I expected CriteriaBuilder.equal(Expression<?> x, Object y) to always evaluate to false when y is null, but to my surprise that's not what happens: it evaluates to true for rows where the relevant field is also null (I've only tested this for Strings).
The only thing I've been able to find on this is this explanation, which corresponds to my mistaken initial assumption.
What is really happening here? Is CriteriaBuilder.equal(Expression<?> x, Object y) transformed into CriteriaBuilder.isNull(Expression<?> x) when y is null? Are null Strings transformed to 'null'? Or am I somehow mistaken?
It is provider specific, but EclipseLInk can tell that your parameter is null and so turn the equality check into an isNull check when it creates the expression from the criteria query. This is not the case if you are using parameters, as the expression may be prepared before the parameter is passed in.