15. Criteria

Criteria queries offer a type-safe alternative to HQL, JPQL and native SQL queries.

Criteria queries are a programmatic, type-safe way to express a query. They are type-safe in terms of using interfaces and classes to represent various structural parts of a query such as the query itself, the select clause, or an order-by, etc. They can also be type-safe in terms of referencing attributes as we will see in a bit. Users of the older Hibernate org.hibernate.Criteria query API will recognize the general approach, though we believe the JPA API to be superior as it represents a clean look at the lessons learned from that API.

Criteria queries are essentially an object graph, where each part of the graph represents an increasing (as we navigate down this graph) more atomic part of query. The first step in performing a criteria query is building this graph. The javax.persistence.criteria.CriteriaBuilder interface is the first thing with which you need to become acquainted to begin using criteria queries. Its role is that of a factory for all the individual pieces of the criteria. You obtain a javax.persistence.criteria.CriteriaBuilder instance by calling the getCriteriaBuilder() method of either javax.persistence.EntityManagerFactory or javax.persistence.EntityManager.

The next step is to obtain a javax.persistence.criteria.CriteriaQuery. This is accomplished using one of the three methods on javax.persistence.criteria.CriteriaBuilder for this purpose:

• <T> CriteriaQuery<T> createQuery( Class<T> resultClass )

• CriteriaQuery<Tuple> createTupleQuery()

• CriteriaQuery<Object> createQuery()

Each serves a different purpose depending on the expected type of the query results.

15.1. Typed criteria queries

The type of the criteria query (aka the <T>) indicates the expected types in the query result. This might be an entity, an Integer, or any other object.

15.2. Selecting an entity

This is probably the most common form of query. The application wants to select entity instances.

Example 314. Selecting the root entity

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Person> criteria = builder.createQuery( Person.class );
Root<Person> root = criteria.from( Person.class );
criteria.select( root );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<Person> persons = entityManager.createQuery( criteria ).getResultList();

The example uses createQuery() passing in the Person class reference as the results of the query will be Person objects.

15.3. Selecting an expression

The simplest form of selecting an expression is selecting a particular attribute from an entity. But this expression might also represent an aggregation, a mathematical operation, etc.

Example 315. Selecting an attribute

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<String> criteria = builder.createQuery( String.class );
Root<Person> root = criteria.from( Person.class );
criteria.select( root.get( Person_.nickName ) );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<String> nickNames = entityManager.createQuery( criteria ).getResultList();

In this example, the query is typed as java.lang.String because that is the anticipated type of the results (the type of the Person#nickName attribute is java.lang.String). Because a query might contain multiple references to the Person entity, attribute references always need to be qualified. This is accomplished by the Root#get method call.

15.4. Selecting multiple values

There are actually a few different ways to select multiple values using criteria queries. We will explore two options here, but an alternative recommended approach is to use tuples as described in Tuple criteria queries, or consider a wrapper query, see Selecting a wrapper for details.

Example 316. Selecting an array

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Object[]> criteria = builder.createQuery( Object[].class );
Root<Person> root = criteria.from( Person.class );

Path<Long> idPath = root.get( Person_.id );
Path<String> nickNamePath = root.get( Person_.nickName);

criteria.select( builder.array( idPath, nickNamePath ) );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<Object[]> idAndNickNames = entityManager.createQuery( criteria ).getResultList();

Technically this is classified as a typed query, but you can see from handling the results that this is sort of misleading. Anyway, the expected result type here is an array.

The example then uses the array method of javax.persistence.criteria.CriteriaBuilder which explicitly combines individual selections into a javax.persistence.criteria.CompoundSelection.

Example 317. Selecting an array using multiselect

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Object[]> criteria = builder.createQuery( Object[].class );
Root<Person> root = criteria.from( Person.class );

Path<Long> idPath = root.get( Person_.id );
Path<String> nickNamePath = root.get( Person_.nickName);

criteria.multiselect( idPath, nickNamePath );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<Object[]> idAndNickNames = entityManager.createQuery( criteria ).getResultList();

Just as we saw in Selecting an array we have a typed criteria query returning an Object array. Both queries are functionally equivalent. This second example uses the multiselect() method which behaves slightly differently based on the type given when the criteria query was first built, but in this case it says to select and return an Object[].

