37.4 Querying Entities

The Java Persistence API provides the following methods for querying entities.

• The Java Persistence query language (JPQL) is a simple, string-based language similar to SQL used to query entities and their relationships. See Chapter 39, "The Java Persistence Query Language" for more information.

• The Criteria API is used to create typesafe queries using Java programming language APIs to query for entities and their relationships. See Chapter 40, "Using the Criteria API to Create Queries" for more information.

Both JPQL and the Criteria API have advantages and disadvantages.

Just a few lines long, JPQL queries are typically more concise and more readable than Criteria queries. Developers familiar with SQL will find it easy to learn the syntax of JPQL. JPQL named queries can be defined in the entity class using a Java programming language annotation or in the application's deployment descriptor. JPQL queries are not typesafe, however, and require a cast when retrieving the query result from the entity manager. This means that type-casting errors may not be caught at compile time. JPQL queries don't support open-ended parameters.

Criteria queries allow you to define the query in the business tier of the application. Although this is also possible using JPQL dynamic queries, Criteria queries provide better performance because JPQL dynamic queries must be parsed each time they are called. Criteria queries are typesafe and therefore don't require casting, as JPQL queries do. The Criteria API is just another Java programming language API and doesn't require developers to learn the syntax of another query language. Criteria queries are typically more verbose than JPQL queries and require the developer to create several objects and perform operations on those objects before submitting the query to the entity manager.