java - 没有名为 X 的 EntityManager 的持久性提供程序

Question

我正在使用 JPA 开发 JavaSE 应用程序。不幸的是，我在调用后得到了 null:Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);

下面你会发现:

• 调用 EntityManagerFactory 并意外返回 null 的代码片段
• 我的persistence.xml 文件
• 我的项目结构

我的代码片段:

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

    public static void main(String[] args) {
        // I get null on this line!!!
       factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);

       EntityManager em = factory.createEntityManager();
       // do stuff with entity manager
       ...
    }
}

我的 persistence.xml:

<?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="MeineJpaPU" transaction-type="RESOURCE_LOCAL">
    <provider>org.apache.openjpa.persistence.PersistenceProviderImpl</provider>
    <class>path.to.package.server.Todo</class>
      <exclude-unlisted-classes>false</exclude-unlisted-classes>
     <properties>       
            <property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver"/>       
            <property name="javax.persistence.jdbc.url"  value="jdbc:postgresql://localhost:5432/test"/>       
            <property name="javax.persistence.jdbc.user" value="postgres"/>       
            <property name="javax.persistence.jdbc.password" value="postgres"/>       
        </properties>
  </persistence-unit>
</persistence>

我的项目结构:

Answer 1

您必须将 persistence.xml 文件移动到适当的位置。

更具体地说，将 META-INF/persistence.xml 文件添加到源文件夹的根目录。

在这种情况下，以下是合适的位置:src\main\java\META-INF\persistence.xml

详情如下:(取自JPA 规范)

A persistence.xml file defines a persistence unit. The persistence.xml file is located in the META-INF directory of the root of the persistence unit.

持久化单元的根是这里的关键。

如果您是非 Java EE 应用

The jar file or directory whose META-INF directory contains the persistence.xml file is termed the root of the persistence unit.

如果您使用的是 Java EE 应用，则以下内容有效

In Java EE environments, the root of a persistence unit must be one of the following:

• an EJB-JAR file
• the WEB-INF/classes directory of a WAR file[80]
• a jar file in the WEB-INF/lib directory of a WAR file
• a jar file in the EAR library directory
• an application client jar file