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10.4. Loading an Object

The load() methods of Session provide a way of retrieving a persistent instance if you know its identifier. load() takes a class object and loads the state into a newly instantiated instance of that class in a 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();
Be aware 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 is 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();
}
return cat;
You can even load an object using an SQL SELECT ... FOR UPDATE, using a LockMode. 
Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);
Any associated instances or contained collections will not be 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)
How much Hibernate loads from the database and how many SQL SELECTs it will use will depend on the fetching strategy. See Section 19.1.1, “About Fetching Strategies”
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