尝试使用Eclipselink JPA + Spring 3打印Parent-Child Parent关系时出现Null Pointer异常

Question

I have the following bidirectional relationship : A Course can have several assignments.

I am getting a Null Pointer exception when I try the following:

I have the following methods in CourseController.java

@RequestMapping(value="view/{id}/assignment/create")
public ModelAndView createAssignment(@PathVariable("id") String id) throws Exception {

    ModelAndView modelAndView = new ModelAndView("course/assignment/create");
    Assignment assignment = new Assignment();
    assignment.setCourse(this.courseManager.getCourse(id));
    //System.out.println(assignment.getCourse().getName()); This prints the course name correctly
    modelAndView.addObject("assignment", assignment);
    return modelAndView;
}

This is the jsp:
<form:form action="/cbass/assignment/check" method="POST" commandName="assignment">

<form:errors path="*" cssClass="errorblock" element="div"/>
Course Name: ${assignment.course.name}
<table>
    <tr>
        <td>Name : </td>
        <td><form:input path="name" /></td>
        <td><form:errors path="name" cssClass="error" /></td>
    </tr>

    <tr>
    <td colspan="3"><input type="submit" /></td>
    </tr>
</table>

ANd this is where the request goes from the JSP.

@RequestMapping(value = "check", method = RequestMethod.POST)
public String processSubmit(
    @ModelAttribute("assignment") Assignment assignment,
    BindingResult result, SessionStatus status) {

    assignmentValidator.validate(assignment, result);
    System.out.println(assignment.getCourse().getName());
    if (result.hasErrors()) {
        //if validator failed
        return "course/assignment/create";
    } else {
        status.setComplete();
        //form success
        System.out.println("Hello"+assignment.getCourse().getName());
        this.assignmentManager.create(assignment);
        return "course/assignment/view";//+assignment.getId();
    }
}

Although I am able to see the course name properly in the JSP page,

Course Name: ${assignment.course.name}

I still get a Null Pointer Exception when I try to execute this:

assignment.getCourse().getName()

I am new to Spring and EclipseLink JPA. Please help.

Answer 1

Seems like your course is null. Ensure you are maintaining both sides of the relationship.