在EJB3中可以使用EntityManager對象的createQuery方法來執行JPQL(類似于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作時的性能。為 此,EJB3提供了命名查詢的概念。命名查詢有些類型于資料庫中的存儲過程,在送出的過程中就已經被編譯處理了。是以,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。但為了便于管理,最好放在相關的實體Bean的上方。如下面的代碼所示:
Code:
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。