JPA (Java Persistence API)

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Java JPA EJB C C++

JPA

JPA是JAVA EE的重要组成部分, 它不再是EJB里面的一种BEAN, 可以在一般的Servlet容器中使用, 并不一定要应用服务器的支持, 可以看
成实体BEAN的替代品. 相比实体BEAN(CMP), JPA开发开发起来更加简化, 也更加轻量级, 使用了Annotation代替了原来繁琐的XML文件的配
置, 在API方面倒是跟Hibernate非常相似.

一. persistence.xml(此文件位于MATE-INF下, 是JPA的主配置文件)
  格式如下:

三. 实体管理
  1. 容器管理(使用@PersistenceContext对EntityManager实行注射, 完成初始化)
    @javax.persistence.PersistenceContext(unitName="persistence_sample")
    private EntityManager em ;
    .......................................................................

  2. 应用管理的实体管理(Application-Managed Entity Managers)
   此时, 由于取不到上下文的信息, 我们只能采用这种方式实现得到EntityManager
   @PersistenceUnit
   EntityManagerFactory emf;
   然后, 从EntityManagerFactory获取一个EntityManager实例:
   EntityManager em = emf.createEntityManager();

四. 实体的生命周期(四种状态)
  1. new 创建
  2. managed 管理
  3. detached 托管
  4. removed 删除

五. 实体操作
  1. 实体的查找

  2. 持久化实体实例(Persisting Entity Instances)
  调用persist方法, 使新的实体类成为被管理的持久对象
  3. 实体类中的实体关系(@OneToMany, @ManyToMany, @OneToOne, @ManytoOne)

4. 删除实体
  public void removeOrder(Integer orderId) {
  try {
  Order order = em.find(Order.class, orderId);
  em.remove(order);
  }...

  5. 实体的查询
   1.) createQuery
   public List findWithName(String name) {
    return em.createQuery(
    "SELECT c FROM Customer c WHERE c.name LIKE :custName")
    .setParameter("custName", name)
    .setMaxResults(10)
    .getResultList();
   }

   2.) createNamedQuery
   使用Annotation定义NamedQuery
   @NamedQuery(
   name="findAllCustomersWithName",
   query="SELECT c FROM Customer c WHERE c.name LIKE :custName"
   )

   @PersistenceContext
   public EntityManager em;
   ...
   customers = em.createNamedQuery("findAllCustomersWithName")
   .setParameter("custName", "Smith")
   .getResultList();

   指定参数名字
   public List findWithName(String name) {
    return em.createQuery(
    "SELECT c FROM Customer c WHERE c.name LIKE :custName")
    .setParameter("custName", name)
    .getResultList();
   }
   指定参数位置
   public List findWithName(String name) {
    return em.createQuery(
    "SELECT c FROM Customer c WHERE c.name LIKE ?1")
    .setParameter(1, name)
    .getResultList();
   }

  6. 用实体更新数据库记录
   我们首先找到这个实体对象, 然后对它进行操作
   public void buyBooks(ShoppingCart cart) throws OrderException{
    Collection items = cart.getItems();
    Iterator i = items.iterator();
    try {
     while (i.hasNext()) {
     ShoppingCartItem sci = (ShoppingCartItem)i.next();
     Book bd = (Book)sci.getItem();
     String id = bd.getBookId();
     int quantity = sci.getQuantity();
     buyBook(id, quantity);
     }
    } catch (Exception ex) {
     throw new OrderException("Commit failed: "
     + ex.getMessage());
    }
   }

   public void buyBook(String bookId, int quantity)
   throws OrderException {
    try {
     Book requestedBook = em.find(Book.class, bookId);
     if (requestedBook != null) {
     int inventory = requestedBook.getInventory();
     if ((inventory - quantity) >= 0) {
     int newInventory = inventory - quantity;
     requestedBook.setInventory(newInventory);
     } else{
     throw new OrderException("Not enough of "
     + bookId + " in stock to complete order.");
     }
     }
    } catch (Exception ex) {
     throw new OrderException("Couldn't purchase book: "
     + bookId + ex.getMessage());
    }
   }

