Automatically serialize Rest controllers to CSV instead of JSON in Spring Boot

Question

I am looking for a quick/easy solution how to automatically serialize Rest controllers output to CSV instead of JSON. I have the simplest possible Spring boot application:

@SpringBootApplication
public class CsvExportApplication {

    public static void main(String[] args) {
        SpringApplication.run(CsvExportApplication.class, args);
    }
}

class User {
    String name;
    String surname;

    public User(String name, String surname) {
        this.name = name;
        this.surname = surname;
    }

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

    public void setSurname(String surname) {
        this.surname = surname;
    }

    public String getName() {
        return name;
    }

    public String getSurname() {
        return surname;
    }
}

@RestController
class UserController {
    @GetMapping(value = "/users")
    List<User> list() {
        return Arrays.asList(new User("adam", "kowalsky"), new User("john", "smith"));
    }
}

I have used jackson-dataformat-csv and came up with the following code that serializes List<User> to String, but ideally I do not want to change the rest controller code:

CsvMapper mapper = new CsvMapper();
CsvSchema schema = mapper.schemaFor(User.class).withHeader();
mapper.writerFor(List.class).with(schema).writeValueAsString(users);

Ideally I would like my controllers to be able to return output in JSON or CSV depending on the Accept header in the request.

Hope this answer helps you to properly define the return format for your rest controller https://stackoverflow.com/a/33417769/8553816 — Khwaja Sanjari, Nov 06 '18 at 19:25

score 4 · Accepted Answer · answered Nov 06 '18 at 23:55

I manage to achieve what I want by:

defining a custom converter for application/csv
the converter can only write CSV (it does not support read)
the converter uses Jackon's ObjectMapper (to make sure CSV and JSON outputs use e.g. the same format of the dates)
the converter builds the jackson-dateformat-csv schema on-the-fly as currently schema-less writing is not supported: https://github.com/FasterXML/jackson-dataformats-text/issues/114

code:

class CsvConverter<T> extends AbstractHttpMessageConverter<T> {

    private final ObjectMapper objectMapper;

    CsvConverter(ObjectMapper objectMapper) {
        super(new MediaType("application", "csv"));
        this.objectMapper = objectMapper;
    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return true;
    }

    @Override
    protected T readInternal(Class<? extends T> clazz, HttpInputMessage inputMessage)
            throws IOException, HttpMessageNotReadableException {
        return null;
    }

    @Override
    protected void writeInternal(T object, HttpOutputMessage outputMessage) 
           throws IOException, HttpMessageNotWritableException {
        try {
            ObjectWriter objectWriter = getCsvWriter(object);
            try (PrintWriter outputWriter = new PrintWriter(outputMessage.getBody())) {
                outputWriter.write(objectWriter.writeValueAsString(object));
            }
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    ObjectWriter getCsvWriter(T object) {
        Set<String> fields = getUniqueFieldNames(object);
        CsvSchema.Builder schemaBuilder = CsvSchema.builder().setUseHeader(true);
        for (String field : fields) {
            schemaBuilder.addColumn(field);
        }
        return new CsvMapper().writerFor(List.class).with(schemaBuilder.build());
    }

    Set<String> getUniqueFieldNames(T object) {
        try {
            JsonNode root = objectMapper.readTree(objectMapper.writeValueAsString(object));
            Set<String> uniqueFieldNames = new LinkedHashSet<>();
            root.forEach(element -> {
                Iterator<String> it = element.fieldNames();
                while (it.hasNext()) {
                    String field = it.next();
                    uniqueFieldNames.add(field);
                }
            });
            return uniqueFieldNames;
        } catch (IOException ex) {
            throw new RuntimeException(ex);
        }
    }
}

@Configuration
class AppConfig implements WebMvcConfigurer {

    private final ObjectMapper objectMapper;

    AppConfig(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
    }

    @Override
    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(new CsvConverter<>(objectMapper));
    }
}

Automatically serialize Rest controllers to CSV instead of JSON in Spring Boot

1 Answers1