How to persist a JPA field as a clob?

Question

I've a class that doesn't serialize very well to a relational entity. As so, I'd like to persist it using JPA as a CLOB (I can serialize it to a String). How would I do this using JPA?

Example Entity:

@Entity
@Table(name = "MY_TABLE")
public class Foo {
   @Id
   private Long id;
   private String simpleString;
   private Bar bar;
}

Bar is the class that I'd like persisted as a CLOB inside MY_TABLE.

Answer 1

You could annotate the string with javax.persistence.Lob.

@Lob
@Basic(fetch = FetchType.LAZY)
private String simpleString;

See Lob Javadoc:

Specifies that a persistent property or field should be persisted as a large object to a database-supported large object type. Portable applications should use the Lob annotation when mapping to a database Lob type. The Lob annotation may be used in conjunction with the Basic annotation or the ElementCollection annotation when the element collection value is of basic type. A Lob may be either a binary or character type.

The Lob type is inferred from the type of the persistent field or property, and except for string and character-based types defaults to Blob.

Next, you could set the Bar field to transient so it's not persisted:

@Transient
private Bar bar

Then in your Bar getter, you could deserialise it if required:

public Bar getBar()
{
  if (this.bar == null)
  {
    this.bar = deserialise(this.simpleString);
  }

  return this.bar;
}

Thread safety is left as an exercise for the reader.

Additionally, you could also use @PrePersist to serialise Bar into its string form if you needed to do this immediately prior to persisting.

Answer 2

I had a similar problem like you where I needed to store a JSON in a field so when I used BLOB I was causing myself a lot of unnecessary headaches. You are using blob for a content kind of data, I respectfully advise you to use CLOB for data if it's in Character format.

to wrap my answer up, if you are using the ORACLE database(which is a database that always causes problems of speaking its language) use bellow format as a guide or best practice, which is based on oracle documentation itself, to solve your problem:

@Lob @Basic(fetch=LAZY)
@Column(name="REPORT")
protected String report;

to retrieve data or persist data you can use spring Crudrepository and use save and findall methods.

Good luck!