This document explains how to implement an OAuth 2.0 authorization callback handler using Java servlets through a sample web application that will display the user's tasks using the Google Tasks API . The sample application will first request authorization to access the user's Google Tasks and will then display the user's tasks in the default tasks list.
Audience
This document is tailored to people who are familiar with Java and J2EE web application architecture. Some knowledge of the OAuth 2.0 authorization flow is recommended.
Contents
In order to have such fully working sample several steps are necessary, you need to:
- Declare servlet mappings in the web.xml file
- Authenticate the users on your system and request authorization to access its Tasks
- Listen for the Authorization code from the Google Authorization endpoint
- Exchange the authorization code for a refresh and access token
- Read the user's tasks and display them
Declare servlet mappings in the web.xml file
We will be using 2 servlets in our application:
- PrintTasksTitlesServlet(mapped to / ): The entry point of the application that will handle user authentication, and will display the user's tasks
- OAuthCodeCallbackHandlerServlet(mapped to /oauth2callback ): The OAuth 2.0 callback which handles the response from the OAuth authorization endpoint
Below is the web.xml file which maps these 2 servlets to URLs in our application:
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
<servlet>
<servlet-name>PrintTasksTitles</servlet-name>
<servlet-class>com.google.oauthsample.PrintTasksTitlesServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>PrintTasksTitles</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>OAuthCodeCallbackHandlerServlet</servlet-name>
<servlet-class>com.google.oauthsample.OAuthCodeCallbackHandlerServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>OAuthCodeCallbackHandlerServlet</servlet-name>
<url-pattern>/oauth2callback</url-pattern>
</servlet-mapping>
</web-app>
/WEB-INF/web.xml file
Authenticate the users on your system and request authorization to access its tasks
The user enters the application through the root '/' URL which is mapped to the PrintTaskListsTitlesServlet servlet. In that servlet the following tasks are performed:
- Checks if the user is authenticated on the system
- If the user is not authenticated he is redirected to the authentication page
- If the user is authenticated, we check if we have a refresh token already in our data storage - which is handled by the OAuthTokenDao below. If there is no refresh token in store for the user this means the user has not yet granted the application authorization to access its tasks. In that case the user is redirected to Google's OAuth 2.0 Authorization endpoint.
package com . google . oauthsample ; import ... /** * Simple sample Servlet which will display the tasks in the default task list of the user . */ @SuppressWarnings ( "serial" ) public class PrintTasksTitlesServlet extends HttpServlet { /** * The OAuth Token DAO implementation , used to persist the OAuth refresh token . * Consider injecting it instead of using a static initialization . Also we are * using a simple memory implementation as a mock . Change the implementation to * using your database system . */ public static OAuthTokenDao oauthTokenDao = new OAuthTokenDaoMemoryImpl (); public void doGet ( HttpServletRequest req , HttpServletResponse resp ) throws IOException { // Getting the current user // This is using App Engine 's User Service but you should replace this to // your own user / login implementation UserService userService = UserServiceFactory . getUserService (); User user = userService . getCurrentUser (); // If the user is not logged - in it is redirected to the login service , then back to this page if ( user == null ) { resp . sendRedirect ( userService . createLoginURL ( getFullRequestUrl ( req ))); return ; } // Checking if we already have tokens for this user in store AccessTokenResponse accessTokenResponse = oauthTokenDao . getKeys ( user . getEmail ()); // If we don 't have tokens for this user if ( accessTokenResponse == null ) { OAuthProperties oauthProperties = new OAuthProperties (); // Redirect to the Google OAuth 2.0 authorization endpoint resp . sendRedirect ( new GoogleAuthorizationRequestUrl ( oauthProperties . getClientId (), OAuthCodeCallbackHandlerServlet . getOAuthCodeCallbackHandlerUrl ( req ), oauthProperties . getScopesAsString ()) . build ()); return ; } } /** * Construct the request 's URL without the parameter part. * * @param req the HttpRequest object * @return The constructed request 's URL */ public static String getFullRequestUrl ( HttpServletRequest req ) { String scheme = req . getScheme () + "://" ; String serverName = req . getServerName (); String serverPort = ( req . getServerPort () == 80 ) ? "" : ":" + req . getServerPort (); String contextPath = req . getContextPath (); String servletPath = req . getServletPath (); String pathInfo = ( req . getPathInfo () == null ) ? "" : req . getPathInfo (); String queryString = ( req . getQueryString () == null ) ? "" : "?" + req . getQueryString (); return scheme + serverName + serverPort + contextPath + servletPath + pathInfo + queryString ; } }
PrintTasksTitlesServlet.java file
Note: The above implementation uses some App Engine libraries, these are used as a matter of simplification. If you are developing for another platform, feel free to re-implement the UserService interface which handles user authentication.