15.5. Selecting a wrapper

Another alternative to Selecting multiple values is to instead select an object that will "wrap" the multiple values. Going back to the example query there, rather than returning an array of [Person#id, Person#nickName], instead declare a class that holds these values and use that as a return object.

Example 318. Selecting a wrapper

public class PersonWrapper {

    private final Long id;

    private final String nickName;

    public PersonWrapper(Long id, String nickName) {
        this.id = id;
        this.nickName = nickName;
    }

    public Long getId() {
        return id;
    }

    public String getNickName() {
        return nickName;
    }
}

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<PersonWrapper> criteria = builder.createQuery( PersonWrapper.class );
Root<Person> root = criteria.from( Person.class );

Path<Long> idPath = root.get( Person_.id );
Path<String> nickNamePath = root.get( Person_.nickName);

criteria.select( builder.construct( PersonWrapper.class, idPath, nickNamePath ) );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<PersonWrapper> wrappers = entityManager.createQuery( criteria ).getResultList();

First we see the simple definition of the wrapper object we will be using to wrap our result values. Specifically, notice the constructor and its argument types. Since we will be returning PersonWrapper objects, we use PersonWrapper as the type of our criteria query.

This example illustrates the use of the javax.persistence.criteria.CriteriaBuilder method construct which is used to build a wrapper expression. For every row in the result we are saying we would like a PersonWrapper instantiated with the remaining arguments by the matching constructor. This wrapper expression is then passed as the select.

15.6. Tuple criteria queries

A better approach to Selecting multiple values is to use either a wrapper (which we just saw in Selecting a wrapper) or using the javax.persistence.Tuple contract.

Example 319. Selecting a tuple

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Tuple> criteria = builder.createQuery( Tuple.class );
Root<Person> root = criteria.from( Person.class );

Path<Long> idPath = root.get( Person_.id );
Path<String> nickNamePath = root.get( Person_.nickName);

criteria.multiselect( idPath, nickNamePath );
criteria.where( builder.equal( root.get( Person_.name ), "John Doe" ) );

List<Tuple> tuples = entityManager.createQuery( criteria ).getResultList();

for ( Tuple tuple : tuples ) {
    Long id = tuple.get( idPath );
    String nickName = tuple.get( nickNamePath );
}

//or using indices
for ( Tuple tuple : tuples ) {
    Long id = (Long) tuple.get( 0 );
    String nickName = (String) tuple.get( 1 );
}

This example illustrates accessing the query results through the javax.persistence.Tuple interface. The example uses the explicit createTupleQuery() of javax.persistence.criteria.CriteriaBuilder. An alternate approach is to use createQuery( Tuple.class ).

Again we see the use of the multiselect() method, just like in Selecting an array using multiselect. The difference here is that the type of the javax.persistence.criteria.CriteriaQuery was defined as javax.persistence.Tuple so the compound selections in this case are interpreted to be the tuple elements.

The javax.persistence.Tuple contract provides three forms of access to the underlying elements:

• typed

• The Selecting a tuple example illustrates this form of access in the tuple.get( idPath ) and tuple.get( nickNamePath ) calls. This allows typed access to the underlying tuple values based on the javax.persistence.TupleElement expressions used to build the criteria.

• positional

• Allows access to the underlying tuple values based on the position. The simple Object get(int position) form is very similar to the access illustrated in Selecting an array and Selecting an array using multiselect. The <X> X get(int position, Class<X> type form allows typed positional access, but based on the explicitly supplied type which the tuple value must be type-assignable to.

• aliased

• Allows access to the underlying tuple values based an (optionally) assigned alias. The example query did not apply an alias. An alias would be applied via the alias method on javax.persistence.criteria.Selection. Just like positional access, there is both a typed (Object get(String alias)) and an untyped (<X> X get(String alias, Class<X> type form.

15.7. FROM clause

A CriteriaQuery object defines a query over one or more entity, embeddable, or basic abstract schema types. The root objects of the query are entities, from which the other types are reached by navigation.

