Java Enums, JPA and Postgres enums - How do I make them work together?

Question

We have a postgres DB with postgres enums. We are starting to build JPA into our application. We also have Java enums which mirror the postgres enums. Now the big question is how to get JPA to understand Java enums on one side and postgres enums on the other? The Java side should be fairly easy but I'm not sure how to do the postgres side.

Answer 1

This involves making multiple mappings.

First, a Postgres enum is returned by the JDBC driver as an instance of type PGObject. The type property of this has the name of your postgres enum, and the value property its value. (The ordinal is not stored however, so technically it's not an enum anymore and possibly completely useless because of this)

Anyway, if you have a definition like this in Postgres:

CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');

Then the resultset will contain a PGObject with type "mood" and value "happy" for a column having this enum type and a row with the value 'happy'.

Next thing to do is writing some interceptor code that sits between the spot where JPA reads from the raw resultset and sets the value on your entity. E.g. suppose you had the following entity in Java:

public @Entity class Person {

  public static enum Mood {sad, ok, happy}

  @Id Long ID;
  Mood mood;

}

Unfortunately, JPA does not offer an easy interception point where you can do the conversion from PGObject to the Java enum Mood. Most JPA vendors however have some proprietary support for this. Hibernate for instance has the TypeDef and Type annotations for this (from Hibernate-annotations.jar).

@TypeDef(name="myEnumConverter", typeClass=MyEnumConverter.class)
public @Entity class Person {

  public static enum Mood {sad, ok, happy}

  @Id Long ID;
  @Type(type="myEnumConverter") Mood mood;

These allow you to supply an instance of UserType (from Hibernate-core.jar) that does the actual conversion:

public class MyEnumConverter implements UserType {

    private static final int[] SQL_TYPES = new int[]{Types.OTHER};

    public Object nullSafeGet(ResultSet arg0, String[] arg1, Object arg2) throws HibernateException, SQLException {

     Object pgObject = arg0.getObject(X); // X is the column containing the enum

     try {
      Method valueMethod = pgObject.getClass().getMethod("getValue");
      String value = (String)valueMethod.invoke(pgObject);   
      return Mood.valueOf(value);  
     }
     catch (Exception e) {
      e.printStackTrace();
     }

     return null;
    }

    public int[] sqlTypes() {  
     return SQL_TYPES;
    }

    // Rest of methods omitted

}

This is not a complete working solution, but just a quick pointer in hopefully the right direction.

Answer 2

I filed a bug report with a patch included for Hibernate: HHH-5188

The patch works for me to read a PostgreSQL enum into a Java enum using JPA.