Firebase Cloud Messaging sender client in java

FcmSender.java

import org.jsonbuddy.JsonNode;
import org.jsonbuddy.JsonObject;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.Signature;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Base64;

public class FcmSender {
    private static final String MESSAGING_SCOPE = "https://www.googleapis.com/auth/firebase.messaging";
    private static final String[] SCOPES = {MESSAGING_SCOPE};
    private String projectId;
    public String keyId;
    public static Base64.Encoder base64Encoder = Base64.getUrlEncoder();
    private String clientEmail;
    private PrivateKey privateKey;

    public static void main(String[] args) throws IOException, GeneralSecurityException {
        JsonObject serviceAccount = JsonObject.read(new FileInputStream("service-account.json"));

        FcmSender fcmSender = new FcmSender();
        fcmSender.setClientEmail(serviceAccount.requiredString("client_email"));
        fcmSender.setProjectId(serviceAccount.requiredString("project_id"));
        fcmSender.setKeyId(serviceAccount.requiredString("private_key_id"));
        fcmSender.setPrivateKey(serviceAccount.requiredString("private_key"));

        String clientToken = "....";
        fcmSender.send(clientToken, "Hello", "Message");
    }

    private void send(String clientToken, String title, String message) throws IOException, GeneralSecurityException {
        HttpURLConnection connection = (HttpURLConnection) new URL("https://fcm.googleapis.com/v1/projects/" + projectId + "/messages:send").openConnection();

        connection.setRequestProperty("Authorization", "Bearer " + getAccessToken());
        connection.setRequestProperty("Content-Type", "application/json; UTF-8");
        connection.setDoOutput(true);
        try (OutputStreamWriter writer = new OutputStreamWriter(connection.getOutputStream(), StandardCharsets.UTF_8)) {
            writer.write(createMessage(clientToken, title, message).toString());
        }

        JsonObject response = JsonObject.read(connection);
        System.out.println(response);
    }

    private JsonObject createMessage(String clientToken, String title, String body) {
        return new JsonObject().put("message", new JsonObject()
                .put("notification", new JsonObject()
                        .put("title", title)
                        .put("body", body))
                .put("token", clientToken));
    }

    private String getAccessToken() throws IOException, GeneralSecurityException {
        HttpURLConnection connection = (HttpURLConnection) new URL("https://oauth2.googleapis.com/token").openConnection();
        connection.setRequestMethod("POST");
        connection.setDoOutput(true);
        connection.getOutputStream().write(
                ("grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer" +
                "&assertion=" + getJwtAssertion()).getBytes(StandardCharsets.UTF_8));

        JsonObject tokenResponse = JsonObject.read(connection);
        return tokenResponse.requiredString("access_token");
    }

    private String getJwtAssertion() throws GeneralSecurityException {
        JsonObject jwtHeader = new JsonObject()
                .put("alg", "RS256")
                .put("typ", "JWT")
                .put("kid", keyId);
        JsonObject jwtPayload = new JsonObject()
                .put("aud", "https://oauth2.googleapis.com/token")
                .put("iss", clientEmail)
                .put("scope", String.join(" ", SCOPES))
                .put("iat", System.currentTimeMillis() / 1000)
                .put("exp", System.currentTimeMillis() / 1000 + 3600);
        return createSignedJwt(jwtHeader, jwtPayload, privateKey);
    }

    private static String createSignedJwt(JsonObject jwtHeader, JsonObject jwtPayload, PrivateKey privateKey) throws GeneralSecurityException {
        String jwtContent = base64Encode(jwtHeader) + "." + base64Encode(jwtPayload);
        Signature signature = Signature.getInstance("SHA256withRSA");
        signature.initSign(privateKey);
        signature.update(jwtContent.getBytes());
        return jwtContent + "." + base64Encode(signature.sign());
    }


    public void setProjectId(String projectId) {
        this.projectId = projectId;
    }

    public void setClientEmail(String clientEmail) {
        this.clientEmail = clientEmail;
    }

    public void setKeyId(String keyId) {
        this.keyId = keyId;
    }

    private void setPrivateKey(String privateKey) throws NoSuchAlgorithmException, InvalidKeySpecException {
        privateKey = privateKey
                .replace("-----BEGIN PRIVATE KEY-----", "")
                .replace("-----END PRIVATE KEY-----", "")
                .replace("\n", "");
        setPrivateKey(Base64.getDecoder().decode(privateKey));
    }

    private void setPrivateKey(byte[] bytes) throws NoSuchAlgorithmException, InvalidKeySpecException {
        setPrivateKey(KeyFactory.getInstance("RSA").generatePrivate(new PKCS8EncodedKeySpec(bytes)));
    }

    private void setPrivateKey(PrivateKey privateKey) {
        this.privateKey = privateKey;
    }

    private static String base64Encode(JsonNode json) {
        return base64Encode(json.toString().getBytes(StandardCharsets.UTF_8));
    }

    private static String base64Encode(byte[] bytes) {
        return base64Encoder.encodeToString(bytes);
    }
}