Question

我有三个项目，Web，EJB3，JPA实体。 EJB3和JPA-entities项目将作为jar文件导出到Web / WebContent / WEB-INF / lib中。我还将MySQL驱动程序jar文件放在“WEB-INF / lib”中。

但是...

jboss找不到MySQL驱动程序... 以下是persistence.xml：

<persistence-unit name="HandShakeEntity">
        <class>net.magidea.handshake.entity.User</class>
        <class>net.magidea.handshake.entity.UserItem</class>
        <properties>
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect" />
            <property name="hibernate.connection.username" value="root" />
            <property name="hibernate.connection.password" value="dj/4ej03" />
            <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/handshake" />
            <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver" />
        </properties>
    </persistence-unit>

并记录：

13:09:26,070 ERROR [org.jboss.msc.service.fail] (ServerService Thread Pool -- 48) MSC000001: Failed to start service jboss.persistenceunit."HandShakeWeb.war#HandShakeEntity": org.jboss.msc.service.StartException in service jboss.persistenceunit."HandShakeWeb.war#HandShakeEntity": javax.persistence.PersistenceException: [PersistenceUnit: HandShakeEntity] Unable to build EntityManagerFactory
    at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1.run(PersistenceUnitServiceImpl.java:103) [jboss-as-jpa-7.2.0.Final-redhat-8.jar:7.2.0.Final-redhat-8]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110) [rt.jar:1.7.0_07]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603) [rt.jar:1.7.0_07]
    at java.lang.Thread.run(Thread.java:722) [rt.jar:1.7.0_07]
    at org.jboss.threads.JBossThread.run(JBossThread.java:122) [jboss-threads-2.1.0.Final-redhat-1.jar:2.1.0.Final-redhat-1]
Caused by: javax.persistence.PersistenceException: [PersistenceUnit: HandShakeEntity] Unable to build EntityManagerFactory
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:930)
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:904)
    at org.hibernate.ejb.HibernatePersistence.createContainerEntityManagerFactory(HibernatePersistence.java:92)
    at org.jboss.as.jpa.service.PersistenceUnitServiceImpl.createContainerEntityManagerFactory(PersistenceUnitServiceImpl.java:200) [jboss-as-jpa-7.2.0.Final-redhat-8.jar:7.2.0.Final-redhat-8]
    at org.jboss.as.jpa.service.PersistenceUnitServiceImpl.access$600(PersistenceUnitServiceImpl.java:57) [jboss-as-jpa-7.2.0.Final-redhat-8.jar:7.2.0.Final-redhat-8]
    at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1.run(PersistenceUnitServiceImpl.java:99) [jboss-as-jpa-7.2.0.Final-redhat-8.jar:7.2.0.Final-redhat-8]
    ... 4 more
Caused by: org.hibernate.service.classloading.spi.ClassLoadingException: Specified JDBC Driver com.mysql.jdbc.Driver could not be loaded
    at org.hibernate.service.jdbc.connections.internal.DriverManagerConnectionProviderImpl.configure(DriverManagerConnectionProviderImpl.java:111)
    at org.hibernate.service.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:75)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:159)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:131)
    at org.hibernate.engine.jdbc.internal.JdbcServicesImpl.buildJdbcConnectionAccess(JdbcServicesImpl.java:223)
    at org.hibernate.engine.jdbc.internal.JdbcServicesImpl.configure(JdbcServicesImpl.java:89)
    at org.hibernate.service.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:75)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:159)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:131)
    at org.hibernate.cfg.SettingsFactory.buildSettings(SettingsFactory.java:78)
    at org.hibernate.cfg.Configuration.buildSettingsInternal(Configuration.java:2283)
    at org.hibernate.cfg.Configuration.buildSettings(Configuration.java:2279)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1748)
    at org.hibernate.ejb.EntityManagerFactoryImpl.<init>(EntityManagerFactoryImpl.java:94)
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:920)
    ... 9 more
Caused by: org.hibernate.service.classloading.spi.ClassLoadingException: Unable to load class [com.mysql.jdbc.Driver]
    at org.hibernate.service.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:149)
    at org.hibernate.service.jdbc.connections.internal.DriverManagerConnectionProviderImpl.configure(DriverManagerConnectionProviderImpl.java:106)
    ... 23 more
Caused by: java.lang.ClassNotFoundException: Could not load requested class : com.mysql.jdbc.Driver
    at org.hibernate.service.classloading.internal.ClassLoaderServiceImpl$AggregatedClassLoader.findClass(ClassLoaderServiceImpl.java:296)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:423) [rt.jar:1.7.0_07]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:356) [rt.jar:1.7.0_07]
    at java.lang.Class.forName0(Native Method) [rt.jar:1.7.0_07]
    at java.lang.Class.forName(Class.java:264) [rt.jar:1.7.0_07]
    at org.hibernate.service.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:146)
    ... 24 more

我找到了一个意外的解决方案。如果我将MySQL驱动程序jar放入[JBoss EAP 6.1 Home] \ modules \ system \ layers \ base \ org \ hibernate \ main，并更新module.xml以包含驱动程序jar，它工作正常，但我不知道为什么我的Web应用程序与hibernate模块有任何关系。

Answer 1

这是因为类加载在JEE环境中的工作方式。您会看到，存在嵌套的类加载器，Web应用程序类加载器位于层次结构的末尾。这意味着虽然web-app类加载器可以看到由其父级加载的类，但它的父级无法看到子级加载的类（web-app CL）。

在JEE环境中，您拥有系统类加载器（一如既往），然后会有一个应用程序服务器类加载器（加载API - 例如javax.servlet.*和特定于应用程序服务器的实现）。最后，每个EAR都有一个类加载器，然后是每个WAR的类加载器，如：

system CL
|
+- Application server CL (loads e.g. Hibernate as the JPA implementation)
   |
   +- EAR 1 CL
   |  |
   |  +- WAR 1_1 CL
   |  |
   |  +- WAR 1_2 CL
   |
   +- WAR 1 CL (In your case, this loads the MySQL driver)

（这是过于简单的，只是为了演示原则）上面树中的每个孩子都看到由其父母加载的类，但 NOT 类由其自己的孩子或同伴加载。

在你的情况下，MySQL驱动程序JAR放在WEB-INF/lib中，即由web-app CL加载，但JPA实现（Hibernate）由父CL加载。此CL尝试查找驱动程序的类，并因上述原因而失败。这也是将驱动程序放在modules/时工作的原因，因为这是父CL的类路径，即加载Hibernate的类。

在任何情况下，我认为解决方案是将驱动程序放在modules/中，如this的“安装JDBC驱动程序”部分所述。如果您有充分的理由在应用程序中捆绑东西，那么您也必须捆绑JPA实现（即将Hibernate JAR放在EB-INF/lib中）。

为什么JBoss在Hibernate + JPA下找不到MySQL驱动程序

1 个答案: