Can't find a way to save record of onetomany relation correctly

Question

I want to save a Student record with say following form

<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>

In controller I have a binder as

@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);               
        }
    });     
}

the above initSubjectBinder() saves Subject object, but subject is not assigned to Student. In POST method I want to add Student to those selected subjects

@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";
    }

I get Hibernate results in console as

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());)

and the stack-trace is

java.lang.NullPointerException at com.app.web.HomeController.saveStd(HomeController.java:76) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at java.lang.reflect.Method.invoke(Unknown Source) at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221) at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137) at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:110) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:776) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:705) at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966) at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:868) at javax.servlet.http.HttpServlet.service(HttpServlet.java:650) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842) at javax.servlet.http.HttpServlet.service(HttpServlet.java:731) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103) at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:957) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423) at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079) at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:620) at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316) at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Unknown Source)

Now that is hard to understand where and why NPE occurs and how should I assign subjects to Student?

Answer 1

Thanks to @PavelHoral, He pointed out my mistake after making some changes I came up with

@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         
        }
    });     
}

and in controller method

@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";
    }

and it worked and my issue was resolved.