在EJB3中可以使用EntityManager对象的createQuery方法来执行JPQL(类似于Hibernate中的HQL),这非常简单。但使用createQuery方法处理JPQL时,在每次执行JPQL的过程中系统都需要对JPQL进行分析,这在一定程度上降低了系统运行时的性能。为此,EJB3提供了命名查询的概念。命名查询有些类型于数据库中的存储过程,在提交的过程中就已经被编译处理了。因此,在执行效率上要高一些。
我们可以使用@NamedQuery注释来定义命名查询。这个注释可以放在任何一个实体Bean的上方。但为了便于管理,最好放在相关的实体Bean的上方。如下面的代码所示:
package entity;
import java.util.Collection;
import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.NamedQuery;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;
@Entity
@Table(name = "t_customers")
@NamedQuery(name="MyQuery",query="select c from Customer c where id=:id")
public class Customer
{
private int id;
private String name;
private Referee referee;
private Collection<Order> orders;
private Collection<Address> addresses;
@OneToOne(cascade = CascadeType.ALL)
@PrimaryKeyJoinColumn
public Referee getReferee()
{
return referee;
}
public void setReferee(Referee referee)
{
this.referee = referee;
}
@ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)
@JoinTable(name = "t_customers_addresses",
joinColumns = @JoinColumn(name = "customer_id", referencedColumnName = "id"),
inverseJoinColumns = @JoinColumn(name = "address_id", referencedColumnName = "id"))
public Collection<Address> getAddresses()
{
return addresses;
}
public void setAddresses(Collection<Address> addresses)
{
this.addresses = addresses;
}
@OneToMany(mappedBy = "customer", cascade = CascadeType.ALL)
public Collection<Order> getOrders()
{
return orders;
}
public void setOrders(Collection<Order> orders)
{
this.orders = orders;
}
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
}
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代码中的命名查询使用了命名参数。我们可以使用下面的代码来执行该JPQL:
private Customer queryCustomer(int id)
{
return (Customer) em.createNamedQuery("MyQuery").setParameter("id", 23)
.getSingleResult();
}
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在Session Bean中调用queryCustomer就可以通过命名查询获得相应的Customer对象了。
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