我花了最后几天寻找错误，但我找不到。我不得不说，我是一个完全新手JPA，这是我的第一个项目，我创建了一个教程，但有一个无法解释的错误。
我使用Eclipse和eclipseline连接了一个运行良好的mySql数据库
我的代码：
package com.tutorialspoint.eclipselink.service;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.tutorialspoint.eclipselink.entity.Employee;
public class CreateEmployee {
public static void main(String []args){
EntityManagerFactory emfactory = Persistence.createEntityManagerFactory("tutorialspoint_JPA_Eclipselink");
EntityManager entitymanager = emfactory.createEntityManager();
entitymanager.getTransaction().begin();
Employee e = new Employee();
e.setId(1201);
e.setName("Gopal");
e.setSalary(40000);
e.setDegree("Technical Manager");
entitymanager.persist(e);
entitymanager.getTransaction().commit();
entitymanager.close();
emfactory.close();
}
}
persistence.xml文件：<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" x xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="tutorialspoint_JPA_Eclipselink" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>com.tutorialspoint.eclipselink.entity.Employee</class>
<properties>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/demo"/>
<property name="javax.persistence.jdbc.user" value="root"/>
<property name="javax.persistence.jdbc.password" value="removed"/>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
</properties>
</persistence-unit>
</persistence>
以及错误：
Exception in thread "main" Local Exception Stack:
Exception [EclipseLink-30005] (Eclipse Persistence Services - 2.6.0.v20150309-bf26070): org.eclipse.persistence.exceptions.PersistenceUnitLoadingException
Exception Description: An exception was thrown while searching for persistence archives with ClassLoader: sun.misc.Launcher$AppClassLoader@1b1f1f12
Internal Exception: javax.persistence.PersistenceException: Exception [EclipseLink-28018] (Eclipse Persistence Services - 2.6.0.v20150309-bf26070): org.eclipse.persistence.exceptions.EntityManagerSetupException
Exception Description: Predeployment of PersistenceUnit [tutorialspoint_JPA_Eclipselink] failed.
Internal Exception: java.lang.NullPointerException
at org.eclipse.persistence.exceptions.PersistenceUnitLoadingException.exceptionSear chingForPersistenceResources(PersistenceUnitLoadingException.java:127)
at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactoryImpl(P ersistenceProvider.java:111)
at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactory(Persi stenceProvider.java:183)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:79)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:54)
at com.tutorialspoint.eclipselink.service.CreateEmployee.main(CreateEmployee.java:1 1)
Caused by: javax.persistence.PersistenceException: Exception [EclipseLink- 28018] (Eclipse Persistence Services - 2.6.0.v20150309-bf26070): org.eclipse.persistence.exceptions.EntityManagerSetupException
Exception Description: Predeployment of PersistenceUnit [tutorialspoint_JPA_Eclipselink] failed.
Internal Exception: java.lang.NullPointerException
at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.createPredeployFaile dPersistenceException(EntityManagerSetupImpl.java:2023)
at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.predeploy(EntityMana gerSetupImpl.java:2014)
at org.eclipse.persistence.internal.jpa.deployment.JPAInitializer.callPredeploy(JPA Initializer.java:100)
at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactoryImpl(P ersistenceProvider.java:100)
... 4 more
Caused by: Exception [EclipseLink-28018] (Eclipse Persistence Services - 2.6.0.v20150309-bf26070): org.eclipse.persistence.exceptions.EntityManagerSetupException
Exception Description: Predeployment of PersistenceUnit [tutorialspoint_JPA_Eclipselink] failed.
Internal Exception: java.lang.NullPointerException
at org.eclipse.persistence.exceptions.EntityManagerSetupException.predeployFailed(E ntityManagerSetupException.java:231)
... 8 more
Caused by: java.lang.NullPointerException
at org.eclipse.persistence.platform.server.NoServerPlatformDetector.checkPlatform(N oServerPlatformDetector.java:28)
at org.eclipse.persistence.platform.server.ServerPlatformUtils.detectServerPlatform (ServerPlatformUtils.java:44)
at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.updateServerPlatform (EntityManagerSetupImpl.java:973)
at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.predeploy(EntityMana gerSetupImpl.java:1718)
... 6 more
最佳答案
你可能放错了EclipseLink jar文件。
NPE（NullPointerException）源于org.eclipse.persistence.platform.server.NoServerPlatformDetector中的第28行（请参阅您发布的错误中的最后一个Caused by）。该行显示在GrepCodehere上，并显示：PrivilegedAccessHelper.getClassLoaderForClass(NoServerPlatformDetector.class).toString();
通过查看PrivilegedAccessHelper.getClassLoaderForClass方法，可以将该行重写为：NoServerPlatformDetector.class.getClassLoader().toString();
现在，getClassLoader可以返回null在这种情况下，toString()将导致NPE。getClassLoader如果EclipseLink jar文件是使用引导类加载器加载的，则返回null（请参见关于引导类加载器的this wiki-page）。这使我相信您将eclipseelinkjar文件放在<JAVA_HOME>/jre/lib目录中，或者用Eclipse术语来说，使它成为“JRE系统库”的一部分。
右键单击Eclipse中的项目，选择Properties，选择“Java Build Path”，打开“Libraries”选项卡，展开“JRE系统库”（或类似的东西），并确保没有显示eclipselinejar文件。使用“Add（external）JARs”按钮添加EclipseLink jar文件（这应该确保使用系统类加载器而不是引导类加载器加载jar文件）。
在侧节点上，导致NPE的NoServerPlatformDetector有缺陷：下一行显示if (loaderStr != null，但此检查是多余的，因为toString()永远不会返回null。换句话说：程序员知道getClassLoader可以返回null，但是忘记了toString()在这种情况下会导致NPE。
关于java - PersistenceUnitLoadingException-创建EntityManagerFactory时出错，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/30143887/