Question

我想在表单中放置一个对象列表：选择并将选定的Discipline值链接到路径：提交时Grade对象的“subject”。

这就是我现在的做法，将grade.discipline.id链接到discipline.id。它正在工作，我在服务器上使用了调试，并看到了成绩对象的规则如何获得规则对象。但是我想知道是否有“出书的方式”，而不仅仅是我的即兴创作。

addGrade.jsp：

<form:form commandName="grade">
    <table>
        <tr>
            <th>Value = </th>
            <td><form:input path="value"/></td>
            <td><form:errors path="value"/></td>
        </tr>
        <tr>
            <th>Select discipline: </th>
            <td colspan = "2">
                <form:select path="discipline.id">
                    <c:forEach items="${disciplines}" var="d">
                        <option value="${d.id}">${d.name}</option>"
                    </c:forEach>
                </form:select>
            </td>
            <td><form:errors path="discipline"/></td>
        </tr>
        <tr>
            <td colspan="2"><input type="submit" value="Submit"/></td>
        </tr>
    </table>
</form:form>

GradeController.java

@RequestMapping(value = "/addGrade", method = RequestMethod.GET)
public String addGradeGet (Model model, HttpSession session){
    //this.populateDefaultModel(session);
    List<Discipline> disciplines = disciplineService.findAll();
    Student student = (Student)session.getAttribute("student");
    if(student==null)
        return "redirect:/showStudent.html";
    model.addAttribute("disciplines",disciplines);
    model.addAttribute("currentStudent",student);
    model.addAttribute("grade",new Grade());
    return "addGrade";
}   
@RequestMapping(value = "/addGrade", method = RequestMethod.POST)
public String addGrade(@Valid @ModelAttribute ("grade") Grade grade,
        BindingResult result, Model model){
    List<Discipline> disciplines = disciplineService.findAll();
    model.addAttribute("disciplines",disciplines);

    /*if we found a grade of the current student
     that has the same discipline as the new grade that we try to add
     then we are not allowed to add it
     */
    Student student = (Student) model.asMap().get("currentStudent");

    grade.setStudent(student);

    if(gradeService.findByStudentAndDiscipline(grade)!=null){
        result.rejectValue("discipline", "duplicate");
    }
    if(!(result.hasErrors()) ){
        gradeService.save(grade);
        return "redirect:/students.html";
    }
    else{
        return "addGrade";
    }
}

对象Discipline：

@Entity
public class Discipline {
@Id
@GeneratedValue
private Long id;
@NotNull
private String name;
@OneToMany(mappedBy="discipline", cascade=CascadeType.ALL,fetch=FetchType.LAZY,orphanRemoval=true)
private List<Grade> grades = new ArrayList<Grade>();
// getters and setters ..... 
}

对象等级：

@Entity
public class Grade {
@Id
@GeneratedValue
private Long id;
@Range(min=1, max=10)
@NotNull
private int value;
@ManyToOne
private Student student;
@ManyToOne
private Discipline discipline;
// getters and setters .... 
}

我知道我应该使用：

<form:select path="discipline">
    <form:options items="${disciplines}"/>
</form:select>

我确实尝试过，将discipHashMap放在纪律属性中：

List<Discipline> disciplines = disciplineService.findAll();
Map<Discipline,String> disciplinesHashMap = new LinkedHashMap<Discipline,String>();
for (Discipline temp:disciplines){
    disciplinesHashMap.put(temp, temp.getName());           
}
model.addAttribute("disciplines",disciplinesHashMap);

我得到了包含所有正确标签的页面但是当我尝试提交时，我在GradeService中的一个方法中获取了一个nullPointer异常，该方法正在搜索我的成绩对象的学科值，在POST方法中调用的方法addGrade（也在服务器上的调试中我看到成绩对象的学科字段为null）...所以，这意味着我没有设法将学科值链接到对象：

java.lang.NullPointerException
com.app.service.GradeServiceImpl.findByStudentAndDiscipline(GradeServiceImpl.java:31)

如何在Spring表单中正确放置List <object>：select form：options？

0 个答案: