Stream Functions in a JPA Entity?

Written on: 26 Feb 2015

A quick little post on a random issue I ran into today. I don’t exactly have a good explanation for this as of yet but here’s a quick tip if you run into this problem.

So let’s define an entity for persistence (we’re using JPA and Eclipselink, to a Postgres database).

@Entity
class Foo implements Serializable {
  private final long serializeUID = 1L;

  @Id @GeneratedValue
  private long id;

  public long bar;
  public long baz;
}

Simple. Now, what if we’re developing a front-end that queries the database, and we want to list out the fields of Foo. So let’s use reflection to list the fields, and use Java 8 stream operators.

@Entity
class Foo implements Serializable {
  ...

  public static final List<String> getMemberNames() {
    return Arrays.asList(Foo.class.getFields()).
      stream().map(field -> field.getName()).collect(Collectors.toList());
  }
}

And so that looks pretty good. Except now JPA does not generate this into the database (fails validation).

However this would work:

@Entity
class Foo implements Serializable {
  ...

  public static final List<String> getMemberNames() {
    ArrayList<String> fields = new ArrayList<String>();
    for (Field field : Foo.class.getFields()) {
      fields.add(field.getName());
    }
    return fields;
  }
}

But that isn’t as cool as using streams. So this also works, but moving the reflection to another class; in this case the DAO.

class JpaDAO {
  public static final List<String> getFooMemberNames() {
    return Arrays.asList(Foo.class.getFields()).
      stream().map(field -> field.getName()).collect(Collectors.toList());
  }
}