错误：java.lang.NoSuchMethodException：spring MVC 中的 java.lang.Long.<init>()

Question

我在从数据库中读取 student 对象时遇到此错误。

org.springframework.beans.BeanInstantiationException: Failed to instantiate [java.lang.Long]: No default constructor found; nested exception is java.lang.NoSuchMethodException: java.lang.Long.<init>()
org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:107)

完整的堆栈跟踪：

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [java.lang.Long]: No default constructor found; nested exception is java.lang.NoSuchMethodException: java.lang.Long.<init>()
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause

org.springframework.beans.BeanInstantiationException: Failed to instantiate [java.lang.Long]: No default constructor found; nested exception is java.lang.NoSuchMethodException: java.lang.Long.<init>()
    org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:107)
    org.springframework.web.bind.annotation.support.HandlerMethodInvoker.resolveModelAttribute(HandlerMethodInvoker.java:775)
    org.springframework.web.bind.annotation.support.HandlerMethodInvoker.resolveHandlerArguments(HandlerMethodInvoker.java:368)
    org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:172)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:446)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:434)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause

java.lang.NoSuchMethodException: java.lang.Long.<init>()
    java.lang.Class.getConstructor0(Class.java:3082)
    java.lang.Class.getDeclaredConstructor(Class.java:2178)
    org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:104)
    org.springframework.web.bind.annotation.support.HandlerMethodInvoker.resolveModelAttribute(HandlerMethodInvoker.java:775)
    org.springframework.web.bind.annotation.support.HandlerMethodInvoker.resolveHandlerArguments(HandlerMethodInvoker.java:368)
    org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:172)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:446)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:434)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:943)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:877)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)

文件 Student.java

@Entity
@Table(name="Student")
public class Student implements Serializable{
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="studentId")
    Long studentId;

    @Column(name="studentName")
    String studentName;

文件 Controller.java

    @RequestMapping(value = "/read.html")
    public String readStudent(Model model, @ModelAttribute("studentId") Long studentId) {
        Student student = null;
        studentId = 2l;
        try{
            student = serviceFile.readStudent(studentId);
        }catch(Exception e){
            model.addAttribute("message", "Some thing went wrong !!!! Exception occoured");
            return "message";
        }
        model.addAttribute("student", student);
        return "read";
    }

文件daoImpl.java

@Repository
@Transactional
public class DaoImplFile implements DaoFile {

    private EntityManager entityManager;

    public EntityManager getEntityManager() {
        return entityManager;
    }

    @PersistenceContext
    public void setEntityManager(EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    @Override
    public Student read(Long studentId) throws NullPointerException {
        Student student = entityManager.find(Student.class, studentId);
        if (student!=null) {
            return student;
        } else {
            return null;
        }
    }

Answer 1

您应该将studentId 字段类型从Long 更改为long。它应该修复它。

另外一件让我眼前一亮的不相关的事情是：

if (student!=null) {
  return student;
} else {
  return null;
}

就是这么说的：

return student;

Answer 2

@ModelAttribute("studentId") Long studentId 是问题的根源，因为 Spring 没有找到可以提供此 Long 对象的方法，因此它尝试实例化一个并将其作为方法参数传递。 要解决此问题，您可以：

• 从方法参数中删除@ModelAttribue

   @RequestMapping(value = "/read.html")
   public String readStudent(Model model,Long studentId) {
       Student student = null;
       studentId = 2l;
       try {
           student = serviceFile.readStudent(studentId);
       } catch(Exception e){
           model.addAttribute("message", "Some thing went wrong !!!! Exception occured");
           return "message";
       }
       model.addAttribute("student", student);
       return "read";
   }
  

• 创建一个提供Long 对象的方法 在你的控制器中

   @ModelAttribute
   public void provideStudentId(Model model){
       model.addAttribute("studentId", new Long(1));
   }
  

---

官方文档

@RequestMapping(path = "/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
public String processSubmit(@ModelAttribute Pet pet) { }

根据上面的例子，Pet 实例是从哪里来的？有几种选择：

1. 由于使用了@SessionAttributes，它可能已经在模型中 — 见 名为“使用@SessionAttributes 存储模型属性”的部分 在请求之间的 HTTP 会话中”。
2. 由于同一控制器中的 @ModelAttribute 方法，它可能已经在模型中 — 如上一节所述。
3. 可以根据 URI 模板变量和类型转换器检索它（下面将详细说明）。
4. 可以使用其默认构造函数对其进行实例化。

如果 studentId 是 UI 发送的参数名称，你可以像这样使用@RequestParam

@RequestMapping(value = "/read.html")
public String readStudent(Model model, @RequestParam("studentId") Long studentId) {
    Student student = null;
    studentId = 2l;
    try {
        student = serviceFile.readStudent(studentId);
    } catch(Exception e) {
        model.addAttribute("message", "Some thing went wrong !!!! Exception occoured");
        return "message";
    }
    model.addAttribute("student", student);
    return "read";
}