通常情况下，我们都是直接将 JPQL 查询语句写到 createNamedQuery("JPQL 语句") 方法中，例如：
EntityManager em = factory.createEntityManager();
Query query = em.createQuery("select t from User t where t.id=?1");
query.setParameter(1, 1);JPQL 命名查询指将上面示例中的 JPQL 通过 @NamedQuery 注解配置在实体类或映射的超类上。给该 JPQL 指定一个唯一名称，然后通过 createNamedQuery("JPQL 命名名称") 方法进行调用。
以下是 JPQL 中命名查询的定义示例：
@NamedQuery( name="findAllCustomersWithName", query="SELECT c FROM Customer c WHERE c.name LIKE :custName" )
上面定义了一个名为 findAllCustomersWithName 的 JPQL 查询语句。以下是使用命名查询的示例：
@PersistenceContext
public EntityManager em;
...
customers = em.createNamedQuery("findAllCustomersWithName")
.setParameter("custName", "Smith")
.getResultList();本示例还是以用户和用户的图书为例，演示 JPQL 命名查询的用法。详细步骤如下：
（1）配置 persistence.xml
<persistence xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1.0"> <persistence-unit name="openJPA" transaction-type="RESOURCE_LOCAL"> <!-- JPA提供者 --> <provider>org.apache.openjpa.persistence.PersistenceProviderImpl</provider> <!-- 声明实体类 --> <class>com.hxstrive.openjpa.entity.User</class> <class>com.hxstrive.openjpa.entity.Book</class> <!-- 配置JPA数据库属性 --> <properties> <property name="openjpa.ConnectionURL" value="jdbc:mysql://localhost:3306/openjpa_learn?useSSL=false& serverTimezone=UTC&useUnicode=true&characterEncoding=UTF-8"/> <property name="openjpa.ConnectionDriverName" value="com.mysql.jdbc.Driver"/> <property name="openjpa.ConnectionUserName" value="root"/> <property name="openjpa.ConnectionPassword" value="aaaaaa"/> <property name="openjpa.Log" value="SQL=TRACE"/> <!-- 自动生成表 --> <property name="openjpa.jdbc.SynchronizeMappings" value="buildSchema(ForeignKeys=true)"/> <!-- 不使用加载时强化和编译时强化，使用运行时Unenhanced（不能发挥OpenJPA的最大效能，所以也不推荐） --> <property name="openjpa.ClassLoadEnhancement" value="false"/> <property name="openjpa.DynamicEnhancementAgent" value="false"/> <property name="openjpa.RuntimeUnenhancedClasses" value="supported"/> </properties> </persistence-unit> </persistence>
（2）用户表实体映射，其中使用 @OneToMany 指定一对多映射，一个用户拥有多本图书。代码如下：
@Data
@Entity
@Table
@NamedQueries({
@NamedQuery(name = "find_users", query = "Select u from User u"),
@NamedQuery(name = "find_byId", query = "Select u from User u where u.id=:id")
})
public class User {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
@Column
private String name;
@Column
private Integer age;
@Column
private Float salary;
@OneToMany(cascade = CascadeType.ALL)
private List<Book> bookList;
}上面代码中，通过 @NamedQueries 或者 @NamedQuery 注解定义了两个命名 JPQL 查询。
（3）图书表实体映射，代码如下：
@Data
@Entity
@Table
public class Book {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
@Column
private String name;
@Column
private float price;
}（4）客户端代码
import com.hxstrive.openjpa.entity.User;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
import java.util.List;
public class JPQLDemo2 {
/** 持久化单元名称 */
private static final String PERSISTENCE_NAME = "openJPA";
public static void main(String[] args) {
EntityManagerFactory factory = Persistence.createEntityManagerFactory(
PERSISTENCE_NAME, System.getProperties());
EntityManager em = factory.createEntityManager();
Query query = em.createNamedQuery("find_byId");
query.setParameter("id", 1);
List<User> userList = query.getResultList();
for (User user : userList) {
System.out.println(user);
}
em.close();
factory.close();
System.out.println("finished.");
}
}运行客户端输出的 SQL 日志如下：
SELECT t0.id, t0.age, t0.name, t0.salary FROM User t0 WHERE (t0.id = ?) [params=?]
SELECT t1.id, t1.name, t1.price FROM User_Book t0 INNER JOIN Book t1 ON t0.BOOKLIST_ID = t1.id WHERE t0.USER_ID = ? [params=?]
User{id=1, name='用户-0', age=56, salary=7251.132, bookList=[{"id":1,"name":"图书-用户-0-0","price":40.912487},{"id":2,"name":"图书-用户-0-1","price":25.45687},{"id":3,"name":"图书-用户-0-2","price":87.681984},{"id":4,"name":"图书-用户-0-3","price":94.50937}]}
finished.
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