— JPA Specification, section 6.5.2 Query Roots, pg 262

15.8. Roots

Roots define the basis from which all joins, paths and attributes are available in the query. A root is always an entity type. Roots are defined and added to the criteria by the overloaded from methods on javax.persistence.criteria.CriteriaQuery:

Example 320. Root methods

<X> Root<X> from( Class<X> );

<X> Root<X> from( EntityType<X> );

Example 321. Adding a root example

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Person> criteria = builder.createQuery( Person.class );
Root<Person> root = criteria.from( Person.class );

Criteria queries may define multiple roots, the effect of which is to create a cartesian product between the newly added root and the others. Here is an example of cross joining Person and Partner entities;

Example 322. Adding a root example

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Tuple> criteria = builder.createQuery( Tuple.class );

Root<Person> personRoot = criteria.from( Person.class );
Root<Partner> partnerRoot = criteria.from( Partner.class );
criteria.multiselect( personRoot, partnerRoot );

Predicate personRestriction = builder.and(
    builder.equal( personRoot.get( Person_.address ), address ),
    builder.isNotEmpty( personRoot.get( Person_.phones ) )
);
Predicate partnerRestriction = builder.and(
    builder.like( partnerRoot.get( Partner_.name ), prefix ),
    builder.equal( partnerRoot.get( Partner_.version ), 0 )
);
criteria.where( builder.and( personRestriction, partnerRestriction ) );

List<Tuple> tuples = entityManager.createQuery( criteria ).getResultList();

15.9. Joins

Joins allow navigation from other javax.persistence.criteria.From to either association or embedded attributes. Joins are created by the numerous overloaded join methods of the javax.persistence.criteria.From interface.

Example 323. Join example

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Phone> criteria = builder.createQuery( Phone.class );
Root<Phone> root = criteria.from( Phone.class );

// Phone.person is a @ManyToOne
Join<Phone, Person> personJoin = root.join( Phone_.person );
// Person.addresses is an @ElementCollection
Join<Person, String> addressesJoin = personJoin.join( Person_.addresses );

criteria.where( builder.isNotEmpty( root.get( Phone_.calls ) ) );

List<Phone> phones = entityManager.createQuery( criteria ).getResultList();

15.10. Fetches

Just like in HQL and JPQL, criteria queries can specify that associated data be fetched along with the owner. Fetches are created by the numerous overloaded fetch methods of the javax.persistence.criteria.From interface.

Example 324. Join fetch example

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Phone> criteria = builder.createQuery( Phone.class );
Root<Phone> root = criteria.from( Phone.class );

// Phone.person is a @ManyToOne
Fetch<Phone, Person> personFetch = root.fetch( Phone_.person );
// Person.addresses is an @ElementCollection
Fetch<Person, String> addressesJoin = personFetch.fetch( Person_.addresses );

criteria.where( builder.isNotEmpty( root.get( Phone_.calls ) ) );

List<Phone> phones = entityManager.createQuery( criteria ).getResultList();

15.11. Path expressions

15.12. Using parameters

Example 325. Parameters example

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Person> criteria = builder.createQuery( Person.class );
Root<Person> root = criteria.from( Person.class );

ParameterExpression<String> nickNameParameter = builder.parameter( String.class );
criteria.where( builder.equal( root.get( Person_.nickName ), nickNameParameter ) );

TypedQuery<Person> query = entityManager.createQuery( criteria );
query.setParameter( nickNameParameter, "JD" );
List<Person> persons = query.getResultList();

Use the parameter method of javax.persistence.criteria.CriteriaBuilder to obtain a parameter reference. Then use the parameter reference to bind the parameter value to the javax.persistence.Query.

15.13. Using group by

Example 326. Group by example

CriteriaBuilder builder = entityManager.getCriteriaBuilder();

CriteriaQuery<Tuple> criteria = builder.createQuery( Tuple.class );
Root<Person> root = criteria.from( Person.class );

criteria.groupBy(root.get("address"));
criteria.multiselect(root.get("address"), builder.count(root));

List<Tuple> tuples = entityManager.createQuery( criteria ).getResultList();

for ( Tuple tuple : tuples ) {
    String name = (String) tuple.get( 0 );
    Long count = (Long) tuple.get( 1 );
}