Google drive API（Driver.Builder）身份验证问题：NullPointerException前置条件检查

Question

public class GoogleDriveService {

/** Global instance of the JSON factory. */
private static final JsonFactory JSON_FACTORY =
  JacksonFactory.getDefaultInstance();

/** Global instance of the HTTP transport. */
private static HttpTransport HTTP_TRANSPORT;

/** Client Email */
private static final String SERVICE_ACCOUNT_EMAIL =
  "[e-mail]";

/** Path to the Service Account's Private Key file */
private static final String SERVICE_ACCOUNT_PKCS12_FILE_PATH = "/gdrive-p12.p12";

/**
 * Build and return an authorized Drive client service.
 * @return an authorized Drive client service
 * @throws IOException
 */
public static Drive getDriveService() throws IOException, GeneralSecurityException {

  GoogleCredential credential = new GoogleCredential.Builder()
    .setTransport(HTTP_TRANSPORT)
    .setJsonFactory(JSON_FACTORY)
    .setServiceAccountId(SERVICE_ACCOUNT_EMAIL)
    .setServiceAccountPrivateKeyFromP12File(new java.io.File(SERVICE_ACCOUNT_PKCS12_FILE_PATH))
    .setServiceAccountScopes(Collections.singleton(DriveScopes.DRIVE))
    .build();

  System.out.println(credential);

  return new Drive.Builder(
    HTTP_TRANSPORT, JSON_FACTORY, credential)
    .build();
}

public static void main(String[] args) throws IOException, GeneralSecurityException {
  // Build a new authorized API client service.
  Drive service = getDriveService();

  // Print the names and IDs for up to 10 files.
  FileList result = service.files().list()
    .setPageSize(10)
    .setFields("nextPageToken, files(id, name)")
    .execute();
  List<File> files = result.getFiles();
  if (files == null || files.size() == 0) {
    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());
    }
  }
}

Exception in thread "main" java.lang.NullPointerException
    at com.google.api.client.repackaged.com.google.common.base.Preconditions.checkNotNull(Preconditions.java:213)
    at com.google.api.client.util.Preconditions.checkNotNull(Preconditions.java:127)
    at com.google.api.client.googleapis.services.AbstractGoogleClient$Builder.<init>(AbstractGoogleClient.java:333)
    at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClient$Builder.<init>(AbstractGoogleJsonClient.java:74)
    at com.google.api.services.drive.Drive$Builder.<init>(Drive.java:7783)
    at com.lumos.xlsx_manager.es.da.services.GoogleDriveService.getDriveService(GoogleDriveService.java:59)
    at com.lumos.xlsx_manager.es.da.services.GoogleDriveService.main(GoogleDriveService.java:66)

Answer 1

我想错误可能在这里：

/** Global instance of the HTTP transport. */
private static HttpTransport HTTP_TRANSPORT;

您没有将http_transport设置为任何内容，因此它保留为null。我猜这就是Google Drive API所抱怨的。

与json_factory和service_account_email不同，您也没有声明它final。如果这样做了，您可能会收到一个关于final字段从未初始化的编译器警告。