java - JPA 新手 - 可以写入，但无法从数据库读取

Question

我正在努力学习JPA。如果我执行写入操作，然后在调用 close 之前立即读取，则它可以工作。但如果我尝试自己阅读，却不会。这是我在用户中的代码:

@Entity
@Table(name = "JPA_Users")
public class User {

    @Id @GeneratedValue
    @Column(name = "user_id")
    private int id;

    @Column(name = "username")
    private String username;

    @Column(name = "password")
    private String password;

    public User(){}

    public User(String username, String password){
        this.username = username;
        this.password = password;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString(){
        return Integer.toString(id) + ":" + username + ":" + password;
    }
}

这是 JPA 代码:

public void write(){
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("helloworld");

    EntityManager em = emf.createEntityManager();
    EntityTransaction et = em.getTransaction();
    et.begin();

    User user = new User("Chris", "123456");
    User user2 = new User("Dave", "password");
    User user3 = new User("Peter", "asfdfsdf");
    em.persist(user);
    em.persist(user2);
    em.persist(user3);
    em.flush();

    List users = em.createQuery("SELECT u FROM User u").getResultList();
    System.out.println(users.size() + " user(s) found");
    for(Object o : users){
        User u = (User) o;
        System.out.println(u.getId());
        System.out.println(u.getUsername());
        System.out.println(u.getPassword());
        System.out.println("--------------------------");
    }

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

以上代码有效，并打印出用户详细信息。但如果我单独运行下面的方法，它就不起作用。它打印出“找到 0 个用户”。我已在 SQL Developer 中检查该表是否存在并已填充，确实如此。

public void read(){
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("helloworld");
    EntityManager em = emf.createEntityManager();
    List users = em.createQuery("SELECT u FROM User u").getResultList();
    System.out.println(users.size() + " user(s) found");
    for(Object o : users){
        User u = (User) o;
        System.out.println(u.getId());
        System.out.println(u.getUsername());
        System.out.println(u.getPassword());
        System.out.println("--------------------------");
    }
    em.close();
    emf.close();
}

这本身也不起作用:

public void read(){
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("helloworld");
    EntityManager em = emf.createEntityManager();
    User u = em.find(User.class, 1);
    System.out.println(u);
    em.close();
    emf.close();
}

最后，这是 persistence.xml。我不明白下面的大部分内容，所以我猜问题与这里的内容有关。谢谢，我感谢任何帮助。

<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="helloworld">
    <properties>
        <!-- Scan for annotated classes and Hibernate mapping XML files -->
        <property name="hibernate.archive.autodetection" value="class, hbm" />

        <!-- SQL stdout logging -->
        <property name="hibernate.show_sql" value="true" />
        <property name="hibernate.format_sql" value="true" />
        <property name="use_sql_comments" value="true" />
        <property name="hibernate.dialect" value="org.hibernate.dialect.OracleDialect" />
        <property name="hibernate.connection.driver_class" value="oracle.jdbc.driver.OracleDriver" />
        <property name="hibernate.connection.url"
            value="jdbc:My_Database_location" />
        <property name="hibernate.connection.username" value="My_Username" />
        <property name="hibernate.connection.password" value="My_Password"/>

        <property name="hibernate.hbm2ddl.auto" value="create" />
        <property name="hibernate.c3p0.min_size" value="2" />
        <property name="hibernate.c3p0.max_size" value="30" />
        <property name="hibernate.c3p0.timeout" value="300" />
        <property name="hibernate.c3p0.max_statements" value="50" />
        <property name="hibernate.c3p0.idle_test_period" value="3000" />
    </properties>
</persistence-unit>

Answer 1

 User u = em.find(User.class, 1);

并且您 100% 确定您的用户表中确实存在 ID =1 的实体，对吧？...