java - Select using hibernate

Question

I saw few examples over internet of Hibernate using transaction.commit() for select statements. Below is the example code.

public static List<?> list(Class<?> className,int start,int limit,SearchFilter[] searchFilter){
    Session session = HibernateUtil.getSessionFactory().openSession();
    Transaction transaction = null; 

    try {
        transaction = session.beginTransaction();

        Criteria criteria = session.createCriteria(className);
        criteria.setFirstResult(start);
        criteria.setMaxResults(limit);

        for(SearchFilter sf : searchFilter){
            String[] values = sf.getValue();
            if(values != null){
                if(values.length == 1) {
                    criteria.add(Restrictions.eq(sf.getField(), values[0]));
                }else{
                    criteria.add(Restrictions.in(sf.getField(), values));
                }
            }
        }

        List<?> Objects = criteria.list();
        transaction.commit();

        return Objects;
    }catch (Exception e) {
        transaction.rollback();
        e.printStackTrace();
    }finally{
        session.close();
    }

    return null;
}

Why do we do beginning and committing a transaction for select statement?

Answer 1

I highly recommend reading Non-transactional data access and the auto-commit mode. Let me quote a small part:

(...)

Many more issues must be considered when you introduce nontransactional data access in your application. We’ve already noted that introducing a new type of transaction, namely read-only transactions, can significantly complicate any future modification of your application. The same is true if you introduce nontransactional operations.

You would then have three different kinds of data access in your application: in regular transactions, in read-only transactions, and now also nontransactional, with no guarantees. Imagine that you have to introduce an operation that writes data into a unit of work that was supposed to only read data. Imagine that you have to reorganize operations that were nontransactional to be transactional.

Our recommendation is to not use the autocommit mode in an application, and to apply read-only transactions only when there is an obvious performance benefit or when future code changes are highly unlikely. Always prefer regular ACID transactions to group your data-access operations, regardless of whether you read or write data.

Having said that, Hibernate and Java Persistence allow nontransactional data access. In fact, the EJB 3.0 specification forces you to access data nontransactionally if you want to implement atomic long-running conversations. We’ll approach this subject in the next chapter. Now we want to dig a little deeper into the consequences of the autocommit mode in a plain Hibernate application. (Note that, despite our negative remarks, there are some good use cases for the autocommit mode. In our experience autocommit is often enabled for the wrong reasons and we wanted to wipe the slate clean first.)

Working nontransactionally with Hibernate

Look at the following code, which accesses the database without transaction boundaries:

Session session = sessionFactory.openSession(); 
session.get(Item.class, 123l); 
session.close(); 

By default, in a Java SE environment with a JDBC configuration, this is what happens if you execute this snippet:

1. A new Session is opened. It doesn’t obtain a database connection at this point.
2. The call to get() triggers an SQL SELECT. The Session now obtains a JDBC Connection from the connection pool. Hibernate, by default, immediately turns off the autocommit mode on this connection with setAutoCommit(false). This effectively starts a JDBC transaction!
3. The SELECT is executed inside this JDBC transaction. The Session is closed, and the connection is returned to the pool and released by Hibernate — Hibernate calls close() on the JDBC Connection. What happens to the uncommitted transaction?

The answer to that question is, “It depends!” The JDBC specification doesn’t say anything about pending transactions when close() is called on a connection. What happens depends on how the vendors implement the specification. With Oracle JDBC drivers, for example, the call to close() commits the transaction! Most other JDBC vendors take the sane route and roll back any pending transaction when the JDBC Connection object is closed and the resource is returned to the pool.
Obviously, this won’t be a problem for the SELECT you’ve executed (...)