java - Vaadin EntityManager NullReferenceException

Question

我试图让程序在 vaadin 和 spring-boot 中运行，但无法让 EntityManager 运行。获取 NullReferenceException

对于初学者来说，这是我第一次尝试使用 Spring-boot 和 Vaadin 制作一些东西。我正在尝试使 EntityManager 正常工作，但出现 NullReferenceException。似乎我的 EntityManager 抛出了异常，但我找不到任何有关如何正确使用它的示例。我考虑过使用 EntityManagerFactory，但同样，任何地方都没有示例。findAll() 方法有效，所以我不确定该怎么做。

我有一个 Repository_Room 接口(interface)

public interface Repository_Room
{
    void save(Room room);

    void delete(Room room);

    void update(Room room);

    List<Room> findAll();

    List<Course> getCourses(Room room);
}

在 Repository_Room_Impl 类中实现

@Transactional
@Repository
public class Repository_Room_Impl implements Repository_Room{

    @PersistenceContext
    private EntityManager em;

    public Repository_Room_Impl()
    {

    }

    @Override
    public List<Room> findAll() {
        return em.createQuery("select n from " + Room.class.getSimpleName() + " n").getResultList();
    }

    @Override
    public void save(Room room) {
        em.persist(room);
    }

    public void delete(Room room) {
        em.remove(em.find(Room.class, room.getId()));
    }

    public void update(Room room) {
        em.merge(em);
    }

    public List<Course> getCourses(Room room)
    {
        List<Course> courses = em.createQuery("select n from " + Course.class.getSimpleName() + " n WHERE n.Room = :roomID", Course.class)
                .setParameter("roomID", room.getId())
                .getResultList();
        return courses;
    }
}

尝试在我的表单中使用存储库来添加新元素

@SpringComponent
@UIScope
public class Form_Room extends VerticalLayout {

    private Room room;

    private Reloader reloader;

    private TextField val_id;
    private TextField val_name;
    private TextField val_level;

    private Binder<Room> binder;

    @Autowired
    private Repository_Room repository;

    public Form_Room()
    {
        binder = new Binder<>(Room.class);

        val_id = new TextField("ID:");
        val_name = new TextField("Name:");
        val_level = new TextField(("Level:"));

        binder.forField(val_id)
                .withConverter(
                        new StringToIntegerConverter("Must enter a number"))
                .bind(Room::getId, Room::setId);

        binder.forField(val_name).bind(Room::getName, Room::setName);

        binder.forField(val_level)
                .withConverter(
                        new StringToIntegerConverter("Must enter a number"))
                .bind(Room::getLevel, Room::setLevel);

        add(val_id);
        add(val_name);
        add(val_level);
    }

    public Room getRoom() {
        return room;
    }

    public void setRoom(Room room) {
        if (room == null) {
            this.room = null;
        }
        else {
            this.room = room;
            binder.readBean(this.room);
        }
    }

    public Reloader getReloader() {
        return reloader;
    }

    public void setReloader(Reloader reloader) {
        this.reloader = reloader;
    }

    public void updateRoom()
    {
        try
        {
            if(room == null)
            {
                repository.save(this.room);
                System.out.println("Created");
            }
            else
            {
                binder.writeBean(this.room);
                repository.update(this.room);
                System.out.println("Updated");
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

这个表单是在我的 View 中创建的

@Route(value = "rooms")
public class View_CRUD_Room extends VerticalLayout implements Reloader{

    @Autowired
    private Repository_Room repository;

    @Autowired
    private Form_Room form;

    private Grid<Room> grid;
    private Button button_view;
    private Button button_remove;
    private Button button_execute;
    private HorizontalLayout buttonLayout;

    private Room selected;

    @PostConstruct
    private void init(){
        form = new Form_Room();
        grid = new Grid<>();
        button_view = new Button();
        button_remove = new Button();
        buttonLayout = new HorizontalLayout();
        selected = null;

        button_execute = new Button();
        button_execute.setText("Végrehajt");

        button_view.setText("Részletek");
        button_remove.setText("Töröl");
        buttonLayout.add(button_view);
        buttonLayout.add(button_remove);

        form.setVisible(true);

        button_execute.addClickListener(event -> {
            form.updateRoom();
        });

        button_view.addClickListener(event -> {
            // Display details
        });

        button_remove.addClickListener(event -> {
            // Display form with confirmation message and YesNo buttons
        });

