How can I access private class members in Java?

Question

I have data model classes that contain private fields which are meant to be read-only (via a getter function). These fields are set by my JPA persistence provider (eclipselink) during normal operation, using the contents of the database. For unit tests, I want to set them to fake values from a mockup of the persistence layer. How can I do that? How does eclipselink set these values, anyway?

Simplified example:

@Entity
class MyEntity
{
    @Id
    private Integer _ix;

    public Integer ixGet()
    {
        return this._ix;
    }
}

Answer 1

Can you just Mock the Entity itself, providing your own implemenations of the getters?

You could create an anonymous extension in your mock persistence layer:

MyEntity x = new MyEntity() {
    public Integer ixGet() { return new Integer(88); }
};

Answer 2

You need to use the Reflection API. Use Class.getField() to get the field, then call setAccessable(true) on that field so that you may write to it, even though it is private, and finally you may call set() on it to write a new value.

For example:

public class A {
    private int i;
}

You want to set the field 'i' to 3, even though it is private:

void forceSetInt(Object o, String fieldName, int value) {
    Class<?> clazz = o.getClass();
    Field field = clazz.getDeclaredField(fieldName);
    field.setAccessible(true);
    field.set(o, value);
}

There are a number of exceptions that you will need to handle.