轉換爲json時對象變爲空

這是當我使用改進將對象發送到服務器時出現問題的原因。我這樣做是爲了創建JSON對象：轉換爲json時對象變爲空

HashMap<String, UserModel> map = new HashMap<>(); 
map.put("user", user);

但是，當JSON在我得到了這樣的服務器到達：

{"user":null}

然後我印有該行紐約JSON文件：

Log.d("TAG", new JSONObject(map).toString());

而且我看到了同樣的空對象。

那麼，這是我的問題，爲什麼會發生這種情況？我該如何解決這個問題？

這裏去了解我的項目的一些信息：

Retrofit version: 2.0.0 
Retrofit serializer: jackson version 2.0.0 
using also jackson to convert JodaTime version 2.4.0

在這裏不用我如何得到改造實例：

public T buildServiceInstance(Class<T> clazz){ 
     return new Retrofit.Builder().baseUrl(BuildConfig.API_HOST) 
            .addConverterFactory(JacksonConverterFactory.create()) 
            .build().create(clazz); 
    }

我認爲這種方法在這裏：

public static final IUserApi serviceInstance = new ApiBuildRequester<IUserApi>() 
                   .buildServiceInstance(IUserApi.class);

方法聲明在接口IUserApi上：

@POST("User.svc/Save") 
Call<ResponseSaveUserApiModel> save(@Body HashMap<String, UserModel> map);

，最後，但我想，不是那麼重要：

public class UserModel implements Parcelable { 


    private String idUser; 
    private String name; 
    private String email; 
    @JsonProperty("password") 
    private String safePassword; 
    private String salt; 
    private String phoneNumber; 
    private String facebookProfilePictureUrl; 
    private String facebookUserId; 

    public UserModel() { 
    } 

    public String getName() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 

    public String getIdUser() { 
     return idUser; 
    } 

    public void setIdUser(String idUser) { 
     this.idUser = idUser; 
    } 

    public String getEmail() { 
     return email; 
    } 

    public void setEmail(String email) { 
     this.email = email; 
    } 

    public String getSafePassword() { 
     return safePassword; 
    } 

    public void setSafePassword(String safePassword) { 
     this.safePassword = safePassword; 
    } 

    public String getPhoneNumber() { 
     return phoneNumber; 
    } 

    public void setPhoneNumber(String phoneNumber) { 
     this.phoneNumber = phoneNumber; 
    } 

    public String getFacebookProfilePictureUrl() { 
     return facebookProfilePictureUrl; 
    } 

    public void setFacebookProfilePictureUrl(String facebookProfilePictureUrl) { 
     this.facebookProfilePictureUrl = facebookProfilePictureUrl; 
    } 

    public String getFacebookUserId() { 
     return facebookUserId; 
    } 

    public void setFacebookUserId(String facebookUserId) { 
     this.facebookUserId = facebookUserId; 
    } 

    @Override 
    public int describeContents() { 
     return 0; 
    } 

    public UserModel(Parcel in) { // Deve estar na mesma ordem do "writeToParcel" 
     setIdUser(in.readString()); 
     setName(in.readString()); 
     setEmail(in.readString()); 
     setSafePassword(in.readString()); 
     setPhoneNumber(in.readString()); 
     setFacebookProfilePictureUrl(in.readString()); 
     setFacebookUserId(in.readString()); 
    } 

    @Override 
    public void writeToParcel(Parcel dest, int flags) { //Deve estar na mesma ordem do construtor que recebe parcel 
     dest.writeString(idUser); 
     dest.writeString(name); 
     dest.writeString(email); 
     dest.writeString(safePassword); 
     dest.writeString(phoneNumber); 
     dest.writeString(facebookProfilePictureUrl); 
     dest.writeString(facebookUserId); 
    } 

    public static final Parcelable.Creator<UserModel> CREATOR = new Parcelable.Creator<UserModel>(){ 
     @Override 
     public UserModel createFromParcel(Parcel source) { 
      return new UserModel(source); 
     } 

     @Override 
     public UserModel[] newArray(int size) { 
      return new UserModel[size]; 
     } 
    }; 

    public String getSalt() { 
     return salt; 
    } 

    public void setSalt(String salt) { 
     this.salt = salt; 
    } 
}

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