我一直在学习一些关于谷歌应用引擎教程的 jpa。在所有教程中，都没有在 persistence.xml 中指定实体类。但是我的本地环境抛出了 NoPersistenceInformationException。“de.vogella.gae.java.todo.model.Todo”类必须是可持久的，但找不到此类的元数据/注释。请检查元数据/注释是否定义在有效的文件位置。
如果我在 persistence.xml 中指定实体类，问题就会消失。
<?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="transactions-optional">
<provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>
<class>de.vogella.gae.java.todo.model.Todo/class>
<properties>
<property name="datanucleus.NontransactionalRead" value="true"/>
<property name="datanucleus.NontransactionalWrite" value="true"/>
<property name="datanucleus.ConnectionURL" value="appengine"/>
<property name="datanucleus.singletonEMFForName" value="true"/>
<property name="datanucleus.appengine.datastoreEnableXGTransactions" value="true"/>
</properties>
有人可以向我解释为什么我需要这样做吗？GAE 教程中的 JPA 都不需要这样做。一个区别是我使用 google appengine maven 插件而不是 eclipse 插件。
这是我的pom文件
http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<packaging>war</packaging>
<version>1.0-SNAPSHOT</version>
<groupId>de.vogella.gae.java.todo</groupId>
<artifactId>todo</artifactId>
<pluginRepositories>
<pluginRepository>
<id>google-snapshots</id>
<name>Google Snapshots</name>
<url>https://oss.sonatype.org/content/repositories/google-snapshots/</url>
<snapshots>
<enabled>true</enabled>
</snapshots>
<releases>
<enabled>false</enabled>
</releases>
</pluginRepository>
</pluginRepositories>
<properties>
<appengine.app.version>1</appengine.app.version>
<appengine.target.version>1.7.4</appengine.target.version>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
<!-- Compile/runtime dependencies -->
<dependency>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-api-1.0-sdk</artifactId>
<version>${appengine.target.version}</version>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>2.5</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>jstl</groupId>
<artifactId>jstl</artifactId>
<version>1.2</version>
</dependency>
<!-- JPA dependencies -->
<dependency>
<groupId>com.google.appengine.orm</groupId>
<artifactId>datanucleus-appengine</artifactId>
<version>2.0.1.1</version>
</dependency>
<dependency>
<groupId>javax.jdo</groupId>
<artifactId>jdo-api</artifactId>
<version>[3.0, 4.0)</version>
</dependency>
<dependency>
<groupId>org.apache.geronimo.specs</groupId>
<artifactId>geronimo-jpa_2.0_spec</artifactId>
<version>1.0</version>
</dependency>
<dependency>
<groupId>org.datanucleus</groupId>
<artifactId>datanucleus-core</artifactId>
<version>[3.0, 3.0.99)</version>
</dependency>
<dependency>
<groupId>org.datanucleus</groupId>
<artifactId>datanucleus-api-jpa</artifactId>
<version>[3.0, 3.0.99)</version>
</dependency>
<dependency>
<groupId>org.datanucleus</groupId>
<artifactId>datanucleus-jpa-query</artifactId>
<version>3.0.2</version>
</dependency>
<!-- Test Dependencies -->
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.10</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-all</artifactId>
<version>1.9.0</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-testing</artifactId>
<version>${appengine.target.version}</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-api-stubs</artifactId>
<version>${appengine.target.version}</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.jboss.resteasy</groupId>
<artifactId>resteasy-jaxrs</artifactId>
<version>2.3.5.Final</version>
</dependency>
<dependency>
<groupId>org.jboss.resteasy</groupId>
<artifactId>resteasy-jackson-provider</artifactId>
<version>2.3.5.Final</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<version>2.5.1</version>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.6</source>
<target>1.6</target>
<!-- compilerArguments>
<processor>org.datanucleus.jpa.query.JPACriteriaProcessor</processor>
</compilerArguments -->
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-war-plugin</artifactId>
<version>2.3</version>
<configuration>
<archiveClasses>true</archiveClasses>
<webResources>
<!-- in order to interpolate version from pom into appengine-web.xml -->
<resource>
<directory>${basedir}/src/main/webapp/WEB-INF</directory>
<filtering>true</filtering>
<targetPath>WEB-INF</targetPath>
</resource>
</webResources>
</configuration>
</plugin>
<plugin>
<groupId>com.google.appengine</groupId>
<artifactId>appengine-maven-plugin</artifactId>
<version>1.7.5-SNAPSHOT</version>
<configuration>
<jvmFlags>
<jvmFlag>-Xdebug</jvmFlag>
<jvmFlag>-Xrunjdwp:transport=dt_socket,address=1044,server=y,suspend=n</jvmFlag>
<jvmFlag>-Ddatastore.default_high_rep_job_policy_unapplied_job_pct=20</jvmFlag>
</jvmFlags>
</configuration>
</plugin>
<plugin>
<groupId>org.datanucleus</groupId>
<artifactId>maven-datanucleus-plugin</artifactId>
<version>3.0.2</version>
<configuration>
<api>JPA</api>
<persistenceUnitName>transactions-optional</persistenceUnitName>
<!-- log4jConfiguration>${basedir}/log4j.properties</log4jConfiguration -->
<verbose>true</verbose>
</configuration>
<executions>
<execution>
<phase>process-classes</phase>
<goals>
<goal>enhance</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
这是我正在学习的教程http://www.vogella.com/articles/GoogleAppEngineJava/article.html