错误:java.lang.NoSuchMethodException:java.lang.Long.<init>()在Spring MVC中

Question

从数据库中读取学生对象时出现此错误.

org.springframework.beans.BeanInstantiationException:无法实例化[java.lang.Long]:找不到默认构造函数; 嵌套异常是java.lang.NoSuchMethodException:java.lang.Long.()org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:107)

完整堆栈跟踪:

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)

Student.java

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)

Controller.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;

daoImpl.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";
    }

Answer 1

这@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";
  }
  

• 创建一个方法,Long 在您的控件中提供该Object

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

官方文件

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

鉴于以上示例,Pet实例可以来自何处？有几种选择:

1. 由于使用@SessionAttributes,它可能已经在模型中 - 请参阅"使用@SessionAttributes在请求之间的HTTP会话中存储模型属性"一节.
2. 由于同一控制器中的@ModelAttribute方法,它可能已经在模型中 - 如上一节中所述.
3. 可以基于URI模板变量和类型转换器来检索它(下面更详细地解释).
4. 它可以使用其默认构造函数进行实例化.

编辑
如果studentId是从UI发送的参数名称,您可以@RequestParam像这样使用

@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";
}