找不到正确保存血细胞关系记录的方法

Question

<form:form commandName="std" method="POST" >             
         <form:input type="text" path="name"/>
         <label>Subjects</label>
         <form:select path="subjects" multiple="multiple">
            <form:option value="English">English</form:option>
            <form:option value="Maths">Maths</form:option>
         </form:select>

     <input type="submit" value="Save">
</form:form>

@InitBinder
public void initSubjectBinder(WebDataBinder binder) {
    binder.registerCustomEditor(Subject.class,
    new PropertyEditorSupport() {
        @Override
        public void setAsText(String text) {
            Subject subject = new Subject();
            subject.setSubjectName(text);
            System.out.println("Saving Subject Object...");
            subjectService.save(subject);               
        }
    });     
}

@RequestMapping(value = "/save-std", method = RequestMethod.POST)
    public String saveStd(@ModelAttribute("std") Student std) {
        try {
            System.out.println("List size is " + std.getSubjects().size()); // Here it says List Size is 4 (if four subjects are selected)
            for (Subject subject : std.getSubjects()) {
                subject.setStudent(std); // Here it gives NPE see stackTrace for more info
                subjectService.update(subject);
            }
        } catch (Exception e) {
            System.out.println("Error is " + e.getMessage());
        }
        return "home";
    }

Saving Subject Object...
Hibernate: 
    insert 
    into
        Subject
        (studentId, subjectName) 
    values
        (?, ?)
Saving Subject Object...
Hibernate: 
    insert 
    into
        Subject
        (studentId, subjectName) 
    values
        (?, ?)
Saving Subject Object...
Hibernate: 
    insert 
    into
        Subject
        (studentId, subjectName) 
    values
        (?, ?)
Saving Subject Object...
Hibernate: 
    insert 
    into
        Subject
        (studentId, subjectName) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        Student
        (studentName) 
    values
        (?)
List size is 4 (Result of System.out.println("List size is " + std.getSubjects().size()); )
Error is null (Result of System.out.println("Error is " + e.getMessage());)

java.lang.NullPointerException位于com.app.web.homeController.saveStd（homeController.java:76)位于sun.reflect.nativeMethodAccessorImpl.Invoke0（原生方法）位于sun.reflect.nativeMethodAccessorImpl.Invoke（未知源）位于sun.reflect.nativeMethodAccessorImpl.Invoke（未知源）位于java.lang.reflect.Method.Invoke（未知源）位于在org.springframework.web.servlet.mvc.Method.annotation.servletInvocableHandlerMethod.InvokeandHandle(servletInvocableHandlerMethod.java:110)在org.springframework.web.servlet.mvc.Method.annotation.requestMappingAdapter.invalleinter(requestMappingHandlerAdapter.java:705)在org.springframework.web.servlet.mvc.method.abstractHandlerMethodAdapter.handle（AbstractHandlerMethodAdapter.java:85)在org.springframework.web.servlet.dispatcherServlet.doDispatch（DispatcherServlet.java:959)在org.springframework.web.servlet.dispatcherServlet.doDispatch（DispatcherServlet.java:893)在在org.springframework.web.servlet.frameworkservlet.service（frameworkservlet.java:842)，在javax.servlet.http.httpservlet.service（httpservlet.java:731)，在org.apache.catalina.core.applicationFilterChain.InternaldoFilter（applicationFilterChain.java:303)，在org.apache.catalina.core.applicationFilterChain.doFilter（applicationFilterChain.java:208)，在in.internaldoFilter(ApplicationFilterChain.java:241)，org.apache.catalina.core.applicationFilterChain.doFilter(ApplicationFilterChain.java:208)，org.apache.catalina.core.standardWrapperValve.invoke(StandardWrapperValve.invoke(StandardWrapperValve.java:220)，org.apache.catalina.core.standardContextValve.invoke(StandardContextValve.inache.catalina.valves.errorreportvalve.invoke(errorreportvalve.java:103)在org.apache.catalina.valves.accesslogvalve.invoke(accesslogvalve.java:957)在org.apache.catalina.core.standardenginevalve.invoke(standardenginevalve.java:116)在org.apache.catalina.connector.coyoteadapter.service(coyoteadapter.java:423)在protocol.javA:620)在org.apache.tomcat.util.net.jioEndpoint$socketprocessor.run(jioEndpoint.java:316)在java.util.concurrent.threadpoolexecutor.runworker（未知源）在java.util.concurrent.threadpoolexecutor$worker.run（未知源）在org.apache.tomcat.util.threads.taskthread$wrappingrunnable.run

现在很难理解NPE在哪里发生，为什么发生，我应该如何给学生分配科目？

Answer 1

感谢@PavelHoral，他在做了一些我想出来的修改后指出了我的错误

@InitBinder
public void initSubjectBinder(WebDataBinder binder) {
    binder.registerCustomEditor(Subject.class,
    new PropertyEditorSupport() {
        @Override
        public void setAsText(String text) {
            Subject subject = new Subject();
            subject.setSubjectName(text);
            setValue(subject); // This is the turning point for me         
        }
    });     
}

在控制器方法中

@RequestMapping(value = "/save-std", method = RequestMethod.POST)
    public String saveStd(@ModelAttribute("std") Student std) {
        try {
            System.out.println("List size is " + std.getSubjects().size()); 
            for (Subject subject : std.getSubjects()) {
                subject.setStudent(std); 
                subjectService.save(subject);
            }
        } catch (Exception e) {
            System.out.println("Error is " + e.getMessage());
        }
        return "home";
    }

它奏效了，我的问题解决了。