我正在从事以下项目： Web动态项目+ JPA 2.1 / EclipseLink 2.5.2 Tomcat 8.0.28 / JRE 1.8.0_66 Eclipse Luna
当在一个简单的JPA项目中作为Java应用程序执行时，项目的JPA部分运行正常。 我对代码的这一部分没有做任何改动。
我想在JSP中打印一些请求结果。我生成了一个servlet并尝试打印已在数据库中创建的用户的名称。几个小时我没有找到关于JDBC驱动程序和persistence.xml的错误。
为了解决依赖性问题，我添加了Maven。
现在，我的用户打印在JSP中，这很好，但是我的＆＃34; Run.java＆＃34;我之前使用过的课程已经不再适用了。
当我将应用程序作为一个＆＃34; Java应用程序＆＃34;运行我的错误时，我很安静。 ：
Exception in thread "main" java.lang.ExceptionInInitializerError
at org.eclipse.persistence.internal.jpa.EntityManagerFactoryProvider.<clinit>(EntityManagerFactoryProvider.java:55)
at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactoryImpl(PersistenceProvider.java:92)
at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactory(PersistenceProvider.java:188)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:79)
at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:54)
at dao.LayerDAO.<init>(LayerDAO.java:16)
at exec.Run.main(Run.java:18)
Caused by: java.lang.NullPointerException
at org.eclipse.persistence.platform.server.NoServerPlatformDetector.checkPlatform(NoServerPlatformDetector.java:28)
at org.eclipse.persistence.platform.server.ServerPlatformUtils.detectServerPlatform(ServerPlatformUtils.java:58)
at org.eclipse.persistence.internal.jpa.IsolatedHashMap.<clinit>(IsolatedHashMap.java:48)
... 7 more
显然，当我在DAO类中创建EntityManagerFactory时（与使用JSP相同的那个），会发生错误。
这是我的POM：
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>CitizenWeb</groupId>
<artifactId>com.citizenweb</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>war</packaging>
<build>
<sourceDirectory>src</sourceDirectory>
<resources>
<resource>
<directory>src</directory>
<excludes>
<exclude>**/*.java</exclude>
</excludes>
</resource>
</resources>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.3</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
<plugin>
<artifactId>maven-war-plugin</artifactId>
<version>2.6</version>
<configuration>
<warSourceDirectory>WebContent</warSourceDirectory>
<failOnMissingWebXml>false</failOnMissingWebXml>
</configuration>
</plugin>
</plugins>
</build>
<properties>
<!-- Generic properties -->
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
<!-- Web -->
<jsp.version>2.2</jsp.version>
<jstl.version>1.2</jstl.version>
<servlet.version>2.5</servlet.version>
<!-- Logging -->
<logback.version>1.0.13</logback.version>
<slf4j.version>1.7.5</slf4j.version>
<!-- Test -->
<junit.version>4.11</junit.version>
</properties>
<dependencies>
<!-- Other Web dependencies -->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>jstl</artifactId>
<version>${jstl.version}</version>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>${servlet.version}</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>javax.servlet.jsp</groupId>
<artifactId>jsp-api</artifactId>
<version>${jsp.version}</version>
<scope>provided</scope>
</dependency>
<!-- Logging with SLF4J & LogBack -->
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>${slf4j.version}</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>${logback.version}</version>
<scope>runtime</scope>
</dependency>
<!-- EclipseLink -->
<dependency>
<groupId>org.eclipse.persistence</groupId>
<artifactId>org.eclipse.persistence.core</artifactId>
<version>2.5.2</version>
</dependency>
<dependency>
<groupId>org.eclipse.persistence</groupId>
<artifactId>org.eclipse.persistence.jpa</artifactId>
<version>2.5.2</version>
</dependency>
<!-- MySQL DB -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.37</version>
</dependency>
</dependencies>
</project>
如果需要，这是我的persistence.xml：
<?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" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="CitizenWeb" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<mapping-file>META-INF/orm.xml</mapping-file>
<properties>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/citizen"/>
<property name="javax.persistence.jdbc.user" value="user"/>
<property name="javax.persistence.jdbc.password" value="user"/>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
</properties>
</persistence-unit>
</persistence>
使用orm.xml
管理实体映射这里也是DAO课程的开始：
package dao;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.Persistence;
import javax.persistence.Query;
import entities.Address;
import entities.GroupAssoc;
import entities.RightsGranted;
import entities.User;
public class LayerDAO implements IFLayerDAOLocal {
EntityManager em = Persistence.createEntityManagerFactory("CitizenWeb").createEntityManager();
public void createUser(User user) {
System.out.println("DAO > addUser > "+em.hashCode());
em.getTransaction().begin();
em.persist(user);
em.getTransaction().commit();
}
此处没有任何更改，也没有在persistence.xml中更改
＆＃34; only＆＃34;由于项目性质的变化（JPA - ＆gt; Web Dynamic + JPA），build.path中的更改。 Maven解决了webserver和mysql连接器的问题，但现在应用程序端已经坏了。
你看到我不喜欢的东西吗？
感谢名单。
答案 0 :(得分：0)
在项目的构建路径中，我添加了一个备用＆＃34; JRE系统库＆＃34;。 我以前有：JDK 1.8.0_66 我补充说：JRE 1.8.0_66
现在JSP工作，也是java Run类。