如何把JDBC方式的代码修改JPA方式的代码

欢迎来到李绪成的一亩三分地，有用的拿走，没有用的告诉我。
本文介绍如何把JDBC应用改成JPA应用。
把JDBC应用改成JPA程序，需要把原来通过JDBC API访问数据库的代码替换成使用JPA代码。
JDBC访问数据库的主要工作包括：
n得到JDBC驱动程序；
n使用DriverManager，Connection，Statement，ResultSet等；
而使用JPA完成数据的操作包括：
n得到JDBC驱动程序；
n得到持久性提供者相关类库和配置文件；
n提供实体类；
n使用Persistence、EntityManagerFactory和Entity等接口。
要完成修改，需要完成如下工作：
n提供配置文件persistence.xml文件和持久性提供者相关类库；
n需要把原来表示信息的普通JavaBean修改实体类；
n把JDBC代码修改为JPA代码。
下面以图书添加为例介绍。
1、提供配置文件persistence.xml文件和持久性提供者相关类库
可以参考JP两个相关实验中的过程，添加JPA支持，可以为工程添加类库，并且会生成persistence.xml配置文件：
<?xml version="1.0" encoding="UTF-8"?>
<persistence 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" version="1.0">

<persistence-unit name="bookstore_PU"
transaction-type="RESOURCE_LOCAL">
<provider>
oracle.toplink.essentials.PersistenceProvider
</provider>
<class>beans.Book</class>
<properties>
<property name="toplink.jdbc.driver"
value="com.mysql.jdbc.Driver" />
<property name="toplink.jdbc.url"
value="jdbc:mysql://localhost:3306/entitytest" />
<property name="toplink.jdbc.user" value="root" />
<property name="toplink.jdbc.password" value="root" />
</properties>
</persistence-unit>
</persistence>
2、把原来的JavaBean修改为实体类
可以使用MyEclipse中的功能，参考JPA的两个相关实验。
修改前JavaBean的代码如下：
package beans;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

publicclass Book implements java.io.Serializable {

private String bid;
private String bname;
private Float price;
private String author;
private String press;

public Book() {
}

public Book(String bid, String bname) {
this.bid = bid;
this.bname = bname;
}

public Book(String bid, String bname, Float price, String author, String press) {
this.bid = bid;
this.bname = bname;
this.price = price;
this.author = author;
this.press = press;
}

public String getBid() {
returnthis.bid;
}

publicvoid setBid(String bid) {
this.bid = bid;
}

public String getBname() {
returnthis.bname;
}

publicvoid setBname(String bname) {
this.bname = bname;
}

public Float getPrice() {
returnthis.price;
}

publicvoid setPrice(Float price) {
this.price = price;
}

public String getAuthor() {
returnthis.author;
}

publicvoid setAuthor(String author) {
this.author = author;
}
public String getPress() {
returnthis.press;
}
publicvoid setPress(String press) {
this.press = press;
}
}
修改的内容包括：
n在类上使用@Entity声明Bean类为实体类；
n在类上使用@Table声明与数据库中表的对应关系；
n在主键属性上(或者get方法上)使用@Id标注主键属性；
n在属性上使用@Column标注属性与表中列的对应关系；
修改后JavaBean的代码如下：
package beans;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;


/**
*Bookentity.@authorMyEclipsePersistenceTools
*/
@Entity
@Table(name="book"
,catalog="entitytest"
)

publicclass Book implements java.io.Serializable {


// Fields

private String bid;
private String bname;
private Float price;
private String author;
private String press;


// Constructors

/**defaultconstructor*/
public Book() {
}

/**minimalconstructor*/
public Book(String bid, String bname) {
this.bid = bid;
this.bname = bname;
}

/**fullconstructor*/
public Book(String bid, String bname, Float price, String author, String press) {
this.bid = bid;
this.bname = bname;
this.price = price;
this.author = author;
this.press = press;
}


// Property accessors
@Id

@Column(name="BID", unique=true, nullable=false, length=13)

public String getBid() {
returnthis.bid;
}

publicvoid setBid(String bid) {
this.bid = bid;
}

@Column(name="BNAME", nullable=false, length=30)

public String getBname() {
returnthis.bname;
}

publicvoid setBname(String bname) {
this.bname = bname;
}

@Column(name="PRICE", precision=12, scale=0)

public Float getPrice() {
returnthis.price;
}

publicvoid setPrice(Float price) {
this.price = price;
}

@Column(name="AUTHOR", length=30)

public String getAuthor() {
returnthis.author;
}

publicvoid setAuthor(String author) {
this.author = author;
}

@Column(name="PRESS", length=30)

public String getPress() {
returnthis.press;
}

publicvoid setPress(String press) {
this.press = press;
}
}
3、访问代码修改如下
修改前的代码：
// 使用JDBC
publicvoid addBook(String bookid, String bookname, String author,float price,String press) {

Connection con = null;
PreparedStatement stmt = null;
try {
// 指出连接数据库所需要的驱动程序
Class.forName("oracle.jdbc.driver.OracleDriver");

// 建立与数据库之间的连接
con = DriverManager.getConnection(
"jdbc:oracle:thin:@myserver:1521:mydb", "scott", "tiger");

// 编写查询数据库信息的SQL语句
String sql = "insert into book values(?,?,?,?,?)";

// 创建语句对象，用于执行SQL语句
stmt = con.prepareStatement(sql);
stmt.setString(1, bookid);
stmt.setString(2, bookname);
stmt.setFloat(3, price);
stmt.setString(4, author);
stmt.setString(5, press);

// 执行SQL语句得到结果集
stmt.executeUpdate();

} catch (Exception e) {
System.out.println(e.getMessage());
} finally {
// 关闭相关对象
if (stmt != null)
try {
stmt.close();
} catch (Exception ee) {
}
if (con != null)
try {
con.close();
} catch (Exception ee) {
}
}
}
修改后的代码：
// 使用JPA
publicvoid addBookJPA(String bookid, String bookname, String author,float price,String press) {
// 创建图书对象
Book book = new Book();
book.setBid(bookid);
book.setBname(bookname);
book.setAuthor(author);
book.setPress(press);
book.setPrice(price);

// 使用JPA完成操作
EntityManagerFactory emf = Persistence.createEntityManagerFactory("bookstore_PU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
em.persist(book);
em.getTransaction().commit();
em.close();
emf.close();
}
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