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The ExternalAuthentication class makes up the interface between the external code and the IdP. The general flow is:

1. Call ExternalAuthentication.startExternalAuthentication(HttpServletRequest), saving off the result as a key.

2. Do work as necessary (reading request details from the attributes below). Any redirects must preserve the key value returned in step 1 because it must be used to complete the login later.

3. Set request attributes to communicate the result of the login back.

4. Call ExternalAuthentication.finishExternalAuthentication(String, HttpServletRequest, HttpServletResponse). The first parameter is the key returned in step 1.

Example JSP implementations are below.

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Note that returning a Subject is often paired with setting the shibboleth.authn.External.addDefaultPrincipals bean (V4.0) or idp.authn.External.addDefaultPrincipals property (V4.1+) to false, to dynamically establish Principal(s) representing the authentication method used without having them overwritten.

For example, your External flow's supportedPrincipals property might be defined to include both password and multi-factor authentication Principals (meaning it supports both methods), but you can return the specific method used at runtime in the Subject. For SAML 2.0, this is typically done (programmatically) by using the net.shibboleth.idp.saml.authn.principal.AuthnContextClassRefPrincipal class with the appropriate value from the standard or a deployment. Other classes in that package address SAML 1.1 and unusual SAML 2.0 use cases. For the built-in constants defined by the standard, there are Java constants available via org.opensaml.saml.saml2.core.AuthnContext.

Reference

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