Eclipse+JBoss+EJB3使用命名查詢執行JPQL

在EJB3中可以使用EntityManager對象的createQuery方法來執行JPQL（類似於Hibernate中的HQL），這非常簡單。但使用createQuery方法處理JPQL時，在每次執行JPQL的過程中系統都需要對JPQL進行分析，這在一定程度上降低了系統運行時的性能。為此，EJB3提供了命名查詢的概念。命名查詢有些類型於數據庫中的存儲過程，在提交的過程中就已經被編譯處理了。因此，在執行效率上要高一些。

我們可以使用@NamedQuery注釋來定義命名查詢。這個注釋可以放在任何一個實體Bean的上方。但為了便於管理，最好放在相關的實體Bean的上方。如下面的代碼所示：

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;  
    }  
}  

代碼中的命名查詢使用了命名參數。我們可以使用下面的代碼來執行該JPQL：

private Customer queryCustomer(int id)  
    {  
        return (Customer) em.createNamedQuery("MyQuery").setParameter("id", 23)  
                .getSingleResult();  
    } 

在Session Bean中調用queryCustomer就可以通過命名查詢獲得相應的Customer對象了。