The application uses a DAO to persist and access user's authorization tokens. Below is the interface - OAuthTokenDao - and a mock (in-memory) implementation - OAuthTokenDaoMemoryImpl - that are used in this sample:
package com . google . oauthsample ; import com.google.api.client.auth.oauth2.draft10.AccessTokenResponse ; /** * Allows easy storage and access of authorization tokens . */ public interface OAuthTokenDao { /** * Stores the given AccessTokenResponse using the { @code username }, the OAuth * { @code clientID } and the tokens scopes as keys . * * @param tokens The AccessTokenResponse to store * @param userName The userName associated wit the token */ public void saveKeys ( AccessTokenResponse tokens , String userName ); /** * Returns the AccessTokenResponse stored for the given username , clientId and * scopes . Returns { @code null } if there is no AccessTokenResponse for this * user and scopes . * * @param userName The username of which to get the stored AccessTokenResponse * @return The AccessTokenResponse of the given username */ public AccessTokenResponse getKeys ( String userName ); }
OAuthTokenDao.java file
package com . google . oauthsample ; import com.google.api.client.auth.oauth2.draft10.AccessTokenResponse ; ... /** * Quick and Dirty memory implementation of { @link OAuthTokenDao } based on * HashMaps . */ public class OAuthTokenDaoMemoryImpl implements OAuthTokenDao { /** Object where all the Tokens will be stored */ private static Map < String , AccessTokenResponse > tokenPersistance = new HashMap < String , AccessTokenResponse > (); public void saveKeys ( AccessTokenResponse tokens , String userName ) { tokenPersistance . put ( userName , tokens ); } public AccessTokenResponse getKeys ( String userName ) { return tokenPersistance . get ( userName ); } }
OAuthTokenDaoMemoryImpl.java file
Also the OAuth 2.0 credentials for the application are stored in a properties file. Alternatively you could simply have them as a constant somewhere in one of your java classes, though here is the OAuthProperties class and the oauth.properties file that is being used in the sample:
package com . google . oauthsample ; import ... /** * Object representation of an OAuth properties file . */ public class OAuthProperties { public static final String DEFAULT_OAUTH_PROPERTIES_FILE_NAME = "oauth.properties" ; /** The OAuth 2.0 Client ID */ private String clientId ; /** The OAuth 2.0 Client Secret */ private String clientSecret ; /** The Google APIs scopes to access */ private String scopes ; /** * Instantiates a new OauthProperties object reading its values from the * { @code OAUTH_PROPERTIES_FILE_NAME } properties file . * * @throws IOException IF there is an issue reading the { @code propertiesFile } * @throws OauthPropertiesFormatException If the given { @code propertiesFile } * is not of the right format ( does not contains the keys { @code * clientId }, { @code clientSecret } and { @code scopes }) */ public OAuthProperties () throws IOException { this ( OAuthProperties . class . getResourceAsStream ( DEFAULT_OAUTH_PROPERTIES_FILE_NAME )); } /** * Instantiates a new OauthProperties object reading its values from the given * properties file . * * @param propertiesFile the InputStream to read an OAuth Properties file . The * file should contain the keys { @code clientId }, { @code * clientSecret } and { @code scopes } * @throws IOException IF there is an issue reading the { @code propertiesFile } * @throws OAuthPropertiesFormatException If the given { @code propertiesFile } * is not of the right format ( does not contains the keys { @code * clientId }, { @code clientSecret } and { @code scopes }) */ public OAuthProperties ( InputStream propertiesFile ) throws IOException { Properties oauthProperties = new Properties (); oauthProperties . load ( propertiesFile ); clientId = oauthProperties . getProperty ( "clientId" ); clientSecret = oauthProperties . getProperty ( "clientSecret" ); scopes = oauthProperties . getProperty ( "scopes" ); if (( clientId == null ) || ( clientSecret == null ) || ( scopes == null )) { throw new OAuthPropertiesFormatException (); } } /** * @return the clientId */ public String getClientId () { return clientId ; } /** * @return the clientSecret */ public String getClientSecret () { return clientSecret ; } /** * @return the scopes */ public String getScopesAsString () { return scopes ; } /** * Thrown when the OAuth properties file was not at the right format , i . e not * having the right properties names . */ @SuppressWarnings ( "serial" ) public class OAuthPropertiesFormatException extends RuntimeException { } }
OAuthProperties.java file
Below is the oauth.properties file which contains the OAuth 2.0 credentials of your appliction. You need to change the values below by your own.
