An instance of the default GSSManager subclass may be obtained through the static method getInstance, but applications are free to instantiate other subclasses of GSSManager. The default GSSManager instance will support the Kerberos v5 GSS-API mechanism in addition to any others. This mechanism is identified by the Oid "1.2.840.113554.1.2.2" and is defined in RFC 1964.

A subclass extending the GSSManager abstract class may be implemented as a modular provider based layer that utilizes some well known service provider specification. The GSSManager API allows the application to set provider preferences on such an implementation. These methods also allow the implementation to throw a well-defined exception in case provider based configuration is not supported. Applications that expect to be portable should be aware of this and recover cleanly by catching the exception.

It is envisioned that there will be three most common ways in which providers will be used:

1. The application does not care about what provider is used (the default case).
2. The application wants a particular provider to be used preferentially, either for a particular mechanism or all the time, irrespective of mechanism.
3. The application wants to use the locally configured providers as far as possible but if support is missing for one or more mechanisms then it wants to fall back on its own provider.

The GSSManager class has two methods that enable these modes of usage: addProviderAtFront and addProviderAtEnd. These methods have the effect of creating an ordered list of <provider, oid> pairs where each pair indicates a preference of provider for a given oid.

It is important to note that there are certain interactions between the different GSS-API objects that are created by a GSSManager, where the provider that is used for a particular mechanism might need to be consistent across all objects. For instance, if a GSSCredential contains elements from a provider p for a mechanism m, it should generally be passed in to a GSSContext that will use provider p for the mechanism m. A simple rule of thumb that will maximize portability is that objects created from different GSSManager's should not be mixed, and if possible, a different GSSManager instance should be created if the application wants to invoke the addProviderAtFront method on a GSSManager that has already created an object.

Here is some sample code showing how the GSSManager might be used:

     GSSManager manager = GSSManager.getInstance();

     Oid krb5Mechanism = new Oid("1.2.840.113554.1.2.2");
     Oid krb5PrincipalNameType = new Oid("1.2.840.113554.1.2.2.1");

     // Identify who the client wishes to be
     GSSName userName = manager.createName("duke", GSSName.NT_USER_NAME);

     // Identify the name of the server. This uses a Kerberos specific
     // name format.
     GSSName serverName = manager.createName("nfs/foo.sun.com",
                                             krb5PrincipalNameType);

     // Acquire credentials for the user
     GSSCredential userCreds = manager.createCredential(userName,
                                             GSSCredential.DEFAULT_LIFETIME,
                                             krb5Mechanism,
                                             GSSCredential.INITIATE_ONLY);

     // Instantiate and initialize a security context that will be
     // established with the server
     GSSContext context = manager.createContext(serverName,
                                                krb5Mechanism,
                                                userCreds,
                                                GSSContext.DEFAULT_LIFETIME);
 

The server side might use the following variation of this source:

     // Acquire credentials for the server
     GSSCredential serverCreds = manager.createCredential(serverName,
                                             GSSCredential.DEFAULT_LIFETIME,
                                             krb5Mechanism,
                                             GSSCredential.ACCEPT_ONLY);

     // Instantiate and initialize a security context that will
     // wait for an establishment request token from the client
     GSSContext context = manager.createContext(serverCreds);