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什么是nullpointerexception，如何修复它(12个答案）
三个月前关门了。
我正在开发一个带有mysql数据库的spring-boot javafx桌面应用程序。我正在尝试使用学生服务创建一个新的学生，但是我得到了一个错误 StudentService 似乎为空。我不知道为什么。
学生服务

@Service
public class StudentService {

    @Autowired
    private StudentRepository repo;

    public List<Student> listAll() {

        return repo.findAll();
    }

    public void save(Student student) {

        repo.save(student);
    }

    public Student get(Integer id) {

        return repo.findById(id).get();
    }

    public void delete(Integer id) {

        repo.deleteById(id);
    }
}

控制器

@Autowired
private StudentService studentService;

public void next1() throws Exception {

    String c1=nom.getText();
    String c2=prenom.getText();
    String c3=email.getText();
    String c4=tel.getText();
    String c5 = num.getText();
    String c6 = mdp.getText();
    String c7 = mdp1.getText();

    if (c5.equals("") || c6.equals("") || c7.equals("")) {
        error.setVisible(true);
        new SlideInDown(error).play();
    } else {
        Student newStudent = new Student();
        newStudent.setLastName(c1);
        newStudent.setName(c2);
        newStudent.setEmail(c3);
        newStudent.setPhoneNumber(Integer.valueOf(c4));
        newStudent.setApogee(Integer.valueOf(c5));

        if (mdp.equals(mdp1)) {
            newStudent.setPassword(c6);
            studentService.save(newStudent);
        }
    }
}

... 这就是我得到的错误：

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
        at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1787)
        at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1670)
        at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
        at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
        at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
        at javafx.event.Event.fireEvent(Event.java:198)
        at javafx.scene.Node.fireEvent(Node.java:8879)
        at javafx.scene.control.Button.fire(Button.java:200)
        at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:206)
        at com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
        at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
        at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
        at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
        at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
        at javafx.event.Event.fireEvent(Event.java:198)
        at javafx.scene.Scene$MouseHandler.process(Scene.java:3851)
        at javafx.scene.Scene$MouseHandler.access$1200(Scene.java:3579)
        at javafx.scene.Scene.processMouseEvent(Scene.java:1849)
        at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2588)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:397)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
        at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:434)
        at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:390)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:433)
        at com.sun.glass.ui.View.handleMouseEvent(View.java:556)
        at com.sun.glass.ui.View.notifyMouse(View.java:942)
        at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
        at java.base/java.lang.Thread.run(Thread.java:832)
    Caused by: java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:564)
        at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
        at jdk.internal.reflect.GeneratedMethodAccessor51.invoke(Unknown Source)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:564)
        at com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:273)
        at com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
        at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1784)
        ... 47 more
    Caused by: java.lang.NullPointerException
        at com.gi.controllers.StudentDetailsController.next1(StudentDetailsController.java:62)
        ... 58 more

    Process finished with exit code -1