錯誤：java.lang.NoSuchMethodException: java.lang.Long.() in spring MVC

Question

從數據庫讀取學生對象時出現此錯誤。

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)

完整的堆棧跟蹤：

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;

文件控制器.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

@ModelAttribute("studentId") Long studentId是問題的根源，因為 Spring 沒有找到可以提供這個Long對象的方法，所以它嘗試實例化一個並將其作為方法參數傳遞。 要解決此問題，您可以：

• 從方法參數中刪除@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"; }

• 創建一個方法，將在您的控制器中提供該Long對象

   @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) { }

鑒於上面的例子，Pet 實例來自哪里？ 有幾種選擇：

1. 由於使用了@SessionAttributes，它可能已經在模型中——請參閱“使用@SessionAttributes 在請求之間的 HTTP 會話中存儲模型屬性”一節。
2. 由於同一控制器中的 @ModelAttribute 方法，它可能已經在模型中——如上一節所述。
3. 可以基於 URI 模板變量和類型轉換器（下面更詳細地解釋）來檢索它。
4. 它可以使用其默認構造函數進行實例化。

如果 studentId 是從 UI 發送的參數名稱，則可以像這樣使用@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";
}

Answer 2

您應該將studentId字段類型從Long更改為long 。 它應該修復它。

還有一件不相關的事情突然出現在我眼前的是：

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

就像這樣說：

return student;