注意设置好方言： arp.setDialect(new OracleDialect());

不设计方言，默认是 mysql 方言，所以在 oracle 数据库下会报异常

@JFinal 请问设置方言在哪个类里面？

@JFinal 通过手册发现了修改的方法，arp.addMapping("user", "user_id", User.class);，这个中的addMapping提示错误，User.class也提示错误，User是当前类的名字么？

@JFinal http://localhost/blog跳转这个地址变为500错误，
日志如下：
Starting JFinal 3.2
Starting scanner at interval of 5 seconds.
Starting web server on port: 80
Starting Complete. Welcome To The JFinal World :)

JFinal action report -------- 2017-09-16 13:13:32 ------------------------------
Url : GET /
Controller : com.demo.index.IndexController.(IndexController.java:1)
Method : index
--------------------------------------------------------------------------------
Before invoking /blog

2017-09-16 13:13:36
[ERROR]-[Thread: qtp355235176-22]-[com.jfinal.core.ActionHandler.handle()]: /blog
java.lang.NullPointerException
at com.jfinal.plugin.activerecord.Model.doPaginate(Model.java:339)
at com.jfinal.plugin.activerecord.Model.paginate(Model.java:313)
at com.demo.blog.BlogService.paginate(BlogService.java:22)
at com.demo.blog.BlogController.index(BlogController.java:20)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.jfinal.aop.Invocation.invoke(Invocation.java:73)
at com.demo.blog.BlogInterceptor.intercept(BlogInterceptor.java:17)
at com.jfinal.aop.Invocation.invoke(Invocation.java:67)

NullPointerException 需要单步调试跟踪看是哪个对象为 null，分分钟解决