Question

我想保存学生记录，并注明以下表格

<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>

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);               
        }
    });     
}

以上initSubjectBinder()保存了Subject对象，但主题未分配给Student。在POST方法中，我想将Student添加到选定的主题

@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";
    }

我在控制台中获得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());)

并且堆栈跟踪是

显示java.lang.NullPointerException 在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）在org.springframework.web.method.support.InvocableHandlerMethod.doInvoke（InvocableHandlerMethod.java:221）在org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest（InvocableHandlerMethod.java:137）在org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle（ServletInvocableHandlerMethod.java:110）在org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod（RequestMappingHandlerAdapter.java:776）在org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal（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.doService（DispatcherServlet.java:893）在org.springframework.web.servlet.FrameworkServlet.processRequest（FrameworkServlet.java:966）在org.springframework.web.servlet.FrameworkServlet.doPost（FrameworkServlet.java:868）在javax.servlet.http.HttpServlet.service（HttpServlet.java:650）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org.apache.tomcat.websocket.server.WsFilter.doFilter（WsFilter.java:52）在org.apache.catalina.core.ApplicationFilterChain.internalDoFilter（ApplicationFilterChain.java:241）在org.apache.catalina.core.ApplicationFilterChain.doFilter（ApplicationFilterChain.java:208）在org.apache.catalina.core.StandardWrapperValve.invoke（StandardWrapperValve.java:220）在org.apache.catalina.core.StandardContextValve.invoke（StandardContextValve.java:122）在org.apache.catalina.authenticator.AuthenticatorBase.invoke（AuthenticatorBase.java:505）在org.apache.catalina.core.StandardHostValve.invoke（StandardHostValve.java:170）在org.apache.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）在java.lang.Thread.run（未知来源）

现在很难理解NPE发生的地点和原因以及我应该如何为学生分配科目？

Answer 1

感谢@PavelHoral，他在做出一些改变之后指出了我的错误

@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         
        }
    });     
}

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";
    }

它起作用，我的问题得到了解决。

找不到正确保存onetomany关系记录的方法

1 个答案: