Java 尝试实例化Datepicker（com.vaadin.flow.component.Datepicker.Datepicker）时出现Nullpointerexception

这有点奇怪，但当我尝试实例化DatePicker组件时，我直接得到了Nullpointerexception：

DatePicker date = new DatePicker("Select date", LocalDate.now());

就像那样，我输入一行，我已经在构造函数中尝试了不同的参数，并且默认构造函数中没有任何参数，我的应用程序由于空指针而无法启动

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'lectureForm' defined in file [D:\IDEAProjects\university\target\classes\com\university\ui\views\forms\LectureForm.class]: Bean instantiation via constructor failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.university.ui.views.forms.LectureForm]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:313)
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:294)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1358)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:226)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:893)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551)
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:143)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:758)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:750)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1237)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226)
    at com.university.Main.main(Main.java:13)
    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:566)
    at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.university.ui.views.forms.LectureForm]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:217)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:117)
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:309)
    ... 25 common frames omitted
Caused by: java.lang.NullPointerException: null
    at com.vaadin.flow.component.datepicker.DatePicker.<init>(DatePicker.java:85)
    at com.university.ui.views.forms.LectureForm.configureFormComponents(LectureForm.java:126)
    at com.university.ui.views.forms.LectureForm.<init>(LectureForm.java:68)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:204)
    ... 27 common frames omitted

在

所以我不知道为什么会发生这种情况，为什么不管参数如何，它总是同一个构造函数，为什么标签有问题，最重要的是，如何修复它-：

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非常感谢您提前

问题的根源是我用来运行应用程序的出乎意料的Spring Boot。

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在IDE IntelliJ IDEA行中，顺序不正确，但标记有链接。因此，Nullpointerexception实际上不是在标签设置中抛出的，而是在调用方法UI.getCurrent时抛出的，并且DataPicker尝试设置语言环境。由于Spring@SpringComponent，当方法调用到达时，还没有构建vaadinui，因此返回值为null。解决方案：向类中添加@UIScope注释。

问题的根源出人意料地是我用来运行应用程序的Spring Boot。

---

在IDE IntelliJ IDEA行中，顺序不正确，但标记有链接。因此，Nullpointerexception实际上不是在标签设置中抛出的，而是在调用方法UI.getCurrent时抛出的，并且DataPicker尝试设置语言环境。由于Spring@SpringComponent，当方法调用到达时，还没有构建vaadinui，因此返回值为null。解决方案：将@UIScope注释添加到类。

您的Vaadin版本是什么？@BenjaminD我使用Vaadin 16您是否尝试了仅使用标签的构造函数？@BenjaminD是的，也不起作用我找到了问题的根源-UI.getCurrent返回null，并且由于此方法用于数据选择器对象中-Nullpointerexception是Thorn。但我仍然不知道为什么会发生这种情况。你的Vaadin版本是什么？@BenjaminD我使用Vaadin 16你是否尝试了只带有标签的构造函数？@BenjaminD是的，也不起作用，我找到了问题的根源-UI.getCurrent返回null，因为此方法用于DataPicker对象-Nullpointerexception是Thorn。但是我仍然不知道为什么会发生这种情况。是的，似乎有关于根本问题的报道。是的，似乎有关于根本问题的报道

this.setLabel(label);

 public DatePicker(String label, LocalDate initialDate, ValueChangeListener<ComponentValueChangeEvent<DatePicker, LocalDate>> listener) {
        this(initialDate);
        this.setLabel(label);
        this.addValueChangeListener(listener);
    }