Using the Criteria API and Metamodel API to Create Basic Typesafe Queries

The basic semantics of a Criteria query consists of a SELECT clause, a FROM clause, and an optional WHERE clause, similar to a JPQL query. Criteria queries set these clauses by using Java programming language objects, so the query can be created in a typesafe manner.

Creating a Criteria Query

The javax.persistence.criteria.CriteriaBuilder interface is used to construct

Criteria queries
Selections
Expressions
Predicates
Ordering

To obtain an instance of the CriteriaBuilder interface, call the getCriteriaBuilder method on either an EntityManager or an EntityManagerFactory instance.

The following code shows how to obtain a CriteriaBuilder instance by using the EntityManager.getCriteriaBuilder method.

EntityManager em = ...;
CriteriaBuilder cb = em.getCriteriaBuilder();

Criteria queries are constructed by obtaining an instance of the following interface:

javax.persistence.criteria.CriteriaQuery

CriteriaQuery objects define a particular query that will navigate over one or more entities. Obtain CriteriaQuery instances by calling one of the CriteriaBuilder.createQuery methods. For creating typesafe queries, call the CriteriaBuilder.createQuery method as follows:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);

The CriteriaQuery object’s type should be set to the expected result type of the query. In the preceding code, the object’s type is set to CriteriaQuery<Pet> for a query that will find instances of the Pet entity.

In the following code snippet, a CriteriaQuery object is created for a query that returns a String:

CriteriaQuery<String> cq = cb.createQuery(String.class);

Query Roots

For a particular CriteriaQuery object, the root entity of the query, from which all navigation originates, is called the query root. It is similar to the FROM clause in a JPQL query.

Create the query root by calling the from method on the CriteriaQuery instance. The argument to the from method is either the entity class or an EntityType<T> instance for the entity.

The following code sets the query root to the Pet entity:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);

The following code sets the query root to the Pet class by using an EntityType<T> instance:

EntityManager em = ...;
Metamodel m = em.getMetamodel();
EntityType<Pet> Pet_ = m.entity(Pet.class);
Root<Pet> pet = cq.from(Pet_);

Criteria queries may have more than one query root. This usually occurs when the query navigates from several entities.

The following code has two Root instances:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet1 = cq.from(Pet.class);
Root<Pet> pet2 = cq.from(Pet.class);

Querying Relationships Using Joins

For queries that navigate to related entity classes, the query must define a join to the related entity by calling one of the From.join methods on the query root object or another join object. The join methods are similar to the JOIN keyword in JPQL.

The target of the join uses the Metamodel class of type EntityType<T> to specify the persistent field or property of the joined entity.

The join methods return an object of type Join<X, Y>, where X is the source entity and Y is the target of the join. In the following code snippet, Pet is the source entity, Owner is the target, and Pet_ is a statically generated metamodel class:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);

Root<Pet> pet = cq.from(Pet.class);
Join<Pet, Owner> owner = pet.join(Pet_.owners);

Joins can be chained together to navigate to related entities of the target entity without having to create a Join<X, Y> instance for each join:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);

Root<Pet> pet = cq.from(Pet.class);
Join<Owner, Address> address = cq.join(Pet_.owners).join(Owner_.addresses);

Path Navigation in Criteria Queries

Path objects are used in the SELECT and WHERE clauses of a Criteria query and can be query root entities, join entities, or other Path objects. The Path.get method is used to navigate to attributes of the entities of a query.

The argument to the get method is the corresponding attribute of the entity’s Metamodel class. The attribute can either be a single-valued attribute, specified by @SingularAttribute in the Metamodel class, or a collection-valued attribute, specified by one of @CollectionAttribute, @SetAttribute, @ListAttribute, or @MapAttribute.

The following query returns the names of all the pets in the data store. The get method is called on the query root, pet, with the name attribute of the Pet entity’s Metamodel class, Pet_, as the argument:

CriteriaQuery<String> cq = cb.createQuery(String.class);

Root<Pet> pet = cq.from(Pet.class);
cq.select(pet.get(Pet_.name));

Restricting Criteria Query Results

The results of a query can be restricted on the CriteriaQuery object according to conditions set by calling the CriteriaQuery.where method. Calling the where method is analogous to setting the WHERE clause in a JPQL query.

The where method evaluates instances of the Expression interface to restrict the results according to the conditions of the expressions. Expression instances are created by using methods defined in the Expression and CriteriaBuilder interfaces.

The `Expression` Interface Methods

An Expression object is used in a query’s SELECT, WHERE, or HAVING clause. Table 35-1 shows conditional methods you can use with Expression objects.

Table 35-1 Conditional Methods in the Expression Interface

Method	Description
`isNull`	Tests whether an expression is null
`isNotNull`	Tests whether an expression is not null
`in`	Tests whether an expression is within a list of values

The following query uses the Expression.isNull method to find all pets where the color attribute is null:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.where(pet.get(Pet_.color).isNull());

The following query uses the Expression.in method to find all brown and black pets:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.where(pet.get(Pet_.color).in("brown", "black"));

The in method also can check whether an attribute is a member of a collection.

