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> ## Application Client-Tier Security

> Authentication requirements for J2EE application clients are the same as the requirements for other J2EE components. Access to protected resources in either the EJB tier or the Web tier requires user authentication, whereas access to unprotected resources does not.

> An application client can use the Java Authentication and Authorization Service (JAAS) for authentication. JAAS implements a Java version of the standard Pluggable Authentication Module (PAM) framework, which permits applications to remain independent from underlying authentication technologies. You can plug new or updated authentication technologies under an application without making any modifications to the application itself. Applications enable the authentication process by instantiating a `LoginContext` object, which, in turn, references a configuration to determine the authentication technologies or login modules that will be used to perform the authentication.

> A typical login module could prompt for and verify a user name and password. Other modules could read and verify a voice or fingerprint sample.

> In some cases, a login module needs to communicate with the user to obtain authentication information. Login modules use a `javax.security.auth.callback.CallbackHandler` for this purpose. Applications implement the `CallbackHandler` interface and pass it to the login context, which forwards it directly to the underlying login modules. A login module uses the callback handler both to gather input (such as a password or smart card PIN number) from users or to supply information (such as status information) to users. By allowing the application to specify the callback handler, an underlying login module can remain independent of the different ways applications interact with users.

> For example, the implementation of a callback handler for a GUI application might display a window to solicit user input. Or, the implementation of a callback handler for a command line tool might simply prompt the user for input directly from the command line.

> The login module passes an array of appropriate callbacks to the callback handler's `handle` method (for example, a `NameCallback` for the user name and a `PasswordCallback` for the password), and the callback handler performs the requested user interaction and sets appropriate values in the callbacks. For example, to process a `NameCallback`, the `CallbackHandler` may prompt for a name, retrieve the value from the user, and call the `setName` method of the `NameCallback` to store the name.

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