  7. JTA事务支持
   @Resource
   UserTransaction utx;
   ...
   try {
    utx.begin();
    bookDBAO.buyBooks(cart);
    utx.commit();
   } catch (Exception ex) {
   try {
    utx.rollback();
   } catch (Exception exe) {
    System.out.println("Rollback failed: "+exe.getMessage());
   }

  8. 主键产生策略
   1.) @Id (含有主键的表, 自己管理主键)

   2.) 主键用一个专门的表产生
      @TableGenerator(name="ADDRESS_ID_GEN",
              table="ID_GEN",
              pkColumnName="GEN_KEY",
              valueColumnName="GEN_VALUE",
              pkColumnValue="ADDRESS_ID",
              allocationSize=1)
              @GeneratedValue(strategy=GenerationType.TABLE,generator="ADDRESS_ID_GEN")
              @Id
              public String getAddressID() {
          return addressID;
      }

    3.) 使用主键类
    package order.entity;
    public final class LineItemKey implements
    java.io.Serializable {
     private Integer orderId;
     private int itemId;
     public int hashCode() {
      return ((this.getOrderId()==null
      ?0:this.getOrderId().hashCode())
      ^ ((int) this.getItemId()));
     }
     public boolean equals(Object otherOb) {
      if (this == otherOb) {
       return true;
      }
      if (!(otherOb instanceof LineItemKey)) {
       return false;
      }
      LineItemKey other = (LineItemKey) otherOb;
      return ((this.getOrderId()==null
      ?other.orderId==null:this.getOrderId().equals
      (other.orderId)) && (this.getItemId ==
      other.itemId));
     }
     public String toString() {
      return "" + orderId + "-" + itemId;
     }
    }

    @IdClass(order.entity.LineItemKey.class)
    @Entity
    ...
    public class LineItem {
    ...
    }

java 代码

@PersistenceContext
EntityManager em;
...
public LineItem createLineItem(Order order, Product product,
int quantity) {
LineItem li = new LineItem(order, product, quantity);
order.getLineItems().add(li);
em.persist(li);
return li;
}

xml 代码

<persistence>
<persistence-unit name="OrderManagement">
<description>This unit manages orders and customers.
It does not rely on any vendor-specific features and can
therefore be deployed to any persistence provider.
description>
<jta-data-source>jdbc/MyOrderDBjta-data-source>
<jar-file>MyOrderApp.jarjar-file>
<class>com.widgets.Orderclass>
<class>com.widgets.Customerclass>
persistence-unit>
persistence>
xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
<persistence-unit name="persistence_sample" transaction-type="JTA">
<provider>oracle.toplink.essentials.ejb.cmp3.EntityManagerFactoryProviderprovider>
<jta-data-source>jdbc/SamplesDBjta-data-source>
<properties>
<property name="toplink.ddl-generation" value="drop-and-create-tables"/>
properties>
persistence-unit>
persistence>

二. 实体类
  使用annotation @Entity标识它是实体类
  使用@Id 标识它是主键
  使用@Table 标识这个实体类对应的数据库表, 如果类名跟表名相同, 则可以省略

java 代码

import java.io.Serializable;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;
@Entity
@Table(name="WEB_BOOKSTORE_BOOKS")
public class Book implements Serializable {
private String bookId;
private String title;
public Book() { }
public Book(String bookId, String title, ...) {
this.bookId = bookId;
this.title = title;
...
}
@Id
public String getBookId() {
return this.bookId;
}
public String getTitle() {
return this.title;
}
...
public void setBookId(String id) {
this.bookId=id;
}
public void setTitle(String title) {
this.title=title;
}
...
}