Expression Methods in the `CriteriaBuilder` Interface

The CriteriaBuilder interface defines additional methods for creating expressions. These methods correspond to the arithmetic, string, date, time, and case operators and functions of JPQL. Table 35-2 shows conditional methods you can use with CriteriaBuilder objects.

Table 35-2 Conditional Methods in the CriteriaBuilder Interface

Conditional Method	Description
`equal`	Tests whether two expressions are equal
`notEqual`	Tests whether two expressions are not equal
`gt`	Tests whether the first numeric expression is greater than the second numeric expression
`ge`	Tests whether the first numeric expression is greater than or equal to the second numeric expression
`lt`	Tests whether the first numeric expression is less than the second numeric expression
`le`	Tests whether the first numeric expression is less than or equal to the second numeric expression
`between`	Tests whether the first expression is between the second and third expression in value
`like`	Tests whether the expression matches a given pattern

The following code uses the CriteriaBuilder.equal method:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.where(cb.equal(pet.get(Pet_.name), "Fido"));
...

The following code uses the CriteriaBuilder.gt method:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
Date someDate = new Date(...);
cq.where(cb.gt(pet.get(Pet_.birthday), date));

The following code uses the CriteriaBuilder.between method:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
Date firstDate = new Date(...);
Date secondDate = new Date(...);
cq.where(cb.between(pet.get(Pet_.birthday), firstDate, secondDate));

The following code uses the CriteriaBuilder.like method:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.where(cb.like(pet.get(Pet_.name), "*do"));

Multiple conditional predicates can be specified by using the compound predicate methods of the CriteriaBuilder interface, as shown in Table 35-3.

Table 35-3 Compound Predicate Methods in the CriteriaBuilder Interface

Method	Description
`and`	A logical conjunction of two Boolean expressions
`or`	A logical disjunction of two Boolean expressions
`not`	A logical negation of the given Boolean expression

The following code shows the use of compound predicates in queries:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.where(cb.equal(pet.get(Pet_.name), "Fido")
    .and(cb.equal(pet.get(Pet_.color), "brown")));

Managing Criteria Query Results

For queries that return more than one result, it’s often helpful to organize those results. The CriteriaQuery interface defines the orderBy method to order query results according to attributes of an entity. The CriteriaQuery interface also defines the groupBy method to group the results of a query together according to attributes of an entity, and the having method to restrict those groups according to a condition.

Ordering Results

The order of the results of a query can be set by calling the CriteriaQuery.orderBy method and passing in an Order object. Order objects are created by calling either the CriteriaBuilder.asc or the CriteriaBuilder.desc method. The asc method is used to order the results by ascending value of the passed expression parameter. The desc method is used to order the results by descending value of the passed expression parameter. The following query shows the use of the desc method:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.select(pet);
cq.orderBy(cb.desc(pet.get(Pet_.birthday)));

In this query, the results will be ordered by the pet’s birthday from highest to lowest. That is, pets born in December will appear before pets born in May.

The following query shows the use of the asc method:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
Join<Owner, Address> address = cq.join(Pet_.owners).join(Owner_.address);
cq.select(pet);
cq.orderBy(cb.asc(address.get(Address_.postalCode)));

In this query, the results will be ordered by the pet owner’s postal code from lowest to highest. That is, pets whose owner lives in the 10001 zip code will appear before pets whose owner lives in the 91000 zip code.

If more than one Order object is passed to orderBy, the precedence is determined by the order in which they appear in the argument list of orderBy. The first Order object has precedence.

The following code orders results by multiple criteria:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
Join<Pet, Owner> owner = cq.join(Pet_.owners);
cq.select(pet);
cq.orderBy(cb.asc(owner.get(Owner_.lastName), owner.get(Owner_.firstName)));

The results of this query will be ordered alphabetically by the pet owner’s last name, then first name.

Grouping Results

The CriteriaQuery.groupBy method partitions the query results into groups. These groups are set by passing an expression to groupBy:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.groupBy(pet.get(Pet_.color));

This query returns all Pet entities and groups the results by the pet’s color.

The CriteriaQuery.having method is used in conjunction with groupBy to filter over the groups. The having method takes a conditional expression as a parameter. By calling the having method, the query result is restricted according to the conditional expression:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
Root<Pet> pet = cq.from(Pet.class);
cq.groupBy(pet.get(Pet_.color));
cq.having(cb.in(pet.get(Pet_.color)).value("brown").value("blonde"));

In this example, the query groups the returned Pet entities by color, as in the preceding example. However, the only returned groups will be Pet entities where the color attribute is set to brown or blonde. That is, no gray-colored pets will be returned in this query.

Executing Queries

To prepare a query for execution, create a TypedQuery<T> object with the type of the query result by passing the CriteriaQuery object to EntityManager.createQuery.

Queries are executed by calling either getSingleResult or getResultList on the TypedQuery<T> object.

Single-Valued Query Results

The TypedQuery<T>.getSingleResult method is used for executing queries that return a single result:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
...
TypedQuery<Pet> q = em.createQuery(cq);
Pet result = q.getSingleResult();

Collection-Valued Query Results

The TypedQuery<T>.getResultList method is used for executing queries that return a collection of objects:

CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);
...
TypedQuery<Pet> q = em.createQuery(cq);
List<Pet> results = q.getResultList();