Persist java.util.Properties with Hibernate?

Question

How would I go about persisting an object that has a java.util.Properties object in it?

Note that a java.util.Properties object has the ability to lookup properties not only in itself, but also in a default properties list. The default properties list is itself another Properties object, so it too can have another default properties list. Looking up a single property could traverse multiple Properties objects before the property is found.

The application I'm building needs override-able properties in a hierarchical manner like Properties provides, so I was thinking of using Properties instead of implementing my own data structure. But I'm confused on how I would persist them.

• Would I need to create a custom UserType? Any pointers on how to do this for this situation?
• Anyone know of a how-to or other resource that demonstrates how to persist Properties in Hibernate?

Answer 1

not sure if it's a "piece of pie" thing, but since you can store the XML from a Properties instance and load an instance from an XML therefore you could have a @Lob property on your entity holding the XML that will be persisted. Something like:

 class MyEntity implements Serializable {
   @Transient
   Properties props;

   @Lob
   byte[] xmlProp; //the xml as a byte[]

   //ids, getters & setters ommited

 }

And then you can implement a DAO to do the hard work of persisting and retrieving your entity:

 class MyEntityDAO {

   public void persist(MyEntity entity){

    try {
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        entity.getProps().storeToXML(bos, "a simple comment if you like");

        byte[] byteStream = bos.toByteArray();
                entity.setXmlProp = byteStream;

                //save your instance with hibernate...

    } catch (IOException e) {
        e.printStackTrace();
    }
   }

   public MyEntity retrieveById(Long id){

      Properties propFromDB = new Properties();

      MyEntity ent = //retrieve the instance with hibernate...

      try{
        Properties propFromDB = new Properties();
        ByteArrayInputStream bais = new ByteArrayInputStream(ent.getXmlProp());
        propFromDB.load(bais);
        ent.setProp(propFromDB);
        return ent;
      } catch (IOException ioe){
       ioe.printStackTrace();
      }
      return null;
   }
 }

Check methods storeToXML and loadFromXML at the java.util.Properties API.

Hope this helps;

Answer 2

The first questions is - do you need to store the hierarchy metadata, or you just want to store all properties. If it's the letter case, then simply add all the properties (using getPropertyNames()) to a Map end persist it (persisting maps is nearly trivial)

If you want to retain the hierarchy, then a question to ask is - how should this be represented in the database.

The simplest way I imagine is to have one table with the following columns:

id | key | value | properties_set_id | parent_properties_set_id

But how to do that - I can't think of immediately. Perhaps persist a Set<PropertyEntry>.