java 代码

package enterprise.customer_cmp_ejb.ejb.session;
import javax.ejb.Stateless;
import javax.ejb.Stateful;
import javax.ejb.SessionContext;
import javax.persistence.*;
import javax.ejb.*;
import java.util.List;
import enterprise.customer_cmp_ejb.persistence.*;
import enterprise.customer_cmp_ejb.common.*;
/**
*
* @author Rahul Biswas
*
* Why a facade?
* 1. session beans are thread safe, and EMs are not necessarily; so injecting a EM into a SessionBean makes it safe.
* 2. Tx management is taken care of by container
* 3. of course, because it's a facade [we can combine operations].
*
*/
@Stateless
@TransactionManagement(value=TransactionManagementType.CONTAINER)
public class CustomerSession implements CustomerSessionLocal, CustomerSessionRemote{
@javax.persistence.PersistenceContext(unitName="persistence_sample")
private EntityManager em ;
public CustomerSession(){
}
public Customer searchForCustomer(String id){
Customer cust = (Customer)em.find(Customer.class, id);
return cust;
}
public Subscription searchForSubscription(String id){
Subscription subscription = (Subscription)em.find(Subscription.class, id);
return subscription;
}
public Address searchForAddress(String id){
Address address = (Address)em.find(Address.class, id);
return address;
}
//This is the default; here as an example of @TransactionAttribute
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public void remove(Object obj){
Object mergedObj = em.merge(obj);
em.remove(mergedObj);
}
public void persist(Object obj){
em.persist(obj);
}
public List findAllSubscriptions(){
List subscriptions = em.createNamedQuery("findAllSubscriptions").getResultList();
return subscriptions;
}
public List findCustomerByFirstName(String firstName){
List customers = em.createNamedQuery("findCustomerByFirstName").setParameter("firstName", firstName).getResultList();
return customers;
}
public List findCustomerByLastName(String lastName){
List customers = em.createNamedQuery("findCustomerByLastName").setParameter("lastName", lastName).getResultList();
return customers;
}
public Customer addCustomerAddress(Customer cust, Address address){
Customer mergedCust = em.merge(cust);
mergedCust.getAddresses().add(address);
return mergedCust;
}
public Customer removeCustomerSubscription(String cust, String subs) throws SubscriptionNotFoundException{
//System.out.println("called remove Customer Subscription.....");
Customer customer = (Customer)em.find(Customer.class, cust);
Subscription subscription = (Subscription)em.find(Subscription.class, subs);
if(!customer.getSubscriptions().contains(subscription)){
System.out.println("remove: did not find a subscription obj for :"+subscription.getTitle());
throw new SubscriptionNotFoundException();
}
customer.getSubscriptions().remove(subscription);
subscription.getCustomers().remove(customer);
return customer;
}
public Customer addCustomerSubscription(String cust, String subs) throws DuplicateSubscriptionException{
//System.out.println("called add Customer Subscription.....");
Customer customer = (Customer)em.find(Customer.class, cust);
Subscription subscription = (Subscription)em.find(Subscription.class, subs);
if(customer.getSubscriptions().contains(subscription)){
System.out.println("add: found an existing subscription obj for :"+subscription.getTitle());
throw new DuplicateSubscriptionException();
}
customer.getSubscriptions().add(subscription);
subscription.getCustomers().add(customer);
return customer;
}
}

java 代码

@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
@OneToMany(cascade=ALL, mappedBy="order")
public Collection <lineitem></lineitem> getLineItems() {
return lineItems;
}
@ManyToMany(fetch=FetchType.EAGER )
@JoinTable(
name="CUSTOMERBEANSUBSCRIPTIONBEAN",
joinColumns=@JoinColumn(name="CUSTOMERBEAN_CUSTOMERID96", referencedColumnName="customerid"),
inverseJoinColumns=@JoinColumn(name="SUBSCRIPTIONBEAN_TITLE", referencedColumnName="TITLE")
)
public Collection<subscription></subscription> getSubscriptions(){
return subscriptions;
}
@OneToOne(optional=false)
@JoinColumn(
name="CUSTREC_ID", unique=true, nullable=false, updatable=false)
public CustomerRecord getCustomerRecord() { return customerRecord; }
@ManyToOne(optional=false)
@JoinColumn(name="CUST_ID", nullable=false, updatable=false)
public Customer getCustomer() { return customer; }

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