// Create the EntityManager
EntityManagerFactory factory = Persistence.createEntityManagerFactory("league");
EntityManager em = factory.createEntityManager();

public class CreatePlayersAndTeams ...{

    /**//** Creates a new instance of CreatePlayersAndTeams */
    public CreatePlayersAndTeams() ...{
    }

    /**//**
     * @param args the command line arguments
     */
    public static void main(String[] args) ...{
        // Create the EntityManager
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("league");
        EntityManager em = emf.createEntityManager();

        em.getTransaction().begin();

        for(Team team: teams) ...{
            em.persist(team);
        }

        for(Player player: players) ...{
            player.setTeam(teams[0]);
            teams[0].addPlayer(player);
            em.persist(player);
        }

        em.getTransaction().commit();

        em.close();
        emf.close();
    }

    private static Player[] players = new Player[] ...{
        // name, number, last quoted statement
        new Player("Lowe", "Derek", 23, "The sinker's been good to me."),
        new Player("Kent", "Jeff", 12, "I wish I could run faster."),
        new Player("Garciaparra", "Nomar", 5,
                "No, I'm not superstitious at all.")

    };

    public static Team[] teams = new Team[] ...{
        new Team("Los Angeles Dodgers", "National"),
        new Team("San Francisco Giants", "National"),
        new Team("Anaheim Angels", "American"),
        new Team("Boston Red Sox", "American")
    };
}

 图一展示了一个 TEAM的表。既然 Team实体已经自动生成了标识符，那么在表中的前四行，从1到4的顺序就是主键的值了。

图一：Team实体变缺省映射为TEAM表

 

图二显示了 PLAYER表。自动生成的标识符是接着 TEAM表下来的。注意到 player每行都有一个外键值，与其球队对应。在此例子中，每个 player都属于 拥有主键值1的Dodgers队。

图二：player表中每行都有一个外键TEAM_ID

 

GlassFish应用实现实际上为你创建了 PLAYER 和 TEAM 表。一旦你使用数据库工具创建了 league数据库，如果数据库中没有这两个表的存在的话，上面的示例代码就会创建所需的实体表。当然，这种操作是可以选择的，它由 persistence.xml文件指定提供商属性 toplink.ddl-generation所决定，在下载的示例代码中你可以看到 create-tables属性值。当然，如果表存在的话，代码执行结果就向存在的表格中插入记录，并且生成很多的警告信息，告诉你存在了那些表。当你需要做改变，测试，删除表，改变代码，重复测试的时候，使用 toplink.ddl-generation属性可以自动创建表，列，和主键，缩短快速开发周期，提高生产力 。

 

如果是需要创建一个大型数据库，仅仅通过简单的提交持久化实体是不好的做法。你或许喜欢使用 SQL语句或者是数据库供应商自己的工具，来生成表格和其它的数据库元素。但是，持久化规范明确的描述了一条规则就是：当你将你的对象实体映射到成它们之间实体关系时，你要使用提供商实现。规范定义了实体、属性、标志符如何在数据库中建立成为表，列，主键，外键。当你在创建数据库表的时候，会用到 Java持久化API，所以你必须记住命名规约。否则，你要使用声明来重写那些缺省的设置。

// The EntityManager, em, has already been created at this point.
...
// Retrieve Player entities using their keys.
for(long primaryKey = 1; primaryKey < 10; primaryKey++) ...{
    Player player = em.find(Player.class, primaryKey);
    if (player != null) ...{
        System.out.println(player.toString());
    }
}

[Jersey Number: 23, Name: Derek Lowe, Team: Los Angeles Dodgers]
[Jersey Number: 12, Name: Jeff Kent, Team: Los Angeles Dodgers]
[Jersey Number: 5, Name: Nomar Garciaparra, Team: Los Angeles Dodgers]

// The EntityManager, em, already exists for this example.
...
// We just happen to know that '5' is one of the
// player identifiers. You shouldn't normally hard-code this
// into any application.
Player p = em.find(Player.class, 5L);
em.clear();
// p is now detached for the convenience of this example

Team t = new Team("Ventura Surfers", "National");
p.setTeam(t);

em.getTransaction().begin();
Player managedPlayer = em.merge(p);
em.getTransaction().commit();

// 'em' is an EntityManager instance
em.getTransaction().begin();
// use a hard-coded player id for convenience in this sample only
Player player = em.find(Player.class, 5L);
if (player != null) ...{
    System.out.println(player.toString());
    em.remove(player);
}
em.getTransaction().commit();