Exploring the Query class

2.1 Introduction to the Query classes

By the way, another important point is the Query helper class. You could see it in this example:

public List<Employee> getAllEmployes() {
    try (Session session = sessionFactory.openSession()) {
            Query<Employee> query = session.createQuery("from Employee", Employee.class);
            return query.list();
    }
}

In fact, Query is an interface and it has several implementations for different cases. But for simplicity, I will continue to call it a class. This is, let's say, a class in a broad sense - in terms of OOP.

Note. There used to be two classes:

• Query to describe the query.
• TypedQuery to describe a query with a known type.

The first appeared when Hibernate already existed, and there were no generics yet. Then, after the release of JDK 5, another class was added to Hibernate - TypedQuery, which already supported the typing of the query result.

But, as far as I remember, starting with the 5th version of Hibernate, only one typed class was left, and it is now called Query.

The standard way to create a Query is:

Query<Employee> query = session.createQuery("from Employee", Employee.class);

You learned how to create Query objects, but how do you execute these queries?

It's even simpler here - we just call the list() method on the Query object:

Query<Employee> query = session.createQuery("from Employee", Employee.class);
List<Employee> resultLіst = query.list();

The list() method has a JPA synonym, a method that does the same thing but is called getResultList() . You can sometimes see it in code written by other programmers.

By the way, if the query implies that the result will be in a single result, then it is easier to use the uniqueResult() method to call the query .

Query<Employee> query = session.createQuery("from Employee where id = 1", Employee.class);
Employee result = query.uniqueResult();

The uniqueResult() method has a JPA synonym, the singleResult() method . It was introduced for Hibernate's compatibility with the JPA standard. He does exactly the same thing.

2.2 Query class methods

In fact, the Query class has a lot of different methods. Below I will talk about three more of them.

The first is the stream() method . And its JPA synonym getResultStream() .

Both of these methods return a stream of data instead of a list. This approach can be very efficient when you don't need all the objects obtained as a result of a query at once. Or there is a possibility that only the first of them will be used.

Example:

Query<Employee> query = session.createQuery("from Employee where id > 100", Employee.class);
Stream<Employee> stream = query.stream();

The second method is the executeUpdate() method . You can write a query that will change something in the database. In this case, it is necessary that Hibernate does not use a read-only transaction when accessing the database.

Request example: we decided to raise the level of all users by 1.

Query<User> query = session.createQuery("update User set level=level+1", User.class);
int count = query.executeUpdate();

The executeUpdate() method will return the number of rows that have actually been modified.

And finally the third method is scroll() . We will tell you a little more about it.

2.3 Scroll class methods

This method is somewhat similar to the stream() method . Only it allows you to move through the list of results without pulling out the results at all. That is, you can execute a query, then scroll it to the millionth line of the result and start reading data from there.

Such an advanced iterator.

Query<Employee> query = session.createQuery("from Employee where id > 100", Employee.class);
ScrollableResults<Employee> scroll = query.scroll();

The ScrollableResults object has the following methods:

Let's say you ran a query and you want to get the last element. Here's how to do it:

Query<Employee> query = session.createQuery("from Employee where id > 100", Employee.class);
ScrollableResults<Employee> scroll = query.scroll();
scroll.last();
Employee lastEmployee = scroll.get();

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