When to use Hibernate projections?

Question

I am a little confused about Hibernate\'s projections and criteria. When to use projections and when to use criteria?

Answer 1

Projection is an Interface given in “org.hibernate.criterion” package, Projections is an class given in same package, actually Projection is an interface, and Projections is an class and is a factory for producing projection objects.

In Projections class, we have all static methods and each method of this class returns Projection interface object.

If we want to add a Projection object to Criteria then we need to call a method setProjection()

Remember, while adding projection object to criteria, it is possible to add one object at a time. It means if we add 2nd projection object then this 2nd one will overrides the first one (first one wont be work), so at a time we can only one projection object to criteria object

Using criteria, if we want to load partial object from the database, then we need to create a projection object for property that is to be loaded from the database

Criteria crit = session.createCriteria(Products.class);
crit.setProjection(Projections.proparty("proName"));
List l=crit.list();
Iterator it=l.iterator();
while(it.hasNext())
{
    String s = (String)it.next();
    // ---- print -----
}

If we add multiple projections to criteria then the last projection added will be considered to execute see…

Criteria crit = session.createCriteria(Products.class);

Projection p1 = Projection.property("proName");
Projection p2 = Projection.property("price");

crit.setProjection(p1):
crit.setProjection(p2):
List l=crit.list();