increment" )
   @org.hibernate.annotations.GenericGenerator( name = "increment", strategy = "increment" )
   private long                 _id;
   @javax.persistence.ManyToOne
   private Container            _container;
   private String               _data;

   private Contained()
   {
   }

   public Contained( final Container container,
                           String    data )
   {
      if ( container == null )
      {
         throw new IllegalArgumentException( "container required" );
      }

      _container = container;
      _data = data;
   }

   public void display( String margin )
   {
      System.out.println( margin + "Contained " + _data );
   }

   public String getData()
   {
      return _data;
   }
}

This is the container class:

@javax.persistence.Entity
public class Container
{
   @javax.persistence.Id
   @javax.persistence.GeneratedValue( generator = "increment" )
   @org.hibernate.annotations.GenericGenerator( name = "increment", strategy = "increment" )
   private long                      _id;
   @javax.persistence.OneToMany( mappedBy = "_container", fetch = javax.persistence.FetchType.EAGER )
   private java.util.Set<Contained>  _children = new java.util.HashSet<Contained>();
   private String                    _data;

   private Container()
   {
   }

   public Container( String data )
   {
      _data = data;
   }

   public Contained addChild( String data )
   {
      Contained result;

      result = new Contained( this,
                              data );

      _children.add( result );

      return result;
   }

   public void display()
   {
      display( "" );
   }

   private void display( String margin )
   {
      System.out.println( margin + "node " + _data );

      for ( Contained child : _children )
      {
         child.display( margin + "   " );
      }
   }

   public java.util.Set<Contained> getChildren()
   {
      return java.util.Collections.unmodifiableSet( _children );
   }

   public String getData()
   {
      return _data;
   }
}

This example illustrates eager loading. It is identical to the lazy loading example I posted earlier, except there are no expected errors.

public class HibernateExample
{
   private org.hibernate.SessionFactory _sessionFactory;

   final static public void main( String arguments[] )
   {
      HibernateExample example;
      long             rootID;
      Container        containers[];

      example = new HibernateExample();
      example.makeContents();
      System.out.println( "In-session display:" );
      System.out.println( "*******************" );
      example.showContents();
      System.out.println();
      System.out.println( "Out-of-session display:" );
      System.out.println( "***********************" );
      containers = example.getContainers();
      for ( Container container : containers )
      {
         container.display();  // Because eager loading is employed, the entire data structure
                               // is available even though there is no active session.
      }
      example.cleanup();
   }

   private HibernateExample()
   {
      org.hibernate.cfg.Configuration configuration;

      java.util.logging.Logger.getLogger( "org.hibernate" ).setLevel( java.util.logging.Level.SEVERE );  // Supress Hibernate's excessive output

      configuration = new org.hibernate.cfg.Configuration();
      configuration.setProperty( "hibernate.dialect",                 "org.hibernate.dialect.H2Dialect" );                                          // Customize this for your particular RDBMS
      configuration.setProperty( "hibernate.connection.driver_class", "org.h2.Driver" );                                                            // Customize this for your particular RDBMS
      configuration.setProperty( "hibernate.connection.url",          "jdbc:h2:file:C:/SFTData/SFTDatabases/SFTInterfaceData;DB_CLOSE_DELAY=-1" );  // Customize this for your particular RDBMS
      configuration.setProperty( "hibernate.connection.username",     "sa" );                                                                       // Customize this for your particular RDBMS
      configuration.setProperty( "hibernate.connection.password",     "" );     // Customize this for your particular RDBMS installation
      configuration.setProperty( "hibernate.connection.pool_size",    "1" );    // Customize this for your particular RDBMS installation
      configuration.setProperty( "hibernate.cache.provider_class",    "org.hibernate.cache.internal.NoCacheProvider" );  // This is not ready for prime-time
      configuration.setProperty( "hibernate.show_sql",                "false" );  // Tell hibernate to not echo the SQL
      configuration.setProperty( "hibernate.hbm2ddl.auto",            "create" );
      configuration.addAnnotatedClass( Container.class );
      configuration.addAnnotatedClass( Contained.class );

      _sessionFactory = configuration.buildSessionFactory();
   }

   final private void makeContents()
   {
      org.hibernate.Session     session;
      boolean                   committed;
      org.hibernate.Transaction transaction;
      Container                 container1;
      Container                 container2;
      Contained                 child11;
      Contained                 child12;
      Contained                 child13;
      Contained                 child21;
      Contained                 child22;

      session = _sessionFactory.openSession();

      try
      {
         committed = false;

         transaction = session.beginTransaction();

         try
         {
            container1 = new Container( "Container1Data" );
            child11 = container1.addChild( "Child11Data");
            child12 = container1.addChild( "Child12Data");
            container2 = new Container( "Container2Data" );
            child21 = container2.addChild( "Child21Data");
            child22 = container2.addChild( "Child22Data");
            child13 = container1.addChild( "Child13Data");
            session.save( container1 );
            session.save( child11 );
            session.save( child12 );
            session.save( child13 );
            session.save( container2 );
            session.save( child21 );
            session.save( child22 );
            transaction.commit();
            committed = true;
         }
         finally
         {
            if ( !committed )
            {
               transaction.rollback();
            }
         }
      }
      finally
      {
         session.close();
      }

      return;
   }

   final private Container[] getContainers()
   {
      org.hibernate.Session     session;
      boolean                   committed;
      org.hibernate.Transaction transaction;
      java.util.List<Container> containers;
      Container                 result[];
      int                       index;

      session = _sessionFactory.openSession();

      try
      {
         committed = false;

         transaction = session.beginTransaction();

         try
         {
            containers = (java.util.List<Container>)( session.createQuery("from Container").list() );

            result = new Container[ containers.size() ];
            index = 0;
            for ( Container container : containers )
            {
               result[ index ] = container;
               index++;
            }

            transaction.commit();
            committed = true;
         }
         finally
         {
            if ( !committed )
            {
               transaction.rollback();
            }
         }
      }
      finally
      {
         session.close();
      }

      return result;
   }

   final private void showContents()
   {
      org.hibernate.Session     session;
      boolean                   committed;
      org.hibernate.Transaction transaction;
      java.util.List<Container> containers;

      session = _sessionFactory.openSession();

      try
      {
         committed = false;

         transaction = session.beginTransaction();

         try
         {
            containers = (java.util.List<Container>)( session.createQuery("from Container").list() );

            for ( Container container : containers )
            {
               container.display();
            }

            transaction.commit();
            committed = true;
         }
         finally
         {
            if ( !committed )
            {
               transaction.rollback();
            }
         }
      }
      finally
      {
         session.close();
      }

      return;
   }

   final private void cleanup()
   {
      if ( _sessionFactory != null )
      {
         _sessionFactory.close();
      }
   }
}