Implement OAuth 2.0 refresh access token

src/main/java/org/scribe/builder/api/DefaultApi20.java

@@ -67,4 +67,11 @@ public OAuthService createService(OAuthConfig config)
     return new OAuth20ServiceImpl(this, config);
   }
 
+  /**
+   * @return the parameter needed to refresh a access token.
+   */
+  public String getRefreshTokenParameterName()
+  {
+    throw new UnsupportedOperationException("Refresh token is not implemented for "+getClass().getSimpleName());
+  }
 }

src/main/java/org/scribe/builder/api/FacebookApi.java

@@ -30,4 +30,10 @@ public String getAuthorizationUrl(OAuthConfig config)
       return String.format(AUTHORIZE_URL, config.getApiKey(), OAuthEncoder.encode(config.getCallback()));
     }
   }
+
+  @Override
+  public String getRefreshTokenParameterName()
+  {
+    return "fb_exchange_token";
+  }
 }

src/main/java/org/scribe/builder/api/LiveApi.java

@@ -37,4 +37,10 @@ public AccessTokenExtractor getAccessTokenExtractor()
 	{
 		return new JsonTokenExtractor();
 	}
+
+  @Override
+  public String getRefreshTokenParameterName()
+  {
+    return "refresh_token";
+  }
 }

src/main/java/org/scribe/model/OAuthConstants.java

@@ -45,5 +45,6 @@ public class OAuthConstants
   public static final String CLIENT_SECRET = "client_secret";
   public static final String REDIRECT_URI = "redirect_uri";
   public static final String CODE = "code";
+  public static final String GRANT_TYPE = "grant_type";
 
 }

src/main/java/org/scribe/oauth/OAuth10aServiceImpl.java

@@ -82,6 +82,14 @@ public Token getAccessToken(Token requestToken, Verifier verifier)
     return api.getAccessTokenExtractor().extract(response.getBody());
   }
 
+  /**
+   * {@inheritDoc}
+   */
+  public Token refreshAccessToken(Token accessToken)
+  {
+    throw new UnsupportedOperationException("Refresh token is not supported in Scribe OAuth 1.0");
+  }
+
   /**
    * {@inheritDoc}
    */

src/main/java/org/scribe/oauth/OAuth20ServiceImpl.java

@@ -37,6 +37,28 @@ public Token getAccessToken(Token requestToken, Verifier verifier)
     return api.getAccessTokenExtractor().extract(response.getBody());
   }
 
+  /**
+   * {@inheritDoc}
+   */
+  public Token refreshAccessToken(Token accessToken)
+  {
+
+    String accessTokenEndpoint = api.getAccessTokenEndpoint();
+    if (accessTokenEndpoint.contains("?grant_type="))
+    {
+      // handle the ugly case where the grant_type parameter is already hardcoded in the constant url
+      accessTokenEndpoint = accessTokenEndpoint.substring(0, accessTokenEndpoint.indexOf("?"));
+    }
+    OAuthRequest request = new OAuthRequest(api.getAccessTokenVerb(), accessTokenEndpoint);
+    request.addQuerystringParameter(OAuthConstants.CLIENT_ID, config.getApiKey());
+    request.addQuerystringParameter(OAuthConstants.CLIENT_SECRET, config.getApiSecret());
+    request.addQuerystringParameter(OAuthConstants.REDIRECT_URI, config.getCallback());
+    request.addQuerystringParameter(OAuthConstants.GRANT_TYPE, api.getRefreshTokenParameterName());
+    request.addQuerystringParameter(api.getRefreshTokenParameterName(), accessToken.getToken());
+    Response response = request.send();
+    return api.getAccessTokenExtractor().extract(response.getBody());
+  }
+
   /**
    * {@inheritDoc}
    */

src/main/java/org/scribe/oauth/OAuthService.java

@@ -27,6 +27,19 @@ public interface OAuthService
    */
   public Token getAccessToken(Token requestToken, Verifier verifier);
 
+  /**
+   * Refresh the access token to extend its expiration date.
+   * <p/>
+   * For the token in parameter, Facebook needs the access_token, while Live
+   * needs the refresh_token (which can be found only in the 
+   * {@link org.scribe.model.Token#getRawResponse()} returned by 
+   * {@link #getAccessToken(org.scribe.model.Token, org.scribe.model.Verifier)})
+   *
+   * @param accessToken access or refresh token, depending on the OAuth provider 
+   * @return fresh access token
+   */
+  public Token refreshAccessToken(Token accessToken);
+
   /**
    * Signs am OAuth request
    *