Eclipse+JBoss+EJB3使用命名查询执行JPQL

　　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;  
    }  
}  

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();  
    } 

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。