XML persistence with Hibernate and Xstream

Not a long time ago in the project I had been working on, we were writing a lot of blobs to a database. The information persisted was not of high interest, so we didn't even bother breaking the blobs into separate tables. Of course on very rare occasions we needed to see the data but couldn't.

The problem seemed to be hard to solve until I had stumbled across a very neat XML persistence library Xstream. It allows you to persist and restore objects of any type to and from XML without any additional configuration. To learn more about the library I recommend you to skip through their very short but expressive tutorial.

We used Hibernate for persistence, so I needed to teach it how to use Xstream. Thankfully Hibernate is an easily extensible library. I had written a persister that was capable of transparently persisting any object as an array of chars in form of XML.

My solution has a major drawback though, which I haven't fixed, max length of character array is restricted in the DBMS we use (in Oracle it is 4000 chars). So if you need to persist some really long sets of data or just big objects you might have to rewirte the persister to use CBLOB or any other alternative in your database.

package cern.oasis.util.persistence;

import java.io.Serializable;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Types;

import org.hibernate.Hibernate;
import org.hibernate.HibernateException;
import org.hibernate.usertype.UserType;

import com.thoughtworks.xstream.XStream;

/**
 * Persists any serializable java object as XML string.
 * 
 * @author Ivan Koblik
 */
public class XMLString implements UserType {

  private static final int[] TYPES = { Types.VARCHAR };

  private final static XStream stream = new XStream();
  static {
    //Here you can define aliases of some classes if you want to
    //stream.alias("Impedance", Impedance.class);
  }

  @Override
  public Object assemble(Serializable cached, Object owner) 
      throws HibernateException {
    return null == cached ? null : stream.fromXML((String) cached);
  }

  @Override
  public Serializable disassemble(Object value) throws HibernateException {
    return null == value ? null : stream.toXML(value);
  }

  @Override
  public Object deepCopy(Object value) throws HibernateException {
    return null == value ? null : stream.fromXML(stream.toXML(value));
  }

  @Override
  public boolean equals(Object x, Object y) throws HibernateException {
    if (x == y) {
      return true;
    } else if (x == null || y == null) {
      return false;
    } else {
      return x.equals(y);
    }
  }

  @Override
  public int hashCode(Object x) throws HibernateException {
    return null == x ? 0 : x.hashCode();
  }

  @Override
  public boolean isMutable() {
    return true;
  }

  @Override
  public Object nullSafeGet(ResultSet rs, String[] names, Object owner) 
      throws HibernateException, SQLException {
    String persistedXml = (String) Hibernate.STRING.nullSafeGet(rs, names[0]);
    return null == persistedXml ? null : stream.fromXML(persistedXml);
  }

  @Override
  public void nullSafeSet(PreparedStatement st, Object value, int index) 
      throws HibernateException, SQLException {
    String xmlToPersist = null == value ? null : stream.toXML(value);
    if (null != xmlToPersist && xmlToPersist.length() > 4000) {
      throw new RuntimeException(
          "Can not persist strings longer then 4000 characters:\n" + xmlToPersist);
    }
    Hibernate.STRING.nullSafeSet(st, xmlToPersist, index);
  }

  @Override
  public Object replace(Object original, Object target, Object owner) 
      throws HibernateException {
    return this.deepCopy(original);
  }

  @Override
  public Class returnedClass() {
    return Serializable.class;
  }

  @Override
  public int[] sqlTypes() {
    return TYPES;
  }

}

That's it, the only thing that is left to do is to tell Hibernate to use this persister. Here's an example with annotations:

  
@Entity
public class EntityClass {
  ...
  @AccessType("field")
  @Type(type = "cern.oasis.util.persistence.XMLString")
  public Map<String, Boolean> getValueThatIsPersistentByXStream() {
      return this.map;
  }
}

Hope you find it useful. Comments are more then welcome!