10.3. Loading an object

The load() methods of Session gives you a way to retrieve a persistent instance if you already know its identifier. load() takes a class object and will load the state into a newly instantiated instance of that class, in persistent state.
Cat fritz = (Cat) sess.load(Cat.class, generatedId);
// you need to wrap primitive identifiers
long id = 1234;
DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new Long(id) );
Alternatively, you can load state into a given instance:
Cat cat = new DomesticCat();
// load pk's state into cat
sess.load( cat, new Long(pkId) );
Set kittens = cat.getKittens();
Note that load() will throw an unrecoverable exception if there is no matching database row. If the class is mapped with a proxy, load() just returns an uninitialized proxy and does not actually hit the database until you invoke a method of the proxy. This behaviour is very useful if you wish to create an association to an object without actually loading it from the database. It also allows multiple instances to be loaded as a batch if batch-size is defined for the class mapping.
If you are not certain that a matching row exists, you should use the get() method, which hits the database immediately and returns null if there is no matching row.
Cat cat = (Cat) sess.get(Cat.class, id);
if (cat==null) {
    cat = new Cat();
    sess.save(cat, id);
}
return cat;
You may even load an object using an SQL SELECT ... FOR UPDATE, using a LockMode. See the API documentation for more information.
Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);
Note that any associated instances or contained collections are not selected FOR UPDATE, unless you decide to specify lock or all as a cascade style for the association.
It is possible to re-load an object and all its collections at any time, using the refresh() method. This is useful when database triggers are used to initialize some of the properties of the object.
sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)
An important question usually appears at this point: How much does Hibernate load from the database and how many SQL SELECT s will it use? This depends on the fetching strategy and is explained in Section 19.1, “Fetching strategies”.