Google Drive API, Oauth and service account

Question

Share your Drive account's folder to your service account.
Your service account's addresss looks like [email protected].
Then your service account can see the shared folder from your Drive account.
So you have 35GB service account.

The answer of drinkmystery is the right way to go in case your account is private and not a part of Google Workspace (formerly GSuite). Otherwise, there's a more elegant way to solve this with the createDelegated() method. It was described here and you should follow all the configuration instructions from there, but the code samples provided there are based on some deprecated packages, so it took me some time to make it work properly combining with the codes from the Drive API tutorial.

So for those who just need a working code sample, here it is (note the use of createDelegated):

import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.HttpRequestInitializer;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.services.drive.Drive;
import com.google.api.services.drive.DriveScopes;
import com.google.api.services.drive.model.File;
import com.google.api.services.drive.model.FileList;
import com.google.auth.http.HttpCredentialsAdapter;
import com.google.auth.oauth2.ServiceAccountCredentials;

import java.io.FileInputStream;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.util.Arrays;
import java.util.List;

public class DelegatedDriveWithServiceAccountQuickstart {
    private static final String APPLICATION_NAME = "your awesome app";
    private static final JsonFactory JSON_FACTORY = JacksonFactory.getDefaultInstance();

    private static final List<String> SCOPES = Arrays.asList(DriveScopes.DRIVE);
    private static final String CREDENTIALS_FILE_PATH = "/path/to/your/service-account-key-downloaded-from-google-api-console.json";

    public static void main(String... args) throws IOException, GeneralSecurityException {
        // Build a new authorized API client service
        final NetHttpTransport HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
        HttpRequestInitializer requestInitializer = new HttpCredentialsAdapter(ServiceAccountCredentials.fromStream(new FileInputStream(CREDENTIALS_FILE_PATH))
                .createScoped(SCOPES)
                .createDelegated("[email protected]"));
        Drive driveService = new Drive.Builder(HTTP_TRANSPORT, JSON_FACTORY, requestInitializer)
                .setApplicationName(APPLICATION_NAME)
                .build();

        // Print the names and IDs for up to 10 files.
        FileList result = driveService.files().list()
                .setPageSize(10)
                .setFields("nextPageToken, files(id, name)")
                .execute();
        List<File> files = result.getFiles();
        if (files == null || files.isEmpty()) {
            System.out.println("No files found.");
        } else {
            System.out.println("Files:");
            for (File file : files) {
                System.out.printf("%s (%s)\n", file.getName(), file.getId());
            }
        }
    }
}

Please, make sure your project dependencies are up to date (don't use blindly those from the Drive API tutorial). Here is my build.gradle:

apply plugin: 'java'
apply plugin: 'application'

mainClassName = 'DelegatedDriveWithServiceAccountQuickstart'
sourceCompatibility = 1.8
targetCompatibility = 1.8
version = '1.0'

repositories {
    mavenCentral()
}

dependencies {
    compile 'com.google.api-client:google-api-client:1.31.1'
    compile 'com.google.apis:google-api-services-drive:v3-rev20201130-1.31.0'
    compile 'com.google.auth:google-auth-library-oauth2-http:0.22.2'
}