Writing Custom Grant Types

OAuth 2.0 authorization servers provide support for four main grant types according to the OAuth 2.0 specification. They also allow you to add custom grant types and extend the existing ones.

See Writing a Custom OAuth 2.0 Grant Type in the WSO2 identity Server documentation to implement custom grant types for the API Manager. Note that API Manager has already customized the Grant Type handlers for authorization_code, password, client_credentials and saml2-bearer grant types. If you require any additional functionality for these grant types, its advisable to extend the following grant handler implementations.

Grant Type	Existing Handler Class (which can be extended if required)
`authorization_code`	`org.wso2.carbon.apimgt.keymgt.handlers.ExtendedAuthorizationCodeGrantHandler`
`password`	`org.wso2.carbon.apimgt.keymgt.handlers.ExtendedPasswordGrantHandler`
`client_credentials`	`org.wso2.carbon.apimgt.keymgt.handlers.ExtendedClientCredentialsGrantHandler`
`urn:ietf:params:oauth:grant-type:saml2-bearer`	`org.wso2.carbon.apimgt.keymgt.handlers.ExtendedSAML2BearerGrantHandler`

Are you using WSO2 Identity Server as the Key Manager? If so, be sure to follow the this compatibility matrix.