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

I've actually been using a simpler way than the one with PGObject and Converters. Since in Postgres enums are converted quite naturally to-from text you just need to let it do what it does best. I'll borrow Arjan's example of moods, if he doesn't mind:

The enum type in Postgres:

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

The class and enum in Java:

public @Entity class Person {    public static enum Mood {sad, ok, happy};    @Enumerated(EnumType.STRING)   Mood mood; 

}

That @Enumerated tag says that serialization/deserialization of the enum should be done in text. Without it, it uses int, which is more troublesome than anything.

At this point you have two options. You either:

1. Add stringtype=unspecified to the connection string, as explained in JDBC connection parameters.This lets Postgres guess the right-side type and convert everything adequately, since it receives something like 'enum = unknown', which is an expression it already knows what to do with (feed the ? value to the left-hand type deserialiser). This is the preferred option, as it should work for all simple UDTs such as enums in one go.

   jdbc:postgresql://localhost:5432/dbname?stringtype=unspecified 

Or:

2. Create an implicit conversion from varchar to the enum in the database. So in this second case the database receives some assignment or comparison like 'enum = varchar' and it finds a rule in its internal catalog saying that it can pass the right-hand value through the serialization function of varchar followed by the deserialization function of the enum. That's more steps than should be needed; and having too many implicit casts in the catalog can cause arbitrary queries to have ambiguous interpretations, so use it sparingly. The cast creation is:

   CREATE CAST (CHARACTER VARYING as mood) WITH INOUT AS IMPLICIT;

Should work with just that.

Answer 2

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.