Glassfish changing master password and saving it to a file

Saving the master password when creating a domain is pretty straight forward just pass the --savemasterpassword flag when executing the create-domain command

${glassfish_install}/bin/asadmin create-domain --savemasterpassword mydomain
Enter admin user name [Enter to accept default "admin" / no password]> admin
Enter the admin password [Enter to accept default of no password]>
Enter the admin password again>
Enter the master password [Enter to accept default password "changeit"]>
Enter the master password again>
Default port 4848 for Admin is in use. Using 55187
Default port 8080 for HTTP Instance is in use. Using 55188
Default port 7676 for JMS is in use. Using 55189
Default port 3700 for IIOP is in use. Using 55190
Default port 8181 for HTTP_SSL is in use. Using 55191
Using default port 3820 for IIOP_SSL.
Using default port 3920 for IIOP_MUTUALAUTH.
Default port 8686 for JMX_ADMIN is in use. Using 55192
Using default port 6666 for OSGI_SHELL.
Using default port 9009 for JAVA_DEBUGGER.
Distinguished Name of the self-signed X.509 Server Certificate is:
[CN=mymachine,OU=GlassFish,O=Oracle Corporation,L=Santa Clara,ST=California,C=US]
Distinguished Name of the self-signed X.509 Server Certificate is:
[CN=mymachine,OU=GlassFish,O=Oracle Corporation,L=Santa Clara,ST=California,C=US]
No domain initializers found, bypassing customization step
Domain mydomain created.
Domain mydomain admin port is 55187.
Domain mydomain admin user is "admin".
Command create-domain executed successfully.

This will create a master-password file under

${glassfish_install}/glassfish/domains/mydomain/master-password

If you forgot to pass the --savemasterpassword flag when creating your domain you can still ask glassfish to generate this file for you by using the change-master-password command more info here

Just run the following command and enter the appropriate passwords when prompted. For this command to work the glassfish instance must be stopped

${glassfish_install}/bin/asadmin change-master-password --savemasterpassword 
Enter the new master password>
Enter the new master password again>
Command change-master-password executed successfully.

Note: If you did not specify a master password when you created your domain, the asadmin tool will use the default glassfish password : changeit