Виктор Потапов

implementation group: 'org.thymeleaf.extras', name: 'thymeleaf-extras-springsecurity5', version: '3.0.4.RELEASE'

<div class="dashboard_rules" sec:authorize="hasAuthority('ADMIN')">
<!-- Увидит только админ-->
</div>

Как в зависимости от роли пользователя, после удачного входа подгружать index.html с разным наполнением?

User user = (User) userService.findUserByEmail(currentUser.getUsername());

@GetMapping("/dashboard")
public String dashboard(
        @AuthenticationPrincipal UserDetails currentUser,
        Model model
) {
    model.addAttribute("pageTitle","Панель управления");
    User user = (User) userService.findUserByEmail(currentUser.getUsername());
    if(user.isAdmin()) {
        model.addAttribute("posts", posts.getAdminPosts());
    }  else {
        model.addAttribute("posts", posts.getUserPosts());
    }

    return "backend/dashboard";
}

import java.io.BufferedReader;
import java.io.Reader;

public class GetSet extends BufferedReader {
    private int intVariable;
    private double doubleVariable;
    private String stringVariable;

    public GetSet(Reader in, int sz) {
        super(in, sz);
    }

    public GetSet(Reader in) {
        super(in);
    }

    public int getIntVariable() {
        return intVariable;
    }

    public void setIntVariable(int newIntVariable) {
        intVariable = newIntVariable;
    }

    public double getDoubleVariable() {
        return doubleVariable;
    }

    public void setDoubleVariable(double newDoubleVariable) {
        doubleVariable = newDoubleVariable;
    }

    public String getStringVariable() {
        return stringVariable;
    }

    public void setStringVariable(String newStringVariable) {
        stringVariable = newStringVariable;
    }

    public void message() {

        System.out.println("Int variable = " + intVariable +
                "\nDouble variable = " + doubleVariable +
                "\nString variable = " + "\"" + stringVariable + "\"");

    }
}

getset.setIntVariable(Integer.parseInt(getset.readLine()));

public void setIntVariable(String s) {
        intVariable = Integer.parseInt(s);
    }

SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
format.setTimeZone(TimeZone.getTimeZone("UTC"));
Date date = format.parse("2015-01-23T09:10:00");

Object object = Class.forName(className).getConstructor().newInstance();

Local variables and parameters cannot have public or private modifier. You can only give final to them.