mybatis-plus 错误 Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/velocity/context/Context

使用mybatis-plus自动生成文件的时候，报下面的错误：

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/velocity/context/Context
at com.baomidou.mybatisplus.generator.AutoGenerator.execute(AutoGenerator.java:103)
at mybatis.MybatisPlusGenerator.build(MybatisPlusGenerator.java:100)
at mybatis.MybatisPlusGenerator.main(MybatisPlusGenerator.java:30)
Caused by: java.lang.ClassNotFoundException: org.apache.velocity.context.Context
at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 3 more

Process finished with exit code 1

环境：mybatis-plus 3.4.0

原因：是缺少了依赖，解决方案如下：

pom.xml文件当中加入velocity的依赖

<!-- 模板引擎 -->
<dependency>
    <groupId>org.apache.velocity</groupId>
    <artifactId>velocity-engine-core</artifactId>
    <version>2.0</version>
</dependency>