# Client ID and secret. They can be found in the APIs console. clientId=1234567890.apps.googleusercontent.com clientSecret=aBcDeFgHiJkLmNoPqRsTuVwXyZ # API scopes. Space separated. scopes=https://www.googleapis.com/auth/tasks
oauth.properties file
The OAuth 2.0 Client ID and Client secret identifies your application and allow the Tasks API apply filters and quota rules that are defined for your application. The client ID and secret can be found in the Google APIs Console . Once on the console you will have to:
- Create or select a project.
- Enable the Tasks API by toggling the Tasks API status to ONin the list of services.
- Under API Accesscreate an OAuth 2.0 Client ID if one hasn't been created yet.
- Make sure that the project's OAuth 2.0 code callback handler URL is registered/whitelisted in the Redirect URIs. For example, in this sample project you would have to register https://www.example.com/oauth2callback if your web application is served from the https://www.example.com domain.
Redirect URI in the APIs Console
Listen for the Authorization code from the Google Authorization endpoint
In the case where the user has not yet authorized the application to access its tasks and therefore have been redirected to Google's OAuth 2.0 Authorization endpoint, the user is displayed an authorization dialog from Google asking the user to grant your application access to its tasks:
Google's authorization Dialog
After granting or denying access, the user will be redirect back to the OAuth 2.0 code callback handler which has been specified as a redirect/callback when constructing the Google authorization URL:
new GoogleAuthorizationRequestUrl(oauthProperties.getClientId(), OAuthCodeCallbackHandlerServlet.getOAuthCodeCallbackHandlerUrl(req), oauthProperties .getScopesAsString()).build()
The OAuth 2.0 code callback handler - OAuthCodeCallbackHandlerServlet - handles the redirect from the Google OAuth 2.0 endpoint. There are 2 cases to handle:
- The user has granted access: parses the request to get the OAuth 2.0 code from the URL parameters
- The user has denied access: shows a message to the user
package com . google . oauthsample ; import ... /** * Servlet handling the OAuth callback from the authentication service . We are * retrieving the OAuth code , then exchanging it for a refresh and an access * token and saving it . */ @SuppressWarnings ( "serial" ) public class OAuthCodeCallbackHandlerServlet extends HttpServlet { /** The name of the Oauth code URL parameter */ public static final String CODE_URL_PARAM_NAME = "code" ; /** The name of the OAuth error URL parameter */ public static final String ERROR_URL_PARAM_NAME = "error" ; /** The URL suffix of the servlet */ public static final String URL_MAPPING = "/oauth2callback" ; public void doGet ( HttpServletRequest req , HttpServletResponse resp ) throws IOException { // Getting the "error" URL parameter String [] error = req . getParameterValues ( ERROR_URL_PARAM_NAME ); // Checking if there was an error such as the user denied access if ( error != null && error . length > 0 ) { resp . sendError ( HttpServletResponse . SC_NOT_ACCEPTABLE , "There was an error: \" " + error [ 0 ] + " \" ." ); return ; } // Getting the "code" URL parameter String [] code = req . getParameterValues ( CODE_URL_PARAM_NAME ); // Checking conditions on the "code" URL parameter if ( code == null || code . length == 0 ) { resp . sendError ( HttpServletResponse . SC_BAD_REQUEST , "The \" code \" URL parameter is missing" ); return ; } } /** * Construct the OAuth code callback handler URL . * * @param req the HttpRequest object * @return The constructed request 's URL */ public static String getOAuthCodeCallbackHandlerUrl ( HttpServletRequest req ) { String scheme = req . getScheme () + "://" ; String serverName = req . getServerName (); String serverPort = ( req . getServerPort () == 80 ) ? "" : ":" + req . getServerPort (); String contextPath = req . getContextPath (); String servletPath = URL_MAPPING ; String pathInfo = ( req . getPathInfo () == null ) ? "" : req . getPathInfo (); return scheme + serverName + serverPort + contextPath + servletPath + pathInfo ; } }
OAuthCodeCallbackHandlerServlet.java file
Exchange the authorization code for a refresh and access token
Then, the OAuthCodeCallbackHandlerServlet exchanges the Auth 2.0 code for a refresh and access tokens, persists it in the datastore and redirects the user back to the PrintTaskListsTitlesServlet URL:
The code added to the file below is syntax highlighted, the already existing code is greyed out.
