# Criteria

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

Hibernate offers an older, legacy org.hibernate.Criteria API which should be considered deprecated. No feature development will target those APIs. Eventually, Hibernate-specific criteria features will be ported as extensions to the JPA javax.persistence.criteria.CriteriaQuery. For details on the org.hibernate.Criteria API, see .

This chapter will focus on the JPA APIs for declaring type-safe criteria 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, or 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 3 methods on javax.persistence.criteria.CriteriaBuilder for this purpose:

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

Chapter 6 Criteria API of the JPA Specification already contains a decent amount of reference material pertaining to the various parts of a criteria query. So rather than duplicate all that content here, lets instead look at some of the more widely anticipated usages of the API.

## 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.

### Selecting an entity

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

Selecting the root entity

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

The call to the CriteriaQuery.select method in this example is unnecessary because personRoot will be the implied selection since we have only a single query root. It was done here only for completeness of an example.

The Person_.eyeColor reference is an example of the static form of JPA metamodel reference. We will use that form exclusively in this chapter. See the documentation for the Hibernate JPA Metamodel Generator for additional details on the JPA static metamodel.

### 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.

Selecting an attribute

In this example, the query is typed as java.lang.Integer because that is the anticipated type of the results (the type of the Person#age attribute is java.lang.Integer). 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.

### Selecting multiple values

There are actually a few different ways to select multiple values using criteria queries. We will explore 2 options here, but an alternative recommended approach is to use tuples as described in . Or consider a wrapper query; see for details.

Selecting an array

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.

Selecting an array (2)

Just as we saw in 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[].

### Selecting a wrapper

Another alternative to 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#age] instead declare a class that holds these values and instead return that.

Selecting an wrapper

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.

## Tuple criteria queries

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

Selecting a tuple

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 passing `Tuple.class`.

Again we see the use of the multiselect method, just like in . 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 3 forms of access to the underlying elements:

typed

  The example illustrates this form of access in the `tuple.get( idPath )` and `tuple.get( agePath )` 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 and . 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.

## FROM clause

> JPA Specification, section 6.5.2 Query Roots, pg 262

> 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.

All the individual parts of the FROM clause (roots, joins, paths) implement the javax.persistence.criteria.From interface.

### 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:

Adding a root

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 matching all single men and all single women:

Adding multiple roots

### 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 with Embedded and ManyToOne

Example with Collections

### 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 with Embedded and ManyToOne

Technically speaking, embedded attributes are always fetched with their owner. However in order to define the fetching of Address#country we needed a javax.persistence.criteria.Fetch for its parent path.

Example with Collections

## Path expressions

Roots, joins and fetches are themselves paths as well.

## Using parameters

Using parameters

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