Hibernate setParamater a class

Question

I am new to Hibernate.

I want to create a public Object getById(Class class,long id) function. I tried creating query like: from :nameEntity where id= :id but I'm facing problems when setting parameter for nameEntity. Hibernate doesn't recognize it as a parameter.

I used session.createQuery function. Could you please help me?

Answer 1

You will either have to do String concatenation to achieve this:

session.createQuery("from " + clazz.getName() " where id=:id").
        setParameter("id", id).
        uniqueResult();

or use the Criteria API:

session.createCriteria(clazz).
        add(Expression.eq("id", id).
        uniqueResult()

Answer 2

I want to create a public Object getById(Class class, long id) function. I tried creating query like:" from :nameEntity where id=:id " but I'm facing problems when setting parameter for 'nameEntity'. Hibernate doesn't recognize it as a parameter.

Don't build a query, use the load() or get() methods from the Session. They are actually heavily overloaded, allowing to pass the class either as Class or String, the persistent object identifier and, if required, a lock option (for pessimistic locking):

get(Class clazz, Serializable id)

load(Class theClass, Serializable id)

get(Class clazz, Serializable id, LockOptions lockOptions)

load(Class theClass, Serializable id, LockOptions lockOptions)

get(String entityName, Serializable id)

load(String entityName, Serializable id)

get(String entityName, Serializable id, LockOptions lockOptions) 

load(String entityName, Serializable id, LockOptions lockOptions)

What are the differences between get() and load()? This is explained in the Reference Documentation:

10.3. 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();
    sess.save(cat, id);
}
return cat;

You can 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);

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 does Hibernate load from the database and how many SQL SELECTs will it use? This depends on the fetching strategy. This is explained in Section 20.1, “Fetching strategies”.

How to choose between them?

Choosing between get() and load() is easy: If you’re certain the persistent object exists, and nonexistence would be considered exceptional, load() is a good option. If you aren’t certain there is a persistent instance with the given identifier, use get() and test the return value to see if it’s null.

See also

• Hibernate Load and Get
• Loading Entities with Hibernate
• Advantage of load() vs get()

Related question

• Hibernate: Difference between session.get and session.load