Authorization Code Flow

/**
 * OAuth 2.0 authorization code flow for an installed Java application that persists end-user credentials.
 * Implementation is thread-safe.
 */
public class AuthorizationCodeInstalledApp {
    
    /**
     * Constructor with default browser support.
     * @param flow authorization code flow
     * @param receiver verification code receiver
     */
    public AuthorizationCodeInstalledApp(
        AuthorizationCodeFlow flow, 
        VerificationCodeReceiver receiver);
    
    /**
     * Constructor with custom browser implementation.
     * @param flow authorization code flow
     * @param receiver verification code receiver  
     * @param browser custom browser implementation
     */
    public AuthorizationCodeInstalledApp(
        AuthorizationCodeFlow flow, 
        VerificationCodeReceiver receiver, 
        Browser browser);
    
    /**
     * Authorizes the installed application to access user's protected data.
     * @param userId user ID or null if not using a persisted credential store
     * @return credential with access token and optional refresh token
     * @throws IOException if authorization fails
     */
    public Credential authorize(String userId) throws IOException;
    
    /** Returns the authorization code flow. */
    public final AuthorizationCodeFlow getFlow();
    
    /** Returns the verification code receiver. */
    public final VerificationCodeReceiver getReceiver();
    
    /**
     * Open a browser at the given URL using Desktop if available, 
     * or alternatively output the URL to System.out for command-line applications.
     * @param url URL to browse
     */
    public static void browse(String url);
}

import com.google.api.client.auth.oauth2.AuthorizationCodeFlow;
import com.google.api.client.auth.oauth2.BearerToken;
import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp;
import com.google.api.client.extensions.java6.auth.oauth2.AbstractPromptReceiver;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.gson.GsonFactory;

// Create a prompt receiver for out-of-band authorization
class OobReceiver extends AbstractPromptReceiver {
    @Override
    public String getRedirectUri() {
        return "urn:ietf:wg:oauth:2.0:oob";
    }
}

// Set up the authorization flow
AuthorizationCodeFlow flow = new AuthorizationCodeFlow.Builder(
    BearerToken.authorizationHeaderAccessMethod(),
    new NetHttpTransport(),
    GsonFactory.getDefaultInstance(),
    new GenericUrl("https://oauth2.googleapis.com/token"),
    new ClientParametersAuthentication("your-client-id", "your-client-secret"),
    "your-client-id",
    "https://accounts.google.com/o/oauth2/auth")
    .setScopes(Arrays.asList("https://www.googleapis.com/auth/drive"))
    .build();

// Authorize the user
OobReceiver receiver = new OobReceiver();
AuthorizationCodeInstalledApp app = new AuthorizationCodeInstalledApp(flow, receiver);
Credential credential = app.authorize("user-id");

// Use the credential for API calls
if (credential.getAccessToken() != null) {
    System.out.println("Authorization successful!");
}

/**
 * Helper interface to allow caller to browse.
 */
public static interface Browser {
    /**
     * Browse to the specified URL.
     * @param url url to browse
     * @throws IOException if browsing fails
     */
    public void browse(String url) throws IOException;
}

/**
 * Default browser that just delegates to AuthorizationCodeInstalledApp.browse(String).
 */
public static class DefaultBrowser implements Browser {
    @Override
    public void browse(String url) throws IOException;
}

/**
 * Handles user authorization by redirecting to the OAuth 2.0 authorization server.
 * Default implementation calls browse(authorizationUrl.build()).
 * @param authorizationUrl authorization URL that can be customized
 * @throws IOException I/O exception
 */
protected void onAuthorization(AuthorizationCodeRequestUrl authorizationUrl) throws IOException;

class CustomInstalledApp extends AuthorizationCodeInstalledApp {
    public CustomInstalledApp(AuthorizationCodeFlow flow, VerificationCodeReceiver receiver) {
        super(flow, receiver);
    }
    
    @Override
    protected void onAuthorization(AuthorizationCodeRequestUrl authorizationUrl) throws IOException {
        // Add state parameter for security
        authorizationUrl.setState("random-state-value");
        // Add additional parameters
        authorizationUrl.set("prompt", "consent");
        super.onAuthorization(authorizationUrl);
    }
}