I decided to write down the answers for some questions which my book's readers emailed to me or asked me via twitter,  so everyone can benefit from the answers. Here is the answer to the first question which involves custom security realms.

GlassFish supports 5 types of security realms out of the box which are sd follow:

1. File Realm: Usefull for development and testing purposes. GlassFish provids a user/ group management interface for this realm. We can add user and groups using the administration console. When using this realm all usernames, passwords and groups are stored in a plain text file.
2. JDBC Realm: In production environment we store user information including but not limited to username, passwords and groups in an RDBMS and then configure a JDBC realm to authenticate the given credentials againts the information stored in the datase.
3. LDAP Realm: Sometimes we have all user details stored in an LDAP like Active Directory or Redhat Directory Server, OpenDS or Sun Java System Directory Server Enterprise Edition.
4. Solaris Realm: This realm is used to authenticate users with a Solaris user directory.
5. Certificate Realm: The certificate realm allows us to conduct mutual SSL authentication based on the client certificates.

Sometimes our users information is stored in a silo different than all of this mentioned storages and we need to use that source for authentication and access control. For example assume that we have our users information including username, passwords and group membership stored in an Object Database and we need to authenticate our enterprise application's users with that storage. In such times we should either think about having a synchronized RDMBS keeping update user information and use JDBC realm for authentication and authorization or we should develop  a custom security realm which uses the object database as a source for authentication.

Setting up synchronization between the e.g object database and RDBS can be tricky while developing a custom authentication realm is much easier using GlassFish provided SPIs.

Second chapter of GlassFish security book discusses GlassFish security realms in details and discuss a sample application which uses these realms for authenticating and authorizing users. In the same chapter, developing custom security realms is discussed along with developing a sample realm.

In the same chapter GlassFish support for  JSR-196 (Java Authentication Service Provider Interface for Containers) is discussed to complete the ring of authentication and authorization in Java EE in general and GlassFish application server in particular.

From http://weblogs.java.net/blog/kalali/archive/2010/05/18/glassfish-security-book-faq-1-custom-security-realm-glassfish