Yes, of course "inverse" - sorry.

Persistent vs RegisteredObject - not a problem but you are calling a simple class method so we don't need any super class. I used this implementation for the IRIS Class:

Class Utils.CSW1JavaFunctions
{
    ClassMethod IrisReturn(user = "user", pass = "pass") As %Stream.GlobalBinary
    {
         try {
             set cswStream=##class(%Stream.GlobalBinary).%New()
             set cswReturn = {"user":(user), "pass":(pass) }
             do cswReturn.%ToJSON(cswStream)
             return cswStream
         } catch exc {
             write !,"Caught Exception on server: ", exc.AsSQLMessage()
         }
    }
}

And this is a crude hack at the Java code - the anonymous InputStream class could use more work but it does run for this simple example. I'll leave the rest of the InputStream coding to you.

package utils;

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.intersystems.jdbc.*;

import java.io.*;
import java.sql.SQLException;


public class Reader {
    public static final String CACHE_CLASS_NAME = "Utils.CSW1JavaFunctions";

    public IRISConnection connection;
    public IRIS iris;

    public Reader(IRISConnection connection) throws SQLException {
        this.connection = connection;
        this.iris = IRIS.createIRIS(connection);
    }
    public static void main(String[] args) throws SQLException {
        IRISDataSource dataSource = new IRISDataSource();
        dataSource.setServerName("localhost");
        dataSource.setPortNumber(51776);
        dataSource.setDatabaseName("USER");
        dataSource.setUser("_SYSTEM");
        dataSource.setPassword("SYS");
        IRISConnection connection = (IRISConnection) dataSource.getConnection();
        Reader reader = new Reader(connection);
        try {
            JsonNode jsonNode = reader.execute("IrisReturn", "java", "jpass");
            System.out.println(jsonNode.toString());
        } catch (Exception exc) {
            exc.printStackTrace();
        }
    }

    public JsonNode execute(String method, Object... args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        JsonNode jsonNode = null;
        try {
            IRISObject data  = (IRISObject) iris.classMethodObject(CACHE_CLASS_NAME, method, args[0], args[1]);
            InputStream is = new InputStream() {
                byte[] buffer;
                int pos = 0;
                int len = -1;

                @Override
                public int read() throws IOException {
                    if (pos >= len) {
                        getBuffer();
                    }
                    if (len == -1) {
                      return -1;
                    }
                    return buffer[pos++];
                }

                void getBuffer() {
                    pos = 0;
                    IRISReference readLen = new IRISReference(3200);
                    String string = (String) data.invoke("Read", readLen);
                    if (readLen.getLong() == -1) {
                        buffer = null;
                        len = -1;
                    } else {
                        buffer = string.getBytes();
                        len = buffer.length;
                    }
                }
            };

            jsonNode = (JsonNode) mapper.readTree(is);
            return jsonNode;
        } catch (Exception ex) {
            ex.printStackTrace();
        }
        return null;
    }
}

Running this produces this output:

/usr/lib/jvm/adoptopenjdk-11-hotspot-amd64/bin/java -javaagent:/home/...
{"user":"java","pass":"jpass"}

Process finished with exit code 0