当使用JDBC连接到数据库时，Rest API忽略数据库中带来的数据时返回一个空列表

Question

Im doing a project on my own and the point of the project is to make an a rest API in java and bring data from database using JDBC. The problem is when i make the connection and retrieve the data using main method everything works fine but when i try to retrieve the data through a get request, it returns an empty list. This is the endpoint

@Path("/users")
public class UserResource {
UserService userService = new UserService();

@GET
@Path("/all")
@Produces(MediaType.APPLICATION_JSON)
public Response getAllUsers() throws InterruptedException {
    List<User> users = userService.getAllUsers();
    GenericEntity<List<User>> wrapper = new GenericEntity<List<User>>(users) {
    };
    return Response.ok(wrapper).build();
}

}

And this is the class that connects to database using JDBC

public class ConnectionJDBC {

private static String USERNAME = "root";
private static String PASSWORD = "root";
private static String CONSTRING = "jdbc:mysql://localhost:3306/";
private static String DATABASE = "users_db";

static Connection connection;

public ConnectionJDBC() {
}

public static Connection getConnectionToDB() {
    try {
        String url = CONSTRING + DATABASE;
        connection = DriverManager.getConnection(url, USERNAME, PASSWORD);
        if (connection != null) {
            System.out.println("Connected");
        } else {
            System.out.println("Connection faild");
        }
    } catch (Exception e) {
        e.printStackTrace();
        System.out.println("Something went wrong before establishing the connection");
    }
    return connection;

}

public List<User> getUsers() {
    connection = getConnectionToDB();
    String query = "SELECT * FROM users_db.user_info;";
    List<User> userListRetrived = new ArrayList<>();

    try {
        PreparedStatement statement = connection.prepareStatement(query);
        ResultSet dataRetrieved = statement.executeQuery();
        while (dataRetrieved.next()) {
            User user = new User(dataRetrieved.getInt("id"), 
                    dataRetrieved.getString(2),
                    dataRetrieved.getString(3));
            userListRetrived.add(user);
            return userListRetrived;

        }
    } catch (Exception e) {
        e.printStackTrace();
        System.err.println("Something went wrong when retriving the data");
    }
    return userListRetrived;

}

}

UserService class is here

public class UserService {
private ConnectionJDBC connectionJDBC = new ConnectionJDBC();

public UserService() {
}

public List<User> getAllUsers() {
    ConnectionJDBC connectionJDBC = new ConnectionJDBC();
    List<User> usersList = connectionJDBC.getUsers();
    return usersList;
}

}

The user class is a normal java bean. It feels like getUsers method is ignoring bringing the data from the database and instead it returns the instantiated empty list instead. I'm using jersey and deploying the project on Glassfish. Any clues on how i can fix that or explanation for this situation?

Many thanks in advance!

Answer 1

You didn't invoke userService.getUsers() method from userResource() class. Please update your code in userResource() class as follows

GenericEntity<List<User>> wrapper = new GenericEntity<List<User>>(userService.getUsers());