        List<Room> list = repository.findAll();
        grid.setItems(repository.findAll());
        grid.addColumn(Room::getId).setHeader("Id");
        grid.addColumn(Room::getName).setHeader("Name");
        grid.addColumn(Room::getLevel).setHeader("Terem");
        grid.asSingleSelect().addValueChangeListener(selectionEvent -> {
            if (selectionEvent.getValue() != null) {
                selected = selectionEvent.getValue();
                form.setRoom(selected);
            } else {
                selected = null;
                form.setRoom(selected);
            }
        });
        add(new Text("Termek"));
        add(grid);
        add(buttonLayout);
        add(form);
        add(button_execute);
    }

    @Override
    public void processRefresh() {
        grid.setItems(repository.findAll());
    }
}

stackTrace 打印以下内容:

java.lang.NullPointerException
    at com.example.beadando.repository.Impl.Repository_Room_Impl.save(Repository_Room_Impl.java:26)
    at com.example.beadando.component.Form_Room.updateRoom(Form_Room.java:93)
    at com.example.beadando.view.View_CRUD_Room.lambda$init$9b1b5227$1(View_CRUD_Room.java:55)
    at com.vaadin.flow.component.ComponentEventBus.fireEventForListener(ComponentEventBus.java:205)
    at com.vaadin.flow.component.ComponentEventBus.handleDomEvent(ComponentEventBus.java:373)
    at com.vaadin.flow.component.ComponentEventBus.lambda$addDomTrigger$dd1b7957$1(ComponentEventBus.java:264)
    at com.vaadin.flow.internal.nodefeature.ElementListenerMap.lambda$fireEvent$2(ElementListenerMap.java:420)
    at java.util.ArrayList.forEach(ArrayList.java:1249)
    at com.vaadin.flow.internal.nodefeature.ElementListenerMap.fireEvent(ElementListenerMap.java:420)
    at com.vaadin.flow.server.communication.rpc.EventRpcHandler.handleNode(EventRpcHandler.java:59)
    at com.vaadin.flow.server.communication.rpc.AbstractRpcInvocationHandler.handle(AbstractRpcInvocationHandler.java:64)
    at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocationData(ServerRpcHandler.java:387)
    at com.vaadin.flow.server.communication.ServerRpcHandler.lambda$handleInvocations$1(ServerRpcHandler.java:368)
    at java.util.ArrayList.forEach(ArrayList.java:1249)
    at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:368)
    at com.vaadin.flow.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:310)
    at com.vaadin.flow.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:89)
    at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:40)
    at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1507)
    at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:242)
    at com.vaadin.flow.spring.SpringServlet.service(SpringServlet.java:81)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:712)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:459)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:352)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:312)
    at org.springframework.web.servlet.mvc.ServletForwardingController.handleRequestInternal(ServletForwardingController.java:141)
    at org.springframework.web.servlet.mvc.AbstractController.handleRequest(AbstractController.java:177)
    at org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter.handle(SimpleControllerHandlerAdapter.java:52)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1039)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005)
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:908)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:200)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:836)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1747)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

我修改了 setRoom() 方法以尝试添加新元素而无需处理我的 TextFields。尝试调用我的 updateRoom() 方法时在表单中获取 NullReferenceException。请指出我正确的方向..谢谢!

Answer 1

抱歉之前看的有点太快了

但这是你的问题，你正在自己创建 Autowiring 类，而不是让 Spring 为你创建它们。

@Autowired
private Repository_Room repository;

public Form_Room() {
    repository = new Repository_Room_Impl(); //remove this line

    //rest of the code
}

然后在您的 View 中再次执行此操作:

@Route(value = "rooms")
public class View_CRUD_Room extends VerticalLayout implements Reloader {

@PostConstruct
private void init(){
    form = new Form_Room(); //remove this line

    //rest of the code
}

你看到 Autowiring 是一种叫做依赖注入(inject)的东西。这是一个奇特的词，表示“框架”(在本例中为 spring)将为您完成所有类的实例化。

Spring会为你做"new"的。并在运行时注入(inject)一个类供您使用，因此您不必这样做。 Spring 还会在它执行“new RepositoryImpl()”之前检查该类具有哪些依赖项并查看它是否需要 EntityManager，然后自动为您实例化一个并将其放入 RepositoryImpl。

这就是使用 Autowire 的全部意义所在，您不必进行新的操作。你只需要相信当你使用它时那里会有类。

然后是关于您的代码的其他一些提示。

在 Java 中，我们使用驼峰式大小写，所以“Repository_Room”应该是“RepositoryRoom”，“Repository_Room_Impl”应该是“RepositoryRoomImpl”