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.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)

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;

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";
    }

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是问题的根源，因为Spring找不到可以提供此Long对象的方法，所以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中

@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) { }

鉴于上面的例子，Pet实例可以来自哪里？有几种选择：


由于使用了@SessionAttributes，它可能已经在模型中 - 请参阅   “使用@SessionAttributes存储模型属性”一节   在请求之间的HTTP会话“。

由于同一个控制器中的@ModelAttribute方法，它可能已经在模型中 - 如上一节所述。

可以基于URI模板变量和类型转换器检索它（在下面更详细地解释）。

可以使用其默认构造函数进行实例化。

修改
如果studentId是从UI发送的参数名称，则可以使用@RequestParam这样的

@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

您应该将studentId字段类型从Long更改为long，它应该修复它。

还有一个与我眼睛无关的事情，这个：

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

就像这样说：

return student;

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

2 个答案: