我有三个项目，web，ejb3，jpa实体。
ejb3和jpa实体项目将作为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>
和日志：
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
我意外地找到了一个解决办法。
如果我将mysql driver jar放入[jboss eap 6.1 home]\modules\system\layers\base\org\hibernate\main，
更新module.xml以包含驱动程序jar，这是可行的，但我不知道为什么我的web应用程序与hibernate模块有任何关系。
最佳答案
这是因为类加载在jee环境中是如何工作的。你看，有嵌套的类加载器，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应用类加载器可以看到父类加载的类，但其父类不能看到子类加载的类（web应用cl）。
在jee环境中，您拥有系统类加载器（一如既往），然后将有一个应用服务器类加载器（加载api-e.g.javax.servlet.*和特定于应用服务器的实现）。最后，每个ear有一个类加载器，然后每个war有一个类加载器，如下所示：
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)
（这过于简单化了，只是为了演示原则）上面树中的每个孩子看到的是父母加载的类，而不是自己的孩子或同龄人加载的类。
在您的例子中，mysql驱动程序jar放在
WEB-INF/lib中，即由web应用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，但由于上述原因失败。这也是将驱动程序放入modules/时工作正常的原因，因为这是加载hibernate的父cl的类路径。无论如何，我认为解决方案是将驱动程序放入
modules/，如this的“安装jdbc驱动程序”部分所述。如果有充分的理由在应用程序中打包东西，那么也必须打包jpa实现（也就是说，将hibernate jars放入EB-INF/lib中）。