Hibernate provides a powerful querying mechanism through Criteria API, which allows developers to construct queries programmatically without writing SQL or HQL statements diretcly.

To initiate a Criteria query, the createCriteria() method of the Session interface is used to generate a Criteria instance for a specific entity class. Subsequently, query conditions are added using the add() method of the Criteria object.

Criteria criteria = session.createCriteria(Student.class);
criteria.add(Restrictions.eq("name", "Bill"));
List results = criteria.list();
Student student = (Student) results.get(0);
System.out.println(student.getName());

Common Restriction Methods

The Restrictions class offers various methods to define query constraints. These include:

• eq() - Equality comparison (=)
• allEq() - Multiple equality checks using a Map
• gt() - Greater than (> )
• lt() - Less than (<)
• le() - Less than or equal (<=)
• between() - Range condition (BETWEEN)
• like() - Pattern matching (LIKE)
• in() - Set membership (IN)
• and() - Logical AND
• or() - Logical OR
• isNull() - Null value check
• isNotNull() - Non-null value check
• Order.asc() - Ascending sort
• Order.desc() - Descending sort

Examples

Filtering by Name Prefix

Criteria criteria = session.createCriteria(Student.class);
criteria.add(Restrictions.like("name", "t%"));
List results = criteria.list();
Student student = (Student) results.get(0);

Alternatively:

Criteria criteria = session.createCriteria(Student.class);
criteria.add(Restrictions.like("name", "t", MatchMode.START));
List results = criteria.list();
Student student = (Student) results.get(0);

Using IN Clause

String[] names = {"Bill", "Jack", "Tom"};
Criteria criteria = session.createCriteria(Student.class);
criteria.add(Restrictions.in("name", names));
List results = criteria.list();
Student student = (Student) results.get(0);

Combining Conditions with OR

Criteria criteria = session.createCriteria(User.class);
criteria.add(Restrictions.or(
    Restrictions.eq("age", new Integer(20)),
    Restrictions.isNull("age")
));
List users = criteria.list();

Sorting Results

Sorting can be applied using Order.asc() or Order.desc() methods:

Criteria criteria = session.createCriteria(User.class);
criteria.addOrder(Order.asc("name"));
List users = criteria.list();

Limiting Result Sets

To restrict the number of returned records, use setMaxResults() and optionally setFirstResult() for pagination:

Criteria criteria = session.createCriteria(User.class);
criteria.setFirstResult(51);
criteria.setMaxResults(50);
List users = criteria.list();

Aggregation Functions

Use Projections to perform aggregations:

Criteria criteria = session.createCriteria(User.class);
criteria.setProjection(Projections.avg("age"));
List results = criteria.list();

Grouping Results

Group results using Projections.groupProperty():

Criteria criteria = session.createCriteria(User.class);
criteria.setProjection(Projections.groupProperty("age"));
List results = criteria.list();

Combine grouping with aggregation:

ProjectionList projectionList = Projections.projectionList();
projectionList.add(Projections.groupProperty("age"));
projectionList.add(Projections.rowCount());
Criteria criteria = session.createCriteria(User.class);
criteria.setProjection(projectionList);
List results = criteria.list();

Example-Based Queries

For querying based on an existing object:

User user = new User();
user.setAge(new Integer(30));
Criteria criteria = session.createCriteria(User.class);
criteria.add(Example.create(user));
List users = criteria.list();

Custom SQL Templates

Custom SQL restricitons can be defined using sqlRestriction():

Criteria criteria = session.createCriteria(User.class);
criteria.add(Restrictions.sqlRestriction(
    "{alias}.name LIKE (?)", "cater%", Hibernate.STRING));
List users = criteria.list();

Join Fetching

Join fetching can be configured using setFetchMode():

Criteria criteria = session.createCriteria(Group.class);
criteria.setFetchMode("students", FetchMode.EAGER);
criteria.add(Restrictions.like("name", "2005", MatchMode.END));
List results = criteria.list();

This approach generates equivalent SQL:

SELECT g.*, s.* FROM Group g LEFT OUTER JOIN Student s ON g.id = s.group_id WHERE g.name LIKE '2005'