Updating JPA entity with reflection does not work?

Question

I have an entity which looks something like this: (I'm coding to the web page so I apologize for any mistakes)

@Entity
public class Entity {
    @Id
    private Long id;
    private String field;

    // Insert getters and setters here...
}

I try to manipulate it using reflection:

Long id = 1;

Entity entity = myDao.getEntity(id);

entity.setField("set directly");

Field[] fields = entity.getClass().getDeclaredFields();

for (Field f : fields) {
    if (f.getName().equals("field")) {
        f.setAccessible(true);
        f.set(entity, "set using reflection");
        f.setAccessible(false);
    }
}

System.out.println(entity.getField());

This program prints "set using reflection". However, in the database the value set using reflection does not get updated:

SELECT * FROM ENTITY WHERE ID = 1
ID     FIELD
1      set directly

This is strange. I could swear that this used to work - but now it isn't. Is it really so that you cannot manipulate entities using reflection?

I'm using EclipseLink 1.1.1 if that matters.

Answer 1

Your class might be instrumented and the setters responsible for recording changes. I'm not familiar with EclipseLink to check if the class returned by myDao.getEntity(id); is your actual class of a sub-class generated by EclipseLink.

Answer 2

I'm pretty sure the Entity you are given by your persistence framework is actually wrapped in another class (possibly the same with stuff tacked on through reflection). Changing the field directly through reflection seems unlikely to work. You might want to check if there's a (generated) setter that you can use. Although if you're going that route one might ask why you don't allow callers to call the setter directly?

Answer 3

Changing values of an entity class by reflection is going to fraught with issues. This is because you're dealing with a class which is persistent and thus the persistence API needs to know about changes to the fields.

If you make changes via reflection, chances are the persistence API will not know about those changes.

A better solution would be to call the setters via reflection.