Spring MVC表单-Long和String值。客户端发送的请求在语法上不正确

简单的例子，检查一下

实体（USER，MOBILEPHONE）

@Entity
@Table(name = "USER")
public class User {

private Long id
private String name;
private Set<Mobilephone> mobilephones= new HashSet<mobilephones>(0);

public User(Long id)
 this.id = id
}

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public Long getId() {
   return this.id;
}

public void setId(long id) {
    this.id = id;
}

//getter and setter for name

@OneToMany(fetch = FetchType.LAZY, mappedBy = "user")
public Set<Mobilephone> getMobilephones() {
    return this.mobilephones;
}

public void setMobilephones(Set<Mobilephone> mobilephones) {
    this.mobilephones= mobilephones;
}

@Entity
@Table(name = "MOBILEPHONE")
public class Mobilephone {

private Long id
private Long number;
private User user

public MobilePhone(Long id)
 this.id = id
}

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public Long getId() {
   return this.id;
}

public void setId(Long id) {
    this.id = id;
}

//getter and setter for number

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "USERID", nullable = false)
public User getUser() {
    return this.user;
}

public void setUser(User user) {
    this.user= user;
}

网页

<form:form modelAttribute="mobilephoneAttribute" action="url" method="post">
   <form:input path="mobilephone"/>
   <form:select path="user">
        <c:forEach items="${userlist}" var="user">
            <form:option value="${user.id}" label="${user.telephone" />
        </c:forEach>
   </form:select>
   <input type="submit"/>
</form:form>

发生了什么。

提交后，我得到此错误：

The request sent by the client was syntactically incorrect.

如果我将用户：“长ID”更改为“字符串ID”（以及方法），问题将消失。

我以为一开始，spring在将Long转换为String时有问题吗？但是可能不是，因为我们有一个数字，Long可以毫无问题地保存下来。

有人知道这个问题吗？