Hibernate memory management

Question

I have an application that uses hibernate. At one part I am trying to retrieve documents. Each document has an account number. The model looks something like this:

    private Long _id;
private String _acct;  
private String _message;  
private String _document;    
private String _doctype;  
private Date _review_date;  

I then retrieve the documents with a document service. A portion of the code is here:

public List<Doc_table> getDocuments(int hours_, int dummyFlag_,List<String> accts) {
        List<Doc_table> documents = new ArrayList<Doc_table>();
    Session session = null;
    Criteria criteria = null;
    try {
        // Lets create a previous Date by subtracting the number of
        // subtractHours_ passed.
        session = HibernateUtil.getSession();
        session.beginTransaction();
        if (accts == null) {
            Calendar cutoffTime = Calendar.getInstance();
            cutoffTime.add(Calendar.HOUR_OF_DAY, hours_);
            criteria = session.createCriteria(Doc_table.class).add(
                    Restrictions.gt("dbcreate_date",  cutoffTime.getTime()))
                    .add(Restrictions.eq("dummyflag", dummyFlag_));
        } else 
        {   criteria = session.createCriteria(Doc_table.class).add(Restrictions.in("acct", accts));
        }
        documents = criteria.list();
        for (int x = 0; x < documents.size(); x++) {
            Doc_table document = documents.get(x);
               ......... more stuff here
                    }

This works great if I'm retrieving a small number of documents. But when the document size is large I get a heap space error, probably because the documents take up a lot of space and when you retrieve several thousand of them, bad things happen.

All I really want to do is retrieve each document that fits my criteria, grab the account number and return a list of account numbers (a far smaller object than a list of objects). If this were jdbc, I would know exactly what to do.

But in this case I'm stumped. I guess I'm looking for a way where I can bring just get the account numbers of the Doc_table object back.

Or alternatively, some way where I can retrieve documents one at a time from the database using hibernate that fit my criteria (instead of bringing back the whole List of objects which uses too much memory).

Answer 1

There are several ways to deal with the problem:

• loading the docs in batches of an smaller size

• (The way you noticed) not to query for the Document, but only for the account numbers:

  List accts = session.createQuery("SELECT d._acct FROM Doc d WHERE ...");

or

 List<String> accts = session.createCriteria(Doc.class).
             setProjection(Projections.property("_acct")).
             list();

• When there is a special field in you Document class that contains the huge amount Document byte data, then you could map this special field as a Lazy loaded field.
• Create a second entity class (read only) that contains only the fields that you need and map it to the same table