使用 json 在 c++ 和 java 之间交换对象

Question

I am trying exchange data stored on objects from a c++/qt project to java/spring using websockets and json. the communication via websockets are working fine, but in the java/spring the data arrives as null .

in the c++ class, i have something like that:

QString Usuario::toString() {
    QString json = "{'email': "+email+"}";
    QJsonDocument document = QJsonDocument::fromJson(json.toUtf8());
    QByteArray prettyPrintedJson = document.toJson(QJsonDocument::Indented);
    return prettyPrintedJson;
}

for classes like that:

usuario.h

class Usuario
{
private:
    QString email;
public:
    Usuario();
    Usuario(QString email);

    QString getEmail();
    void setEmail(QString email);

    QString toString();
};

registro.h

class Registro
{
private:
    QString token;
    Usuario usuario;
    Produto produto;
    Maquina maquina;
public:
    Registro();
    Registro(QString token, Usuario usuario, Produto produto, Maquina maquina);
    Registro(Usuario usuario, Produto produto, Maquina maquina);

    QString getToken();
    void setToken(QString token);

    Usuario getUsuario();
    void setUsuario(Usuario usuario);

    Produto getProduto();
    void setProduto(Produto produto);

    Maquina getMaquina();
    void setMaquina(Maquina maquina);

    QString toString();
};

in the java project, I have something like this:

@Component
public class CheckRegistro extends TextWebSocketHandler {
...
    @Override
    public void handleTextMessage(WebSocketSession session, TextMessage message) throws InterruptedException, IOException {
    Registro value = new Gson().fromJson(message.getPayload(), Registro.class);
    System.out.println("registro -> " + value);

    String email_usuario = value.getUsuario().getEmail();
    Usuario usuario = usuarioServ.findBy("email", email_usuario);

    String nome_produto = value.getProduto().getNome();
    Produto produto = produtoServ.findBy("nome", nome_produto);

    Cliente cliente = clienteServ.findBy("usuario", usuario);
    if(cliente.produtosComprados().contains(produto))
      value = registroServ.findBy("produto", produto);

    String result = new Gson().toJson(value);
        session.sendMessage(new TextMessage(result));
    }
...
}

and classes like that:

usuario.java

@Entity
public class Usuario extends Model implements UserDetails {
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private Integer id;

  @Column
  private String username;

  @Column
  private String password;

  @Column
  private String firstName;

  @Column
  private String lastName;

  @Column
  private String email;
...
}

registro.java

@Entity
public class Registro extends Model{
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private Integer id;

  @Column
  private String token;

  @OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
  private Usuario usuario;

  @OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
  private Produto produto;

  @OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
  private Maquina maquina;
...
}

(for the record, some classes, like usuario have more fields in java than c++; others, like registro have the same fields either on the c++ side or in the java side).

someone can hint me what's wrong here? I know there is some libraries that can serialize the c++ object into json automatically, but I dunno if it's possible do that without add a third-party component to my project, just finding the right format for the string sent to the java socket.

update

I change the toString method in the c++ side to something like that:

QString Registro::toString() {
    QJsonObject json{
      { "token", token },
      { "usuario", usuario.toString() },
      { "produto", produto.toString() },
      { "maquina", maquina.toString() }
    };
    QJsonDocument jsonDoc;
    jsonDoc.setObject(json);
    QByteArray prettyPrintedJson = jsonDoc.toJson(QJsonDocument::Indented);
    return prettyPrintedJson;
}

and now, in the java side, the json string is printed like that:

"registro": "{\n    \"maquina\": \"{\\n    \\\"cpuArch\\\": \\\"x86_64\\\",\\n    \\\"hostName\\\": \\\"DESKTOP-7GAPC4K\\\",\\n    \\\"kernelType\\\": \\\"windows\\\",\\n    \\\"kernelVersion\\\": \\\"10\\\",\\n    \\\"productName\\\": \\\"Windows 10 Version 1909\\\",\\n    \\\"ram\\\": \\\"RAM: 16030 MB\\\",\\n    \\\"uniqueId\\\": \\\"c69d8cc7-8e66-4ea3-964a-792b2c2a6f80\\\"\\n}\\n\",\n    \"produto\": \"{\\n    \\\"nome\\\": \\\"prod1\\\"\\n}\\n\",\n    \"token\": \"\",\n    \"usuario\": \"{\\n    \\\"email\\\": \\\"klebermo@gmail.com\\\"\\n}\\n\"\n}\n"
}

update 2

I change the method toString in the c++ side to something like that:

QString Registro::toString() {
    return "{ \"token\": \""+token+"\", \"usuario\": "+usuario.toString()+", \"produto\": "+produto.toString()+", \"maquina\": "+maquina.toString()+" }";
}

and now in the java side I get a valid json, but here:

Registro value = new Gson().fromJson(message.getPayload(), Registro.class);

String email_usuario = value.getUsuario().getEmail();
Usuario usuario = usuarioServ.findBy("email", email_usuario);

String nome_produto = value.getProduto().getNome();
Produto produto = produtoServ.findBy("nome", nome_produto);

I am geting a NullPointerException for value.getUsuario().getEmail() . I really need send a class from the c++ side with all the fields from the java class? Is there any way to allow me send only the class with fields needed for this query? Or this is not the problem?

Answer 1

json in Usuario::toString is not valid JSON. QJsonDocument::fromJson returns null when parsing fails.

Assuming email is not quoted you need:

QString json = "{\"email\": \""+email+"\"}";

Alternatively, safer and simpler would be to use the JSON API:

QJsonObject json{
  { "email", email }
};
QJsonDocument document = QJsonDocument::fromJson(json);
QByteArray prettyPrintedJson = document.toJson(QJsonDocument::Indented);