我在derby db中使用jpa保存这个实体，每当我发布json并试图在db中保存数据时，我的url字段就会响应为空，而数据的插入进行得很顺利，我不会得到任何错误。
pojo类：

@Entity
public class Message {
@Id 
@GeneratedValue(strategy = GenerationType.SEQUENCE)
private Long id;
private String title;
private String content;
private String sender;
private String URL;
public Message(){
}
public Message(String URL,Long id ,String title, String content, String sender) {
    super();
    this.URL = URL;
    this.id = id;
    this.title = title;
    this.content = content;
    this.sender = sender;
}
public String getURL() {
    return URL;
}
public void setURL(String uRL) {
    URL = uRL;
}
@JsonIgnore
@JsonProperty(value = "id")
public Long getId() {
    return id;
}
public void setId(Long id) {
    this.id = id;
}
public String getTitle() {
    return title;
}
public void setTitle(String title) {
    this.title = title;
}
public String getContent() {
    return content;
}
public void setContent(String content) {
    this.content = content;
}
public String getSender() {
    return sender;
}
public void setSender(String sender) {
    this.sender = sender;
}
}

发布json:

{
    "title":"alpha beta gama",
    "content":"greek letters" ,
    "sender":"laura",
    "URL":"http://abc.efg"
}

响应

{
    "title": "alpha beta gama",
    "content": "greek letters",
    "sender": "laura",
    "url": null
}