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Creation DataMatrix in Java Using the query API and JPQL to retrieve entities

s 8 and 9 discussed how the EJB 3 Java Persistence API (JPA) O/R mapping mechanism shields developers from having to write SQL to directly manipulate data. 8 explained how to map entities to relational tables, and chapter 9 revealed how to manipulate those entities using the EntityManager API. The next couple of topics we ll tackle in our quest for JPA mastery are the query API and the Java Persistence Query Language (JPQL). We ll continue to use ActionBazaar to explore these new concepts, incorporating JPQL into our auction system as well as looking at how the various one-to-many and many-to-one mappings work with JPA. With JPA, you can use the following methods to retrieve entities and related data:

We discussed the first option, retrieving entities using a primary key, in chapter 9. In this chapter we ll focus on retrieving entities using the latter two methods: JPQL and SQL queries. First we ll look at the query API and show you how to execute queries; then we ll explore the basics of JPQL. The chapter concludes by exploring the use of SQL queries with EJB 3.

Queries are as important as storing data. Therefore, a flexible query language is an important aspect of ORM. One of the primary problems in EJB 2 was that its query capabilities were limited. EJB 3 greatly improves the query capabilities of entities. In this section we introduce the query API and two types of queries: dynamic and named.

In the previous chapter, we used the EntityManager.find method to retrieve entities. This method only retrieves entities by their ID or primary key. However, there are many instances when you need more powerful queries. The query API allows developers to write custom queries to retrieve either one or a collection of entities. The EntityManager interface has several methods designed for creating queries to retrieve entities. The EntityManager methods are used together with the javax.persistence.Query interface methods, which perform the actual query definition, parameter binding, execution, and pagination.

The JPA query API allows you to use either JPQL or SQL to create the queries. We ll cover JPQL in detail in section 10.3. SQL is the standard for querying relational data; we ll discuss how to use SQL to create queries with JPA in section 10.4. If you are not familiar with SQL, refer to appendix B. Most of the time when you use the Query interface, you will likely choose to work with JPQL rather than SQL, since a SQL query returns database records and a JPA query returns entities. The EntityManager interface methods for creating query instances, the Query interface methods that define and execute the query, and JPQL are referred to as the query API. This is shown in figure 10.1. As we dive deeper into the query API, you will start to realize that building queries to retrieve entities is often the most challenging task of building any enterprise applications, and almost every aspect of an application requires some data retrieval. The query API supports building and using queries in several ways, as you ll learn in the next few sections. Depending on what you are trying to accomplish, you may devise some interesting ways to utilize queries. The steps you follow to create a JPA query are similar to those of a traditional JBDC query. Table 10.1 compares using a JDBC query to the basic flow of a JPA query. As you can see, the primary differences lie in the way you obtain the database connection and the way in which you manipulate the query itself. In JDBC it is normal to employ a database connection directly. Conversely, JPA hides the database connection behind the EntityManager.

Figure 10.1 The JPA query API includes EntityManager methods to create queries, the methods in the Query interface for executing the query, and the Java Persistence Query Language.