java - 尝试运行 Vogella.de JPA 教程时，我不断收到 "No Persistence provider for EntityManager"

Question

我正在尝试在 http://www.vogella.de/articles/JavaPersistenceAPI/article.html#simple 获取教程上类。事情并非如此简单。我知道这对大多数人来说可能很简单，但我没有任何 JPA 经验或 EJB 经验。我的最终目标是在 Java 桌面类型应用程序中使用 JPA。

当我尝试运行示例时出现此错误:

Exception in thread "main" javax.persistence.PersistenceException: No Persistence provider for EntityManager named todos
    at javax.persistence.Persistence.createEntityManagerFactory(Unknown Source)
    at javax.persistence.Persistence.createEntityManagerFactory(Unknown Source)
    at de.vogella.eclipse.ide.main.Main.main(Main.java:17)

我有/src/META-INF/persistence.xml

<?xml version="1.0" encoding="UTF-8" ?>
<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_2_0.xsd"
    version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
    <persistence-unit name="todos" transaction-type="RESOURCE_LOCAL">
        <class>de.vogella.jpa.simple.model.Todo</class>
        <properties>
            <property name="javax.persistence.jdbc.driver" value="org.apache.derby.jdbc.EmbeddedDriver" />
            <property name="javax.persistence.jdbc.url"
                value="jdbc:derby:/Users/james/Desktop/simpleDb;create=true" />
            <property name="javax.persistence.jdbc.user" value="test" />
            <property name="javax.persistence.jdbc.password" value="test" />

            <!-- EclipseLink should create the database schema automatically -->
            <property name="eclipselink.ddl-generation" value="create-tables" />
            <property name="eclipselink.ddl-generation.output-mode"
                value="database" />
        </properties>

    </persistence-unit>
</persistence>

/jpa_example/src/de/vogella/jpa/simple/model/Todo.java

package de.vogella.jpa.simple.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Todo {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String summary;
    private String description;

    public String getSummary() {
        return summary;
    }

    public void setSummary(String summary) {
        this.summary = summary;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    @Override
    public String toString() {
        return "Todo [summary=" + summary + ", description=" + description
                + "]";
    }

}

/jpa_example/src/de/vogella/eclipse/ide/main/Main.java

package de.vogella.eclipse.ide.main;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

import de.vogella.jpa.simple.model.Todo;

public class Main {
    private static final String PERSISTENCE_UNIT_NAME = "todos";
    private static EntityManagerFactory factory;

    public static void main(String[] args) {
        factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        EntityManager em = factory.createEntityManager();
        // Read the existing entries and write to console
        Query q = em.createQuery("select t from Todo t");
        List<Todo> todoList = q.getResultList();
        for (Todo todo : todoList) {
            System.out.println(todo);
        }
        System.out.println("Size: " + todoList.size());

        // Create new todo
        em.getTransaction().begin();
        Todo todo = new Todo();
        todo.setSummary("This is a test");
        todo.setDescription("This is a test");
        em.persist(todo);
        em.getTransaction().commit();

        em.close();
    }
}

我一直在兜圈子，试图让任何基本的 JPA 教程发挥作用。我没有 Java 数据库经验，之前也没有使用 Java 开发 Web 应用程序，所以大部分行话都让我头疼。顺便说一句，我实际上对将 JPA 与 javaSE 桌面应用程序而不是 Web 应用程序一起使用感兴趣。我真的希望我可以下载一个已经在运行的 eclipse jpa 项目，但我还没有找到一个。我只是找到教程，在花了几个小时试图让它们工作后我放弃并继续下一个。是否有更好的教程包含可下载的项目来帮助我入门？

Answer 1

如错误消息所示，您的 persistence.xml 中缺少一个标记。在 <class> 之前添加以下行- 标记您使用的提供程序实现(显示的示例使用 OpenJPA 实现):

<provider>org.apache.openjpa.persistence.PersistenceProviderImpl</provider>