Specializations only need to implement the Parser.parse(Context, TypedData, Parse) method.

 import ratpack.exec.Promise;
 import ratpack.core.handling.Handler;
 import ratpack.core.handling.Context;
 import ratpack.core.http.TypedData;
 import ratpack.core.parse.Parse;
 import ratpack.core.parse.ParserSupport;
 import ratpack.core.parse.ParseException;
 import ratpack.func.Types;

 import java.io.UnsupportedEncodingException;

 import ratpack.test.handling.HandlingResult;
 import ratpack.test.handling.RequestFixture;

 import static org.junit.jupiter.api.Assertions.assertEquals;

 public class Example {

   // The parse options
   public static class StringParseOpts {
     private int maxLength;

     public StringParseOpts(int maxLength) {
       this.maxLength = maxLength;
     }

     public int getMaxLength() {
       return maxLength;
     }
   }

   // A parser for this type
   public static class MaxLengthStringParser extends ParserSupport<StringParseOpts> {
     public <T> T parse(Context context, TypedData body, Parse<T, StringParseOpts> parse) throws UnsupportedEncodingException {
       if (!parse.getType().getRawType().equals(String.class)) {
         return null;
       }

       String rawString = body.getText();
       StringParseOpts opts = parse.getOpts().orElse(new StringParseOpts(rawString.length()));
       if (rawString.length() < opts.getMaxLength()) {
         return Types.cast(rawString);
       } else {
         return Types.cast(rawString.substring(0, opts.getMaxLength()));
       }
     }
   }

   public static class ToUpperCaseHandler implements Handler {
     public void handle(Context context) throws Exception {
       context.parse(String.class, new StringParseOpts(5)).then(string -> context.render(string));
     }
   }

   // unit test
   public static void main(String[] args) throws Exception {
     HandlingResult result = RequestFixture.handle(new ToUpperCaseHandler(), fixture ->
         fixture
           .body("123456", "text/plain")
           .registry(registry -> registry.add(new MaxLengthStringParser()))
     );

     assertEquals("12345", result.rendered(String.class));
   }
 }