Josh Juneau [Josh Juneau] - Java EE 8 Recipes: A Problem-Solution Approach

Create a CriteriaQuery object by generating a criteria builder from the EntityManager object and calling its createQuery method. Once a CriteriaQuery object has been created, generate a query by calling a series of the CriteriaBuilder methods against the entity that you want to query. Finally, call the EntityManager s createQuery method, passing the query that you have previously built. Return the ResultList from the query to return all the rows from the table. In the following lines of code, you can see this technique performed:

An entity instance can be referred to as a record in the underlying data store. That is, there is an entity instance for each record within a given database table. That said, sometimes it is handy to retrieve all of the instances for a given entity. Some applications may require all objects in order to perform a particular task against each, or perhaps your application needs to simply display all of the instances of an entity for the user. Whatever the case, there are a couple of ways to retrieve all of the instances for a given entity. Each of the techniques should occur within an EJB.

In Solution #1, the JPQL can be used to query an entity for all instances. To create a dynamic query, call the EntityManager s createQuery method, to which you can pass a string-based query that consists of JPQL syntax, or a javax.persistence.Query instance. The Query interface has a sizable number of methods that can be used to work with the query object. Table describes what these methods do.

In the example, a query string is passed to the method, and it reads as follows:

To break this down, the query is selecting all objects from the BookAuthor entity. Any letter could have been used in place of the o character within the query, but o is a bit of a standard since JPQL is referring to objects. All queries contain a SELECT clause , which is used to define the types of entity instances that you want to obtain. In the example, the entire instance is selected from the BookAuthor entity, as opposed to single fields that are contained within the instance. Since the JPA works with objects, queries should always return the entire object; if you want to use only a subset of fields from the object, then you can call upon those fields from the instance(s) returned from the query. The object keyword is optional and is provided mainly for readability. The same JPQL could be written as follows:

The FROM clause can reference one or more identification variables that can refer to the name of an entity, an element of a single-valued relationship, an element of a collection relationship, or a member of a collection that is the multiple side of a one-to-many relationship. In the example, the BookAuthor variable refers to the entity itself.