Quite often, you need to provide users with the same data, but in different forms, like JSON, PDF, XLS, etc. If your application is Spring Framework-based, this task can be achieved using HTTP message converters.

HTTP message converters are applied when an HTTP request (or its parts) needs to be converted into type required for handler method argument (for more info, see Handler methods — method arguments), or when the value is returned by handler method needs to be converted somehow to create HTTP response (for more info, see Handler methods — Return values).

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The Spring Framework provides you with a set of predefined HTTP message converters. For example, for byte arrays, JSON, etc., this set can be modified or extended to best suit your needs.

In this post, we will focus on converting the value returned from the handler method into the desired form, using an example provided by myself a bit later on (see the link below to view the source code repository).

Suppose that we have a controller returning some Team data, like this (yes, I know, I've ignored teamId):

@RestController
public class TeamDetailsController {

    @GetMapping("/teams/{teamId}/")
    public Team read() {
        final Set<TeamMember> members = new LinkedHashSet<>();
        members.add(new TeamMember("Albert Einstein", LocalDate.of(1879, 3, 14)));
        members.add(new TeamMember("Benjamin Franklin", LocalDate.of(1706, 1, 17)));
        members.add(new TeamMember("Isaac Newton", LocalDate.of(1643, 1, 4)));
        return new Team(members);
    }

}

In our example, the handler method response will be, by default, converted into JSON:

{
  "members": [
    {
      "dateOfBirth": "1879-03-14",
      "name": "Albert Einstein"    },
    {
      "dateOfBirth": "1706-01-17",
      "name": "Benjamin Franklin"    },
    {
      "dateOfBirth": "1643-01-04",
      "name": "Isaac Newton"    }
  ]
}

If we would like to convert the data returned by the handler into XLS file, we can simply define a bean being HTTP message converter implementation, which will be activated by the HTTP Accept header:

@Service
public class TeamToXlsConverter extends AbstractHttpMessageConverter<Team> {

    private static final MediaType EXCEL_TYPE = MediaType.valueOf("application/vnd.ms-excel");

    TeamToXlsConverter() {
        super(EXCEL_TYPE);
    }

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

    @Override
    protected boolean supports(final Class<?> clazz) {
        return (Team.class == clazz);
    }

    @Override
    protected void writeInternal(final Team team, final HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        try (final Workbook workbook = new HSSFWorkbook()) {
            final Sheet sheet = workbook.createSheet();
            int rowNo = 0;
            for (final TeamMember member : team.getMembers()) {
                final Row row = sheet.createRow(rowNo++);
                row.createCell(0)
                   .setCellValue(member.getName());
            }
            workbook.write(outputMessage.getBody());
        }
    }

}

You have to keep in mind that in our example, the defined HTTP message converter will always be applied when the handler method returns the value of type Team (see the supports method), and HTTP Accept header matches "application/vnd.ms-excel". In this case, the XLS file generated by the HTTP message converter is returned instead of the JSON representation of Team.

For more code, please check out my GitHub repo.

Further Reading

How Spring MVC Really Works

[DZone Refcard] Introduction to HTTP