Helper component that provides a valid token to access Keycloak's REST API

KeycloakTokenProvider.java

package com.example.demo;

import com.fasterxml.jackson.annotation.JsonProperty;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.web.client.RestTemplate;

import java.math.BigInteger;

import static java.lang.System.currentTimeMillis;
import static org.springframework.http.MediaType.APPLICATION_FORM_URLENCODED;

public class KeycloakTokenProvider {

    private static final long SAFETY_BUFFER_MILLIS = 30_000;
    private static final String URL = "http://localhost:8080/auth/realms/master/protocol/openid-connect/token";

    private final RestTemplate restTemplate = new RestTemplate();

    private SavedKeycloakTokenResponse savedKeycloakTokenResponse;

    public synchronized String getToken() {
        if (savedKeycloakTokenResponse != null && savedKeycloakTokenResponse.stillValid()) {
            return savedKeycloakTokenResponse.keycloakTokenResponse.accessToken;
        }
        MultiValueMap<String, String> body = new LinkedMultiValueMap<>();
        body.add("grant_type", "password");
        body.add("client_id", "admin-cli");
        body.add("username", "admin");
        body.add("password", "admin");
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(APPLICATION_FORM_URLENCODED);
        HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(body, headers);
        KeycloakTokenResponse response = restTemplate.postForObject(URL, request, KeycloakTokenResponse.class);
        long expiresInMillis = response.getExpiresIn().multiply(BigInteger.valueOf(1_000)).longValueExact();
        long validUntil = currentTimeMillis() + expiresInMillis - SAFETY_BUFFER_MILLIS;
        savedKeycloakTokenResponse = new SavedKeycloakTokenResponse(response, validUntil);
        return response.accessToken;
    }

    private static class KeycloakTokenResponse {

        @JsonProperty("access_token")
        private String accessToken;

        @JsonProperty("expires_in")
        private BigInteger expiresIn;

        public String getAccessToken() {
            return accessToken;
        }

        public void setAccessToken(String accessToken) {
            this.accessToken = accessToken;
        }

        public BigInteger getExpiresIn() {
            return expiresIn;
        }

        public void setExpiresIn(BigInteger expiresIn) {
            this.expiresIn = expiresIn;
        }
    }

    private static class SavedKeycloakTokenResponse {

        private final KeycloakTokenResponse keycloakTokenResponse;
        private final long validUntil;

        public SavedKeycloakTokenResponse(KeycloakTokenResponse keycloakTokenResponse, long validUntil) {
            this.keycloakTokenResponse = keycloakTokenResponse;
            this.validUntil = validUntil;
        }

        boolean stillValid() {
            return currentTimeMillis() <= validUntil;
        }
    }
}