Spring MVC - Creating a custom HttpMessageConverter

This example demonstrates how to create and register a custom HttpMessageConverter.

Creating the Custom Converter

Following custom converter converts the message body to the Report object. The converter expects that the first line of message body will be the report name and the rest will be the report content.

public class Report {
  private int id;
  private String reportName;
  private String content;
    .............
}

public class ReportConverter
      extends AbstractHttpMessageConverter<Report> {

  public ReportConverter() {
      super(new MediaType("text", "report"));
  }

  @Override
  protected boolean supports(Class<?> clazz) {
      return Report.class.isAssignableFrom(clazz);
  }

  @Override
  protected Report readInternal(Class<? extends Report> clazz, HttpInputMessage inputMessage)
          throws IOException, HttpMessageNotReadableException {
      String requestBody = toString(inputMessage.getBody());
      int i = requestBody.indexOf("\n");
      if (i == -1) {
          throw new HttpMessageNotReadableException("No first line found");
      }

      String reportName = requestBody.substring(0, i).trim();
      String content = requestBody.substring(i).trim();

      Report report = new Report();
      report.setReportName(reportName);
      report.setContent(content);
      return report;
  }

  @Override
  protected void writeInternal(Report report, HttpOutputMessage outputMessage)
          throws IOException, HttpMessageNotWritableException {
      try {
          OutputStream outputStream = outputMessage.getBody();
          String body = report.getReportName() + "\n" +
                  report.getContent();
          outputStream.write(body.getBytes());
          outputStream.close();
      } catch (Exception e) {
      }
  }

  private static String toString(InputStream inputStream) {
      Scanner scanner = new Scanner(inputStream, "UTF-8");
      return scanner.useDelimiter("\\A").next();
  }
}

A Controller utilizing the conversion

@Controller
public class ReportController {
  private List<Report> reports = new ArrayList<>();

  @RequestMapping(value = "/reports",
          method = RequestMethod.POST,
          consumes = "text/report")
  @ResponseBody
  public String handleRequest(@RequestBody Report report) {
      report.setId(reports.size() + 1);
      reports.add(report);
      return "report saved: " + report;
  }

  @RequestMapping(
          value = "/reports/{id}",
          method = RequestMethod.GET)
  @ResponseBody
  public Report reportById(@PathVariable("id") int reportId) {
      if (reportId > reports.size()) {
          throw new RuntimeException("No found for the id :" + reportId);
      }
      return reports.get(reportId - 1);
  }
}

Registering the Converter

@EnableWebMvc
@ComponentScan
public class AppConfig extends WebMvcConfigurerAdapter {

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

Writing a JUnit test

@RunWith(SpringJUnit4ClassRunner.class)
@WebAppConfiguration
@ContextConfiguration(classes = AppConfig.class)
public class ReportTests {
  @Autowired
  private WebApplicationContext wac;
  private MockMvc mockMvc;

  @Before
  public void setup() {
      DefaultMockMvcBuilder builder = MockMvcBuilders.webAppContextSetup(this.wac);
      this.mockMvc = builder.build();
  }

  @Test
  public void testNewReport() throws Exception {
      System.out.println("-- creating new report --");
      MockHttpServletRequestBuilder builder =
              MockMvcRequestBuilders.post("/reports")
                                    .contentType("text/report")
                                    .content(createTestReport());
      mockMvc.perform(builder)
             .andExpect(MockMvcResultMatchers.status()
                                             .isOk())
             .andDo(MockMvcResultHandlers.print());

      System.out.println("\n-- getting report by id --");
      builder = MockMvcRequestBuilders.get("/reports/1");
      mockMvc.perform(builder)
             .andExpect(MockMvcResultMatchers.status()
                                             .isOk())
             .andDo(MockMvcResultHandlers.print());
  }

  private String createTestReport() {
      return "dummy report name\ndummy report content.";
  }
}

Output

-- creating new report --

MockHttpServletRequest:
      HTTP Method = POST
      Request URI = /reports
       Parameters = {}
          Headers = {Content-Type=[text/report]}

Handler:
             Type = com.logicbig.example.ReportController
           Method = public java.lang.String com.logicbig.example.ReportController.handleRequest(com.logicbig.example.Report)

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = null

ModelAndView:
        View name = null
             View = null
            Model = null

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = {Content-Type=[text/plain;charset=ISO-8859-1], Content-Length=[91]}
     Content type = text/plain;charset=ISO-8859-1
             Body = report saved: Report{id=1, reportName='dummy report name', content='dummy report content.'}
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

-- getting report by id --

MockHttpServletRequest:
      HTTP Method = GET
      Request URI = /reports/1
       Parameters = {}
          Headers = {}

Handler:
             Type = com.logicbig.example.ReportController
           Method = public com.logicbig.example.Report com.logicbig.example.ReportController.reportById(int)

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = null

ModelAndView:
        View name = null
             View = null
            Model = null

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = {Content-Type=[text/report]}
     Content type = text/report
             Body = dummy report name
dummy report content.
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

Example Project

Dependencies and Technologies Used:

• spring-webmvc 4.2.4.RELEASE: Spring Web MVC.
• spring-test 4.2.4.RELEASE: Spring TestContext Framework.
• javax.servlet-api 3.0.1 Java Servlet API
• junit 4.12: JUnit is a unit testing framework for Java, created by Erich Gamma and Kent Beck.
• JDK 1.8
• Maven 3.3.9