The writer interceptors manage the writer interceptor context. This context lets you manage the following things:

• Headers
• Output stream
• Entities

The entity and the output streams represent the result of a REST call. The entity is the serialized object declared in the service methods. Here's a sample of writer interceptor that adds an encrypted MD5 string in the output stream:

@Provider
public class ContentMD5Writer implements WriterInterceptor {
   @Override
   public void aroundWriteTo(WriterInterceptorContext context) throws IOException, WebApplicationException {
      MessageDigest digest = null;
      try {
         digest = getInstance("MD5");
      } catch (NoSuchAlgorithmException e) {
         throw new IllegalArgumentException(e);
      }
      ByteArrayOutputStream buffer = new ByteArrayOutputStream();
      DigestOutputStream digestStream = new DigestOutputStream(buffer, digest);
      OutputStream old = context.getOutputStream();
      context.setOutputStream(digestStream);
      try {
        context.proceed();
        byte[] hash = digest.digest();
        String encodedHash = getEncoder().encodeToString(hash);
        context.getHeaders().putSingle(CONTENT_MD5_STRING, encodedHash);
        byte[] content = buffer.toByteArray();
        old.write(content);
      } finally {
        context.setOutputStream(old);
      }
   }
}

The result can be seen in the client:

Client client = newClient();
client.register(ContentMD5Writer.class);
String md5 = response.getHeaderString(CONTENT_MD5_STRING);

The variable MD5 will return the hcEzFGyuhOARcfBb4bM1sw== encrypted value.