Why does Gmail API Java Quickstart return "PKIX path building failed"? - gradle

Question

I'm trying to access my corporate Gmail account, and I am using the Java Quickstart guide. However, even after following all the directions in the guide, I keep getting the below error:
Exception in thread "main" javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
I've tried to address this issue by adding certificates for both accounts.google.com and gmail.com to cacerts, but it didn't help and the error remained the same.
Here's the code that I'm using:
/* build.gradle */
apply plugin: 'java'
apply plugin: 'application'
mainClassName = 'GmailQuickstart'
sourceCompatibility = 11
targetCompatibility = 11
version = '1.0'
repositories {
mavenCentral()
}
dependencies {
implementation 'com.google.api-client:google-api-client:2.0.0'
implementation 'com.google.oauth-client:google-oauth-client-jetty:1.34.1'
implementation 'com.google.apis:google-api-services-gmail:v1-rev20220404-2.0.0'
}
/* GmailQuickstart.java */
import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp;
import com.google.api.client.extensions.jetty.auth.oauth2.LocalServerReceiver;
import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow;
import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.gson.GsonFactory;
import com.google.api.client.util.store.FileDataStoreFactory;
import com.google.api.services.gmail.Gmail;
import com.google.api.services.gmail.GmailScopes;
import com.google.api.services.gmail.model.Label;
import com.google.api.services.gmail.model.ListLabelsResponse;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.security.GeneralSecurityException;
import java.util.Collections;
import java.util.List;
/* class to demonstrate use of Gmail list labels API */
public class GmailQuickstart {
/**
* Application name.
*/
private static final String APPLICATION_NAME = "Application_Name";
/**
* Global instance of the JSON factory.
*/
private static final JsonFactory JSON_FACTORY = GsonFactory.getDefaultInstance();
/**
* Directory to store authorization tokens for this application.
*/
private static final String TOKENS_DIRECTORY_PATH = "tokens";
/**
* Global instance of the scopes required by this quickstart.
* If modifying these scopes, delete your previously saved tokens/ folder.
*/
private static final List<String> SCOPES = Collections.singletonList(GmailScopes.GMAIL_LABELS);
private static final String CREDENTIALS_FILE_PATH = "/credentials.json";
/**
* Creates an authorized Credential object.
*
* #param HTTP_TRANSPORT The network HTTP Transport.
* #return An authorized Credential object.
* #throws IOException If the credentials.json file cannot be found.
*/
private static Credential getCredentials(final NetHttpTransport HTTP_TRANSPORT)
throws IOException {
// Load client secrets.
InputStream in = GmailQuickstart.class.getResourceAsStream(CREDENTIALS_FILE_PATH);
if (in == null) {
throw new FileNotFoundException("Resource not found: " + CREDENTIALS_FILE_PATH);
}
GoogleClientSecrets clientSecrets =
GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in));
// Build flow and trigger user authorization request.
GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, SCOPES)
.setDataStoreFactory(new FileDataStoreFactory(new java.io.File(TOKENS_DIRECTORY_PATH)))
.setAccessType("offline")
.build();
LocalServerReceiver receiver = new LocalServerReceiver.Builder().setPort(8888).build();
Credential credential = new AuthorizationCodeInstalledApp(flow, receiver).authorize("user");
//returns an authorized Credential object.
return credential;
}
public static void main(String... args) throws IOException, GeneralSecurityException {
// Build a new authorized API client service.
final NetHttpTransport HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
Gmail service = new Gmail.Builder(HTTP_TRANSPORT, JSON_FACTORY, getCredentials(HTTP_TRANSPORT))
.setApplicationName(APPLICATION_NAME)
.build();
// Print the labels in the user's account.
String user = "me";
ListLabelsResponse listResponse = service.users().labels().list(user).execute();
List<Label> labels = listResponse.getLabels();
if (labels.isEmpty()) {
System.out.println("No labels found.");
} else {
System.out.println("Labels:");
for (Label label : labels) {
System.out.printf("- %s\n", label.getName());
}
}
}
}
What can I do to fix this issue?