hibernate-orm/reference/en/modules/batch.xml

<chapter id="batch">
    <title>Batch processing</title>
    
    <para>
        A naive approach to inserting 100 000 rows in the database using Hibernate might 
        look like this:
    </para>

<programlisting><![CDATA[Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
    Customer customer = new Customer(.....);
    session.save(customer);
}
tx.commit();
session.close();]]></programlisting>

    <para>
        This would fall over with an <literal>OutOfMemoryException</literal> somewhere 
        around the 50 000th row. That's because Hibernate caches all the newly inserted 
        <literal>Customer</literal> instances in the session-level cache. 
    </para>

    <para>
        In this chapter we'll show you how to avoid this problem. First, however, if you
        are doing batch processing, it is absolutely critical that you enable the use of
        JDBC batching, if you intend to achieve reasonable performance. Set the JDBC batch 
        size to a reasonable number (say, 10-50):
    </para>
    
<programlisting><![CDATA[hibernate.jdbc.batch_size 20]]></programlisting>

    <para>
        You also might like to do this kind of work in a process where interaction with 
        the second-level cache is completely disabled:
    </para>

<programlisting><![CDATA[hibernate.cache.use_second_level_cache false]]></programlisting>

    <sect1 id="batch-inserts">
        <title>Batch inserts</title>

        <para>
            When making new objects persistent, you must <literal>flush()</literal> and 
            then <literal>clear()</literal> the session regularly, to control the size of
            the first-level cache.
        </para>

<programlisting><![CDATA[Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
   
for ( int i=0; i<100000; i++ ) {
    Customer customer = new Customer(.....);
    session.save(customer);
    if ( i % 20 == 0 ) { //20, same as the JDBC batch size
        //flush a batch of inserts and release memory:
        session.flush();
        session.clear();
    }
}
   
tx.commit();
session.close();]]></programlisting>

    </sect1>

    <sect1 id="batch-update" >
        <title>Batch updates</title>

        <para>
            For retrieving and updating data the same ideas apply. In addition, you need to 
            use <literal>scroll()</literal> to take advantage of server-side cursors for 
            queries that return many rows of data.
        </para>

<programlisting><![CDATA[Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
   
ScrollableResults customers = session.getNamedQuery("GetCustomers")
    .setCacheMode(CacheMode.IGNORE)
    .scroll(ScrollMode.FORWARD_ONLY);
int count=0;
while ( customers.next() ) {
    Customer customer = (Customer) customers.get(0);
    customer.updateStuff(...);
    if ( ++count % 20 == 0 ) {
        //flush a batch of updates and release memory:
        session.flush();
        session.clear();
    }
}
   
tx.commit();
session.close();]]></programlisting>

    </sect1>

</chapter>