在Google Services和Google APIs客户端库上使用服务帐户时，无需阅读签名即可自行构造和构造JWT令牌使用实用程序类可通过OAuth 2.0轻松执行服务帐户授权.

但是，除了Google云端硬盘文档(其中包含多种编程语言的详细说明和代码示例)以外，该文档的文献记录并不十分完善.您应该阅读: https://developers.google.com/drive/service-accounts#google_apis_console_project_service_accounts

您的代码存在一些问题:

• 服务帐户的ID的格式应为:<some-id>@developer.gserviceaccount.com(是电子邮件，而不是客户ID，我知道这很奇怪)
• 您只能在进行 Google Apps域范围内的授权时设置principal，但不能在Gmail上进行设置帐户，当然，仅在管理员已授予您域权限的Google Apps帐户上，因此，在您的情况下:请勿进行设置.

以下是带有Java中的服务帐户的OAuth 2.0的代码示例.

注意:您还需要下载 URL缩短程序库.

/** Email of the Service Account */
private static final String SERVICE_ACCOUNT_EMAIL = "<some-id>@developer.gserviceaccount.com";

/** Path to the Service Account's Private Key file */
private static final String SERVICE_ACCOUNT_PKCS12_FILE_PATH = "/path/to/<public_key_fingerprint>-privatekey.p12";

/**
 * Build and returns a URL Shortner service object authorized with the service accounts.
 *
 * @return URL Shortner service object that is ready to make requests.
 */
public static Drive getDriveService() throws GeneralSecurityException, IOException, URISyntaxException {
  HttpTransport httpTransport = new NetHttpTransport();
  JacksonFactory jsonFactory = new JacksonFactory();
  GoogleCredential credential = new GoogleCredential.Builder()
      .setTransport(httpTransport)
      .setJsonFactory(jsonFactory)
      .setServiceAccountId(SERVICE_ACCOUNT_EMAIL)
      .setServiceAccountScopes(UrlshortenerScopes.URLSHORTENER)
      .setServiceAccountPrivateKeyFromP12File(
          new java.io.File(SERVICE_ACCOUNT_PKCS12_FILE_PATH))
      .build();
  Urlshortener service = new Urlshortener.Builder(httpTransport, jsonFactory, null)
      .setHttpRequestInitializer(credential).build();
  return service;
}

I am using Google APIs (version google-oauth-java-client-1.12.0-beta) to get a OAuth2 access token but got back "invalid_grant". Ref: https://developers.google.com/accounts/docs/OAuth2ServiceAccount

Here is the code:

import com.google.api.client.auth.jsontoken.JsonWebSignature;
import com.google.api.client.auth.jsontoken.JsonWebToken;
import com.google.api.client.auth.jsontoken.RsaSHA256Signer;
import com.google.api.client.auth.oauth2.TokenRequest;
import com.google.api.client.auth.oauth2.TokenResponse;
import com.google.api.client.http.GenericUrl;
import com.google.api.client.http.HttpTransport;
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.client.util.Clock;

import java.io.FileInputStream;
import java.io.IOException;

import java.security.GeneralSecurityException;
import java.security.KeyStore;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.UnrecoverableKeyException;
import java.security.cert.CertificateException;


public class TestClient
{
  private static PrivateKey getPrivateKey(String keyFile, String alias, String password)
    throws KeyStoreException, IOException, NoSuchAlgorithmException, CertificateException, UnrecoverableKeyException
  {
    KeyStore keystore = KeyStore.getInstance("PKCS12");
    keystore.load(new FileInputStream(keyFile), password.toCharArray());
    PrivateKey privateKey = (PrivateKey) keystore.getKey(alias, password.toCharArray());

    return privateKey;
  }

  public static void main(String[] args)
    throws GeneralSecurityException, IOException
  {
    String password = "notasecret";
    String alias = "privatekey";
    String keyFile = "<private key file>.p12";
    String serviceAccountScopes = "https://www.googleapis.com/auth/urlshortener";
    String serviceAccountUser = "user1@gmail.com";
    String serviceAccountId = "<a/c id>.apps.googleusercontent.com";
    JsonWebSignature.Header header = new JsonWebSignature.Header();
    header.setAlgorithm("RS256");
    header.setType("JWT"); 

