Are you integrating existing applications, or do you just want to support your own applications?
Are you looking for actual SSO or simply shared credentials? SSO is logging in to a single application, and having that credential propagate to another application (such as logging in to Gmail and being automatically logged in to Blogger). Shared credential is you can use the same login name and password across applications, but the credential itself is not automatically propagated.
LDAP is a common system used to manage a shared credential. Many systems allow you to point their authentication store to an existing LDAP server.
For example, if you had several apps deployed in a Java EE container, and also, say, an email server and web based email client, all of these diverse applications could be pointed to the same LDAP server and your users would have a single login and password for all of the different systems, all written in different languages, all deployed on different machines. This is a bread and butter use case of LDAP, and pretty much every system can handle this out of the box. Glassfish and Tomcat can both readily validate against an LDAP server. So can Apache (Web server), Postgres (Database), Postfix (email), etc. etc.
So if you want simply a shared credential, you get that "for free", right now, by installing an LDAP server. LDAP is a bit of a different beast than something like a DBMS, but once you study it a little and "get it", it's really quite nice. OpenLDAP is a popular LDAP server, but I'm partial to ApacheDS.
The way to set that up in a Java EE container is to set up a "Realm". GF and Tomcat both have LDAP realms out of the box, I imagine the rest do to. But the nut there is that you need to use Java EE security to leverage the Realm.
See, the detail with a Java EE Realm is that it's an aspect of the CONTAINER, not the application. Just like a connection pool is a container resource that your application leverages. Most people want security to be a part of their application, where they feel they have more control over it.
That's all well and good until you start getting a bunch of different applications and everyone is configured differently and has separate user lists, and password policies, etc. etc.
LDAP can fix a lot of that, since you configure them all to use the same credential store.
The Realm fills that need on a Java EE server. Your application is configured to use a Realm provided by the container. If you have several applications, and a single Realm, then they all get to share the credentials within that Realm (regardless of the Realm type).
Realms can be anything: file based, db based, LDAP, etc. Realms also cluster if the container clusters (which can be handy).
The dark side of Java EE security, and why most apps avoid it is that, since, again, the Realm is part of the container, and not the application, it can be a little ungainly to use, and perhaps not offer the features you like in terms of user management, password policies, etc.
But the bright side of Java EE security is that once you're under its umbrella, you can leverage the credential all over in your code easily. A person logs in to the web site, and that credential can be used there in the web app, or automatically propagated back to the EJB tier (ever a remote EJB tier), and the information is always handy.
You can point your web apps at a realm, you EJBs, your web services. They all leverage the same pieces.
In order to get kind of the best of both worlds is to leverage container specific mechanisms to access the containers security. This is the other dark side of Java EE security.
Things like Realms, and direct access to container security are not portable across containers. GF does it different than Tomcat does it different from WebLogic. It's all really close, but differs in details so your code won't port seamlessly.
The bright side is for in house apps, most folks simply leverage the container they have, put a reasonable abstraction around the container dependent code, and call it day noting that yes, they will have to port this if and when they move to a different container. But, in practice. much like a database, once a container platform is chosen, folks tend to snuggle in tight and stick with it.
Finally, Servlet 3.0 (In GF3 and Tomcat 7) standardizes more of the programmatic login issues to make them more portable across containers, but the underlying concepts are the same.
Now, SSO.
SSO is a different beast. But, out the gate, both GF and Tomcat support SSO for web apps. This lets you log in to one web app and have be able to easily access others without having to log in to them. But the SSO is a little bit limited since it relies more heavily on the container security and its lifecycle, rather than a more flexible one under the control of the application. Mind, not just on Realms (that's a given), but on the actual container based FORM login, rather than a custom programmatic login. FORM login is not spectacular, but it's functional and it works. Implement a Realm, deploy your apps to a single instance of Tomcat or GF (or a cluster in GF 3.1), and you get SSO for free, so if that's important, it's kind of nice really. It's usability is fine for back office apps, but not perhaps the public internet.
If you want a more sophisticated SSO solution, then you need look at custom implementations. OpenSSO is one of those, and it relies on SAML and the SAML web profile. However, there are others. There's CAS, Atlassian Cloud, Kerberos, and OAuth as well. Those are all using different protocols than SAML. If you want to stick with SAML you can also look at Shibboleth, or even SimpleSAML (SimpleSAML is a PHP server that acts as a SAML Identity Provider, among other things, but you still need a Service Provider within your applications).
Whatever protocol you choose, the process is basically the same (detailed here -- Cross Domain Login - How to login a user automatically when transferred from one domain to another).
But the devil is in the details. And, boy, are there devils.
All of these systems are complicated. SSO is complicated. For example, now that you have Single Sign On, what about Single Sign Out? What about Single Time Out? What about credential changes while users are logged in? What about an STS (Secure Token Service) for your Web Services? (STS offers a similar delegated authentication mechanism for web services.)
SAML introduces you to a whole lot of new vocabulary, and a lot of configuration. It's not readily picked up since the documentation isn't stellar and relies a lot on standards documents which talk to a higher level of generic things, and not to you and your application specifically.
If you don't need really need SSO, then you'll likely be content with something like a central LDAP store and going on from there.
All that said, as an example, our applications support both a DB and LDAP backend. They use Glassfish, and Java EE security. We completely control the user experience. We also support SSO via SAML (we wrote our own Identity and Service Providers), and we have both shared credentials via LDAP and SSO across Java and other applications, using our code and third party code. The bright side is this is all standards based. The dark side is that standards are communicated in english, and english is subject to interpretation.
I say this simply to say it can be done. I have also written ad hoc, back of the napkin SSO implementations, both same domain and cross domain (same domain is simple with a shared cookie) using simple Servlet Filters. Password policies, password recovery, keep alive timers, multiple window timeout and session management (that's a hoot), roles, privileges, etc. etc. Been there, done that.
Also, I would be remiss to not mention Spring and Spring Security which offers all of this on top of Spring. I have not used it (I'm not a Spring person), but those folks do know what they are doing so it's worth looking at.
The dark side of Java EE security, and why most apps avoid it is that, since, again, the Realm is part of the container, and not the application- For that reason JASPIC has been added to Java EE 6. It allows for fully portable auth modules that can be installed in either the app (and then be fully part of the app) or like the container specific realms inside the container. See arjan-tijms.blogspot.com/2012/11/… – Anetteaneurin