package com . google . oauthsample ; import ... /** * Servlet handling the OAuth callback from the authentication service . We are * retrieving the OAuth code , then exchanging it for a refresh and an access * token and saving it . */ @SuppressWarnings ( "serial" ) public class OAuthCodeCallbackHandlerServlet extends HttpServlet { /** The name of the Oauth code URL parameter */ public static final String CODE_URL_PARAM_NAME = "code" ; /** The name of the OAuth error URL parameter */ public static final String ERROR_URL_PARAM_NAME = "error" ; /** The URL suffix of the servlet */ public static final String URL_MAPPING = "/oauth2callback" ;
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
// Getting the "error" URL parameter
String[] error = req.getParameterValues(ERROR_URL_PARAM_NAME);
// Checking if there was an error such as the user denied access
if (error != null && error.length > 0) {
resp.sendError(HttpServletResponse.SC_NOT_ACCEPTABLE, "There was an error: \""+error[0]+"\".");
return;
}
// Getting the "code" URL parameter
String[] code = req.getParameterValues(CODE_URL_PARAM_NAME);
// Checking conditions on the "code" URL parameter
if (code == null || code.length == 0) {
resp.sendError(HttpServletResponse.SC_BAD_REQUEST, "The \"code\" URL parameter is missing");
return;
}
/**
* Construct the OAuth code callback handler URL.
*
* @param req the HttpRequest object
* @return The constructed request's URL
*/
public static String getOAuthCodeCallbackHandlerUrl(HttpServletRequest req) {
String scheme = req.getScheme() + "://";
String serverName = req.getServerName();
String serverPort = (req.getServerPort() == 80) ? "" : ":" + req.getServerPort();
String contextPath = req.getContextPath();
String servletPath = URL_MAPPING;
String pathInfo = (req.getPathInfo() == null) ? "" : req.getPathInfo();
return scheme + serverName + serverPort + contextPath + servletPath + pathInfo;
}
OAuthCodeCallbackHandlerServlet.java file
Note: The above implementation uses some App Engine libraries, these are used as a matter of simplification. If you are developing for another platform, feel free to re-implement the UserService interface which handles user authentication.
Read the user's tasks and display them
The user has granted the application access to its tasks. The application has a refresh token that is saved in the datastore accessible through the OAuthTokenDao . The PrintTaskListsTitlesServlet servlet can now use these tokens to access the user's tasks and display them:
The code added to the file below is syntax highlighted, the already existing code is greyed out.
package com . google . oauthsample ; import ... /** * Simple sample Servlet which will display the tasks in the default task list of the user . */ @SuppressWarnings ( "serial" ) public class PrintTasksTitlesServlet extends HttpServlet { /** * The OAuth Token DAO implementation , used to persist the OAuth refresh token . * Consider injecting it instead of using a static initialization . Also we are * using a simple memory implementation as a mock . Change the implementation to * using your database system . */ public static OAuthTokenDao oauthTokenDao = new OAuthTokenDaoMemoryImpl (); public void doGet ( HttpServletRequest req , HttpServletResponse resp ) throws IOException { // Getting the current user // This is using App Engine 's User Service but you should replace this to // your own user / login implementation UserService userService = UserServiceFactory . getUserService (); User user = userService . getCurrentUser (); // If the user is not logged - in it is redirected to the login service , then back to this page if ( user == null ) { resp . sendRedirect ( userService . createLoginURL ( getFullRequestUrl ( req ))); return ; } // Checking if we already have tokens for this user in store AccessTokenResponse accessTokenResponse = oauthTokenDao . getKeys ( user . getEmail ()); // If we don 't have tokens for this user if ( accessTokenResponse == null ) { OAuthProperties oauthProperties = new OAuthProperties (); // Redirect to the Google OAuth 2.0 authorization endpoint resp . sendRedirect ( new GoogleAuthorizationRequestUrl ( oauthProperties . getClientId (), OAuthCodeCallbackHandlerServlet . getOAuthCodeCallbackHandlerUrl ( req ), oauthProperties . getScopesAsString ()) . build ()); return ; }
}
/**
* Construct the request's URL without the parameter part.
*
* @param req the HttpRequest object
* @return The constructed request's URL
*/
public static String getFullRequestUrl(HttpServletRequest req) {
String scheme = req.getScheme() + "://";
String serverName = req.getServerName();
String serverPort = (req.getServerPort() == 80) ? "" : ":" + req.getServerPort();
String contextPath = req.getContextPath();
String servletPath = req.getServletPath();
String pathInfo = (req.getPathInfo() == null) ? "" : req.getPathInfo();
String queryString = (req.getQueryString() == null) ? "" : "?" + req.getQueryString();
return scheme + serverName + serverPort + contextPath + servletPath + pathInfo + queryString;
}
PrintTasksTitlesServlet.java file
The user will be displayed with its tasks:
The user's tasks
Sample application
The code for this sample application can b downloaded here . Feel free to check it out.
