Vaadin EntityManager NullReferenceException答案

【问题标题】：Vaadin EntityManager NullReferenceExceptionVaadin EntityManager NullReferenceException
【发布时间】：2019-06-07 18:27:06
【问题描述】：

我正在尝试使程序在 vaadin 和 spring-boot 中运行，但无法使 EntityManager 运行。获取 NullReferenceException

首先，这是我第一次尝试使用 Spring-boot 和 Vaadin 制作东西。我正在尝试使 EntityManager 工作，但我得到了 NullReferenceException。似乎我的 EntityManager 正在引发异常，但我找不到任何关于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 界面

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 类中实现它

@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();
        }
    }
}

这个表单是在我的视图中创建的

@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() 方法来尝试添加新元素，而不必处理我的 TextField。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！

【问题讨论】：

尝试用“@Component”注解替换“@Repository”注解，看看是否可行
不幸的是，这没有帮助。在 repository.save() 中的 em.persist() 处仍然会抛出相同的 NullReferenceException
快速浏览一下，您的代码中有几个问题。例如，您正在使用 new 实例化 Form_Room 中的存储库，并且在该类中有 @Autowired。您应该重构代码。此外，我看到您更新阅读或将 bean 设置为 Binder，因此您编辑的任何内容都不会被保存，因为您的表单中没有 bean。
补充一下 Tatu 所说的，findAll() 起作用的原因是，在您看来，您不会手动初始化存储库，而是让 Spring 为您注入它，同时注入实体管理器进入存储库。但是，在您的表单中，您手动初始化它，这意味着 Spring 不再管理它，并且无法为您注入您的实体管理器。

标签： java spring-boot vaadin

【解决方案1】：

抱歉我之前看的有点太快了。

但这是你的问题，你自己创建自动装配的类，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
}

在这里，你在你的视野中再次这样做：

@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 autowire 是一种叫做依赖注入的东西。这是一个花哨的词，表示“框架”（在本例中为 spring）将为您完成所有类的实例化。

春天会为你做“新”。并在运行时注入一个类供您使用，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一个 EntityManager，并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，您不必执行新的操作。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一些其他指针。

在 java 中，我们使用驼峰式大小写，因此应该是“RepositoryRoom”而不是“Repository_Room”，“Repository_Room_Impl”应该是“RepositoryRoomImpl”

【讨论】：

感谢您的解释，现在它清楚了自动装配的工作原理，但即使我删除该行并让存储库自动装配，我仍然在同一个地方得到我的异常（repository.update ) 但我找不到为什么，在选择一行时，我调用 setRoom 来设置更新中使用的变量。
将 setRoom 恢复为之前的代码，并尝试仅测试使用我的表单修改现有项目但仍然抛出异常的更新方法。
调试的时候，“Repository_Room”是空的吗？还是 EntityManager 为空，还是 this.room 为空？
存储库为空。 this.room 有价值。所以在尝试合并之前抛出异常。
如果您在调试中运行应用程序，您应该能够在日志中看到它是否设法找到您的存储库并创建它。