无法找到正确保存一个多人关系记录的方法
[英]Can't find a way to save record of onetomany relation correctly
问题描述
I want to save a Student record with say following form 
我想用以下表格保存学生记录
<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 在控制器中，我有一个活页夹
@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. 上面的initSubjectBinder()保存Subject对象，但没有将Subject分配给Student。 In POST method I want to add Student to those selected subjects 在POST方法中，我想将Student添加到那些选定的主题中
@RequestMapping(value = "/save-std", method = RequestMethod.POST)
public String saveStd(@ModelAttribute("std") Student std) {
try {
System.out.println("List size is " + std.getSubjects().size()); // 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 我在控制台中获得了Hibernate结果
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 并且堆栈跟踪是
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.springf
Now that is hard to understand where and why NPE occurs and how should I assign subjects to Student? 现在，很难理解发生NPE的位置和原因，以及如何为学生分配学科？
1 个解决方案
解决方案1
0 2016-06-09 06:35:06
Thanks to @PavelHoral, He pointed out my mistake after making some changes I came up with 感谢@PavelHoral，他在做出一些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
}
});
}
and in controller method 和控制器方法
@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. 它奏效了，我的问题解决了。
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