set赋值的时候报空指针Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exceptio_dispatcherservlet 空指针

Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed;
nested exception is java.lang.NullPointerException] with root causenested exception is java.lang.NullPointerException] with root cause
在编写返回值时，使用set方法给参数赋值时出现空指针异常
开启debug模式 打印给set赋的值

CourseReturn reCpourseList = null;
List<Course> clist = courseServiceImpl.getCourseList( search, courseStartTime, courseEndTime, type, pageIndex);
int size = clist.size();
System.out.println(clist);
System.out.println(size );
reCpourseList.setPageSize(size);
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发现list 和size都有值
这个真的搞得头大的 ，尝试了很多方法 ，结果发现是
因为实体类序列化后，需要创建一个对象，而不是等于null，应该是CourseReturn reCpourseList = new CourseReturn();

CourseReturn reCpourseList = new CourseReturn();
List<Course> clist = courseServiceImpl.getCourseList( search, courseStartTime, courseEndTime, type, pageIndex);
int size = clist.size();
reCpourseList.setPageSize(size);
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这样就没问题了