java - Unable to intercept and manipulate HttpServletResponse in Spring Boot

I have a requirement to Base64 decode every JSON request payload that my Spring Boot service receives. The JSON payload would have been Base64 encoded at the client before posting using the HTTP POST method. Further, I also need to Base64 encode the JSON response before presenting to the calling client application.

I am required to reduce boilerplate code by using handler interceptors.
I have already achieved the request/incoming leg of the operation by the use of interceptors but is yet to achieve this for the response leg.
I have posted the code snippets below. The code to intercept the response and base64 encode it is in the postHandle method of the interceptor class.

What am I doing wrong here?

Interceptor Class:

public class Base64ResponseEncodingInterceptor implements HandlerInterceptor {   

    private static final String DECODED_REQUEST_ATTRIB = "decodedRequest";
    private static final String POST = "POST";


    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView arg3) throws Exception {
          try {
              if (POST.equalsIgnoreCase(request.getMethod())) {
                    CharResponseWrapper res = new CharResponseWrapper(response);
                    res.getWriter();

                    byte[] encoded = Base64.encodeBase64(res.toString().getBytes());
                    byte[] encoded = Base64.encodeBase64(response.getHeader(ENCODED_RESPONSE_ATTRIB).getBytes());
                    response.getWriter().write(new String(encoded));
              }
          } catch (Exception e) {
              throw new Exception(e.getMessage());
          }
    }



    // preHandle and afterCompletion methods
    // Omitted 
}

The CharResponseWrapper Class used above:

public class CharResponseWrapper extends HttpServletResponseWrapper {

    protected CharArrayWriter charWriter;


    protected PrintWriter writer;

    protected boolean getOutputStreamCalled;

    protected boolean getWriterCalled;


    public CharResponseWrapper(HttpServletResponse response) {
        super(response);

        charWriter = new CharArrayWriter();
    }


    @Override
    public ServletOutputStream getOutputStream() throws IOException {
        if (getWriterCalled) {
            throw new IllegalStateException("getWriter already called");
        }
        getOutputStreamCalled = true;

        return super.getOutputStream();
    }


    @Override
    public PrintWriter getWriter() throws IOException {
        if (writer != null) {
            return writer;
        }
        if (getOutputStreamCalled) {

            throw new IllegalStateException("getOutputStream already called");
        }
        getWriterCalled = true;
        writer = new PrintWriter(charWriter);
        return writer;
    }


    @Override
    public String toString() {

        String s = null;
        if (writer != null) {
            s = charWriter.toString();
        }
        return s;
    }
}

JavaConfig Class where Interceptor is registered:

@Configuration
@EnableJpaRepositories(repositoryBaseClass = BaseRepositoryBean.class, basePackages = "")
@EntityScan(basePackages = { "com.companyname", "com.companyname.productname"})
public class RestConfig extends WebMvcConfigurerAdapter {


      @Override
      public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new Base64ResponseEncodingInterceptor());

      }

}

The Controller Class, where the Interceptor is used (Only the working request leg is shown here):

@Autowired
HttpServletRequest request;


String decodedRequest = null;

@ModelAttribute("decodedRequest")
public void getDecodedParam(){
    decodedRequest = (String) request.getAttribute("decodedRequest");
} 

The code in the postHandle method does not work. It is either the HttpServletResponse is null or I get an exception message:

getOutputStream already called

Update: Work around solution to reading the response directly in the ResponseBodyAdvice
In the Controller Class, I added the following:

@RestController
@RequestMapping("/api/ipmanager")
public class IPProfileRestController extends AbstractRestController {


    @Autowired
    HttpServletResponse response;

   String encodedResponse = null;

   @ModelAttribute("encodedResponse")
    public void getEncodedResponse(){
       response.setHeader("encodedResponse", StringUtils.EMPTY);
    } 


    @RequestMapping(value = "/time", method = { RequestMethod.POST }, produces = { MediaType.APPLICATION_JSON_VALUE, MediaType.TEXT_PLAIN_VALUE }, consumes = {
            MediaType.APPLICATION_JSON_VALUE })
    public @ResponseBody String saveAccessClientTime(@RequestBody String ecodedRequest) {

        // Some code here

        String controllerResponse = prettyJson(iPProfileResponse);
        response.setHeader("encodedResponse", controllerResponse);
        return controllerResponse;

    }
}

I have the following in the ResponseBodyAdvice

@ControllerAdvice
public class Base64EncodedResponseBodyAdvice implements ResponseBodyAdvice