Could not open JPA EntityManager for transaction; nested exception is javax.persistence.PersistenceException: org.hibernate.exception.SQLGrammarException: Cannot open connection

Could not open JPA EntityManager for transaction; nested exception is javax.persistence.PersistenceException: org.hibernate.exception.SQLGrammarException: Cannot open connection

检查一下 数据源 配置， 看你的异常应该是 使用 JTA 事物 ，容器没有实例化 EntityManager

 <?xml version="1.0"?>
 <persistence 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" version="1.0">
   <persistence-unit name="jun" transaction-type="RESOURCE_LOCAL">
       <provider>org.hibernate.ejb.HibernatePersistence</provider>
     <properties>
          <property name="hibernate.dialect"
              value="org.hibernate.dialect.MySQLDialect"/><!--数据库方言-->
          <property name="hibernate.connection.driver_class"
              value="com.mysql.jdbc.Driver"/><!--数据库驱动类-->
          <property name="hibernate.connection.username" value=""/><!--数据库用户名-->
          <property name="hibernate.connection.password" value=""/>
          <property name="hibernate.connection.url"
              value="jdbc:mysql://localhost:3306/mytest;"/><!--数据库连接URL-->
          <property name="hibernate.max_fetch_depth" value="3"/><!--外连接抓取树的最大深度 -->
          <property name="hibernate.hbm2ddl.auto" value="update"/><!-- 自动输出schema创建DDL语句 -->
          <property name="hibernate.jdbc.fetch_size" value="18"/><!-- JDBC的获取量大小 -->
          <property name="hibernate.jdbc.batch_size" value="10"/><!-- 开启Hibernate使用JDBC2的批量更新功能  -->
          <property name="hibernate.show_sql" value="true"/><!-- 在控制台输出SQL语句 -->
       </properties>
   </persistence-unit>
 </persistence>