    JsonWebToken.Payload payload = new JsonWebToken.Payload(Clock.SYSTEM);
    long currentTime = Clock.SYSTEM.currentTimeMillis();
    payload.setIssuer(serviceAccountId)
       .setAudience("https://accounts.google.com/o/oauth2/token")
       .setIssuedAtTimeSeconds(currentTime / 1000)
       .setExpirationTimeSeconds(currentTime / 1000 + 3600)
       .setPrincipal(serviceAccountUser);
    payload.put("scope", serviceAccountScopes); 
    System.out.println(payload.toPrettyString());

    PrivateKey serviceAccountPrivateKey = getPrivateKey(keyFile, alias, password);
    String assertion = RsaSHA256Signer.sign(serviceAccountPrivateKey, getJsonFactory(), header, payload);     
    TokenRequest request = new TokenRequest(getTransport(), getJsonFactory(), new GenericUrl(getTokenServerEncodedUrl()), "assertion");     

    request.put("grant_type", "urn:ietf:params:oauth:grant-type:jwt-bearer");     
    request.put("assertion", assertion);     
    TokenResponse resp = request.execute();    
    System.out.println("token : " + resp.getAccessToken());
  }

  private static String getTokenServerEncodedUrl()
  {
    return "https://accounts.google.com/o/oauth2/token";
  }

  private static JsonFactory getJsonFactory()
  {
    return new JacksonFactory();
  }

  private static HttpTransport getTransport()
  {
    return new NetHttpTransport();
  }
}

Result:

Exception in thread "main" com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request
{
  "error" : "invalid_grant"
}
    at com.google.api.client.auth.oauth2.TokenResponseException.from(TokenResponseException.java:103)
    at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:303)
    at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:323)

What is the problem here? any hint would be appreciated.

When using Service Accounts on Google Services and the Google APIs Client library you don't have to create the signature and construct the JWT Token by yourself as there are read-to-use utility classes to simply perform Service accounts authorization via OAuth 2.0.

However this is not very well documented except on the Google Drive documentation which contains detailed explanation and code samples in multiple programming languages. You should read: https://developers.google.com/drive/service-accounts#google_apis_console_project_service_accounts

Some issues with your code:

• The ID of the service account should be in the form: <some-id>@developer.gserviceaccount.com (yes the email instead of the Client ID, I know it's weird)
• You only set the principal when doing Google Apps domain Wide delegation but you can't do that on Gmail accounts of course, only on Google Apps accounts whose domain you have been granted access to by an administrator so in your case: don't set it.

Below is the code sample for OAuth 2.0 w/ Service accounts in Java.

Note: You'll also need to download the URL Shortener library.

/** Email of the Service Account */
private static final String SERVICE_ACCOUNT_EMAIL = "<some-id>@developer.gserviceaccount.com";

/** Path to the Service Account's Private Key file */
private static final String SERVICE_ACCOUNT_PKCS12_FILE_PATH = "/path/to/<public_key_fingerprint>-privatekey.p12";

/**
 * Build and returns a URL Shortner service object authorized with the service accounts.
 *
 * @return URL Shortner service object that is ready to make requests.
 */
public static Drive getDriveService() throws GeneralSecurityException, IOException, URISyntaxException {
  HttpTransport httpTransport = new NetHttpTransport();
  JacksonFactory jsonFactory = new JacksonFactory();
  GoogleCredential credential = new GoogleCredential.Builder()
      .setTransport(httpTransport)
      .setJsonFactory(jsonFactory)
      .setServiceAccountId(SERVICE_ACCOUNT_EMAIL)
      .setServiceAccountScopes(UrlshortenerScopes.URLSHORTENER)
      .setServiceAccountPrivateKeyFromP12File(
          new java.io.File(SERVICE_ACCOUNT_PKCS12_FILE_PATH))
      .build();
  Urlshortener service = new Urlshortener.Builder(httpTransport, jsonFactory, null)
      .setHttpRequestInitializer(credential).build();
  return service;
}