Is this object to table mapping possible?

Hibernate ORM
1

I want to be able to store any Serializable object in a database against a unique ID. So I created a Java object that has a constructor taking those 2 values. The constructor converts the Serializable into bytes and then splits these into chunks of 4096 characters. These bytes are stored in a List in the object. So the object looks like:

public class MyObj {
    private String id;
    private List<byte[]> serData;

    public MyObj(String id, Serializable value) {
        this.id = id;
        this.serData = convertSerializableToByteArray(value);
    }
}

I want to store this into a database table but I want the database table to have 3 columns:

  • ID
  • line_no
  • serialized_data (4096 bytes max)

The primary key of the table should be the ID and line_no combined. As an example, to store an object of ID ‘foo’ to a 10,000 character string, I would expect the database to have the following entries:

ID   line_no   serialized_data
--   -------   ----------------
foo  0         <first 4096 bytes>           
foo  1         <4096  - 8192 bytes>
foo  2         <remaining 808 bytes>

I am having a lot of trouble creating either an hbm file for this or using Annotations. Can anyone help me out? Is this approach even possible? This is the hbm file I have so far (I am aware that this doesn’t work):

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.foo.MyObject" table="settings_ser">

    <list name="serializedData" cascade="all">
            <key column="id" />
            <index column="line_number"/>
            <element type="binary" column="ser_data" length="4096" />
        </list>
    </class>
</hibernate-mapping>

Thanks

2

The best way to serialize this object type is to use JSON. Check out the hibernate-types project for more details.

3

Let us assume that the Java Object and the database table must remain exactly as they are (they are part of a much bigger project). Can a hibernate mapping be created which would perform the mapping I want ?

4

Yes, of course it can. Just make sure that the entity id is a composite key of that id column and line number, if you want to use a child entity.

Or, you can use an embeddable collection too. That’s going to be easier.

5

I fear that I am not explaining myself properly. As you can see from my example there is no line-number in the Java code. I simply have 2 Java objects and wish to map these to 3 columns, using the index of the List as one of the columns.

The mappings I have tried do not work - the example I provided isn’t accepted and I don’t know how to express what I want in either an hbm form or Annotation form. That is what I am looking for.

6

As long as you ttansform the incoming data to match the database table structure, it will work just fine.

The Hibernate User Guide provides plenty of examples of hpw to map a collection of embeddable. Use that as a reference.

7

I abandoned using an hbm file and added Annotations to the class. I added the annotation @ ElementCollection to the List and got it to refer to the same table as the class (is this recommended???) :

@Table(name="myobj_ser")
public class MyObj  {
@Id
@Column(name="id", nullable=false)
private String id;

@ElementCollection
@CollectionTable(name="myobj_ser")
private List<byte[]> serData = new ArrayList<>();

This resulted in a table with 3 columns:

  • id
  • MyObj_id
  • serData

Can you advise as to how I can annotate to get the correct columns?

8

Yes, that’s a good idea.

Can you advise as to how I can annotate to get the correct columns?

The entity should look like this:

Can you advise as to how I can annotate to get the correct columns?

Can you advise as to how I can annotate to get the correct columns?

@Entity
@Table(name="myobj_ser")
public class MyObj  {
	@Id
	@Column(name="id")
	private String id;

	@ElementCollection
	@CollectionTable(name="myobj_ser")
	private List<SerData> serData = new ArrayList<>();
}

And SerData:

@Embeddable
public class SerData  {
	@Id
	@Column(name="ID")
	private String id;
	
	@Column(name="line_no")
	private Integer lineNumber;

	@Column(name="serialized_data")
	private byte[] serializedData;
}
9

It sort-of worked. Your example created a table with 4 columns which were:

  • id
  • myobj_id
  • line_no
  • serialized_data

I tried to work round this and put a joinColumns of "@ JoinColumn(name = “id”)) ". This meant that only 3 columns were created. However when my application started it threw a Foreign Key exception (although it still created the table). I was unable to work around this.

Ultimately I have abandoned my initial approach. The Object “MyObj” has no annotations and simply wrappers the “SerData” object, as you supplied. This is then persisted to the database and I have to ensure that updates, deletions, etc. work on the appropriate object.

Thanks for your help.

10

You need the myobj_id FK column, as otherwise, the JOIN will not work between the parent and child tables.