Custom session solution

EveningServlet.java

package com.pch;


import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.Cookie;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.UUID;

@WebServlet("/evening")
public class EveningServlet extends HttpServlet {

    private static final String GREETING = "Good evening, %s!";
    private static final String BUDDY = "Buddy";
    private static final String CUSTOM_SESSION_ID = "CustomSessionId";
    private final Map<String, String> sessionIdValues = new HashMap<>();

    @Override
    protected void doGet(HttpServletRequest req,
                         HttpServletResponse resp) throws ServletException, IOException {
        String paramValue = getQueryParamValue(req, "name");
        String sessionId = saveToCustomSession(req, paramValue);
        String value = getValue(sessionId, resp);

        var writer = resp.getWriter();
        writer.println(String.format(GREETING, value));
        writer.flush();

    }

    private String saveToCustomSession(HttpServletRequest req,
                                       String paramValue) {
        String customSessionId = getCustomSessionId(req);
        if (customSessionId == null) {
            customSessionId = UUID.randomUUID().toString();
        } else {
            try {
                UUID.fromString(customSessionId);
            } catch (Exception e) {
                System.out.println("Wrong format of custom session id ");
                customSessionId = UUID.randomUUID().toString();
            }
        }

        if (paramValue == null) {
            String sessionValue = sessionIdValues.get(customSessionId);
            paramValue = Objects.requireNonNullElse(sessionValue, BUDDY);
        }

        sessionIdValues.put(customSessionId, paramValue);
        return customSessionId;
    }

    private String getQueryParamValue(HttpServletRequest req, String key) {
        String value = null;
        var parameterMap = req.getParameterMap();
        var queryParamName = parameterMap.get(key);

        if (queryParamName != null && queryParamName[0] != null) {
            value = queryParamName[0];
        }
        return value;
    }

    public String getValue(String sessionId, HttpServletResponse resp) {
        String value = sessionIdValues.get(sessionId);
        resp.addCookie(new Cookie(CUSTOM_SESSION_ID, sessionId));
        return value;
    }

    private String getCustomSessionId(HttpServletRequest req) {
        String customSessionId = null;
        for (Cookie cookie : req.getCookies()) {
            if (cookie.getName().equals(CUSTOM_SESSION_ID)) {
                customSessionId = cookie.getValue();
                return customSessionId;
            }
        }
        return customSessionId;
    }
}