How to create nested json objects with jackson

I am using hibernate to query records of users from my db as shown:

String hql = "SELECT U FROM User U";
List<User> users = this.em.createQuery(hql).getResultList();

How do I convert the returned data into json as shown in the format below:

If no users are found I want my json to look as shown below:

I want to do some manipulation on the retrieved data before creating the json data. Therefore I want to avoid passing the list directly as shown below:

 String data = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(users);

You can simply build json step by step:

At first step I would recommend to create a method in User class which gives name and email of user as json.

public class User{

    ...
 
    public String getJsonUserEmail(){

        return "{\"name\": \"" + this.getName() + "\", " +
                "\"email\": \"" + this.getEmail() + "\"}"

    }

    ...

}

Than transform all users to json and build the last json.

public static void main(String[] args) {

	String finalJson = "{";

	if(!users.isEmpty()){

		finalJson += "\"usersFound\": true, ";
                
        //transform every user to json
        //{"name": "name1", "email": "email1"}
	    List<String> usersJson = users.stream()
									  .map(e -> e.getJsonUserEmail())
									  .collect(Collectors.toList());

        //join all jsons
        //{"name": "name1", "email": "email1"}, {"name": "name2", "email": "email2"}, ...
		String usersJsonJoin = String.join(", ", usersJson);

		finalJson += "\"users\": [" + usersJsonJoin + "]";

	}else{

		finalJson += "\"usersFound\": false, ";
		finalJson += "\"msg\": \"No users found\"";

	}

	finalJson += "}";

}

If you are using Jackson library, you should implement a simple DTO object containing a list of User / UserDto, a stub for an empty list with custom msg property and then create appropriate instance depending on available results.

Also you can set the order of properties in the generated JSON using @JsonPropertyOrder annotation.

public class UserDto {
    private final String name;
    private final String email;

    public UserDto(String name, String email) {
        this.name = name;
        this.email = email;
    }
    public String getName() { return this.name; }
    public String getEmail() { return this.email; }
}

@JsonPropertyOrder({ "usersFound", "users" })
public class ListUserDto {
    private final List<UserDto> users;

    public ListUserDto(List<UserDto> users) {
        this.users = users;
    }

    public List<UserDto> getUsers() {
        return new ArrayList<>(users);
    }

    @JsonProperty("usersFound")
    public boolean isUsersFound() {
        return true;
    }
}

@JsonPropertyOrder({ "usersFound", "msg" })
@JsonIgnoreProperties(value = {"users"})
public class EmptyListUserDto extends ListUserDto {
    public EmptyListUserDto() {
        super(Collections.emptyList());
    }

    public String getMsg() {
        return "No User Found";
    }

    @JsonProperty("usersFound")
    public boolean isUsersFound() {
        return false;
    }
}


// -------------
public ListUserDto buildUserListJson(List<User> users) {
//    String hql = "SELECT U FROM User U";
//    List<User> users = this.em.createQuery(hql).getResultList();

    if (null == users || users.isEmpty()) {
        return new EmptyListUserDto();
    }
    return new ListUserDto(
            users.stream()
                 .map(u -> new UserDto(u.getName(), u.getEmail()))
                 .collect(Collectors.toList())
    );
}

Test code:

ObjectWriter writer = new ObjectMapper().writerWithDefaultPrettyPrinter();

System.out.println(writer.writeValueAsString(buildUserListJson(
    List.of(new User("jack", "jack@mail.com"), new User("john", "john@mail.com"))
)));

System.out.println("null -> " + writer.writeValueAsString(buildUserListJson(null)));

System.out.println("empty -> " + writer.writeValueAsString(buildUserListJson(Collections.emptyList())));

Output:

{
"usersFound" : true,
"users" : [ {
"name" : "jack",
"email" : "jack@mail.com"
}, {
"name" : "john",
"email" : "john@mail.com"
} ]
}
null -> {
"usersFound" : false,
"msg" : "No User Found"
}
empty -> {
"usersFound" : false,
"msg" : "No User Found"
}