


The first step in implementing a security policy is assigning a trusted identity to our users, which means that we must authenticate them. Web applications usually adopt a form-based authentication with a login form that asks user for a unique username and the relative password:

image::../img/wikipedia-login-form.png[]

Wicket supports form-based authentication with session class _AuthenticatedWebSession_ and application class _AuthenticatedWebApplication_, both placed inside package _org.apache.wicket.authroles.authentication_.

=== AuthenticatedWebSession

Class AuthenticatedWebSession comes with the following set of public methods to manage user authentication:

* *authenticate(String username, String password)*: this is an abstract method that must be implemented by every subclass of _AuthenticatedWebSession_. It should contain the actual code that checks for user's identity. It returns a boolean value which is true if authentication has succeeded or false otherwise.
* *signIn(String username, String password)*: this method internally calls authenticate and set the flag signedIn to true if authentication succeeds.
* *isSignedIn()*:getter method for flag signedIn.
* *invalidate()*: sets the flag signedIn to false and invalidates session.
* *signOut()*: an alias of *invalidate()*.

Another abstract method we must implement when we use _AuthenticatedWebSession_ is  getRoles which is inherited from parent class _AbstractAuthenticatedWebSession_. This method can be ignored for now as it will be discussed later when we will talk about role-based authorization.

=== AuthenticatedWebApplication

Class AuthenticatedWebApplication provides the following methods to support form-based authentication:

* *getWebSessionClass()*: abstract method that returns the session class to use for this application. The returned class must be a subclass of _AbstractAuthenticatedWebSession_.
* *getSignInPageClass()*: abstract method that returns the page to use as sign in page when a user must be authenticated.
* *restartResponseAtSignInPage()*: forces the current response to restart at the sign in page. After we have used this method to redirect a user, we can make her/him return to the original page calling _Component_'s method _continueToOriginalDestination()_.

The other methods implemented inside _AuthenticatedWebApplication_ will be introduced when we talk about authorization.

=== A basic example of authentication

Project _BasicAuthenticationExample_ is a basic example of form-based authentication implemented with classes _AuthenticatedWebSession_ and _AuthenticatedWebApplication_.

The homepage of the project contains only a link to page _AuthenticatedPage_ which can be accessed only if user is signed in. The code of _AuthenticatedPage_ is this following:

[source,java]
----
public class AuthenticatedPage extends WebPage {
   @Override
   protected void onConfigure() {
      super.onConfigure();
      AuthenticatedWebApplication app = (AuthenticatedWebApplication)Application.get();
      //if user is not signed in, redirect him to sign in page
      if(!AuthenticatedWebSession.get().isSignedIn())
         app.restartResponseAtSignInPage();
   }
   
   @Override
   protected void onInitialize() {
      super.onInitialize();
      add(new BookmarkablePageLink<Void>("goToHomePage", getApplication().getHomePage()));

      add(new Link<Void>("logOut") {

         @Override
         public void onClick() {
            AuthenticatedWebSession.get().invalidate();
            setResponsePage(getApplication().getHomePage());
         }
      });
   }
}
----

Page _AuthenticatedPage_ checks inside onConfigure if user is signed in and if not, it redirects her/him to the sign in page with method _restartResponseAtSignInPage_. The page contains also a link to the homepage and another link that signs out user. 

The sign in page is implemented in class _SignInPage_ and contains the form used to authenticate users:

[source,java]
----
public class SignInPage extends WebPage {
   private String username;
   private String password;
   
   @Override
   protected void onInitialize() {
      super.onInitialize();
      
      StatelessForm<Void> form = new StatelessForm<Void>("form") {
         @Override
         protected void onSubmit() {
            if(Strings.isEmpty(username))
               return;
            
            boolean authResult = AuthenticatedWebSession.get().signIn(username, password);
            //if authentication succeeds redirect user to the requested page
            if(authResult)
               continueToOriginalDestination();
         }
      };
      
      form.setModel(new CompoundPropertyModel(this));
      
      form.add(new TextField("username"));
      form.add(new PasswordTextField("password"));
      
      add(form);
   }
}
----

The form is responsible for handling user authentication inside its method _onSubmit()_. The username and password are passed to _AuthenticatedWebSession_'s method _signIn(username, password)_ and if authentication succeeds, the user is redirected to the original page with method _continueToOriginalDestination_.

The session class and the application class used in the project are reported here:

*Session class:*

[source,java]
----
public class BasicAuthenticationSession extends AuthenticatedWebSession {

	public BasicAuthenticationSession(Request request) {
		super(request);
	}

	@Override
	public boolean authenticate(String username, String password) {
	      //user is authenticated if both username and password are equal to 'wicketer'
		return username.equals(password) && username.equals("wicketer");
	}

	@Override
	public Roles getRoles() {
		return new Roles();
	}
}
----

*Application class:*

[source,java]
----
public class WicketApplication extends AuthenticatedWebApplication {
	@Override
	public Class<HomePage> getHomePage(){
		return HomePage.class;
	}

	@Override
	protected Class<? extends AbstractAuthenticatedWebSession> getWebSessionClass(){
		return BasicAuthenticationSession.class;
	}

	@Override
	protected Class<? extends WebPage> getSignInPageClass() {
		return SignInPage.class;
	}
}
----

The authentication logic inside authenticate has been kept quite trivial in order to make the code as clean as possible. Please note also that session class must have a constructor that accepts an instance of class _Request_.

=== Redirecting user to an intermediate page

Method _restartResponseAtSignInPage_ is an example of redirecting user to an intermediate page before allowing him to access to the requested page. This method internally throws exception _org.apache.wicket.RestartResponseAtInterceptPageException_ which saves the URL and the parameters of the requested page into session metadata and then redirects user to the page passed as constructor parameter (the sign in page).

Component's method _redirectToInterceptPage(Page)_ works in much the same way as _restartResponseAtSignInPage_ but it allows us to specify which page to use as intermediate page:

[source,java]
----
    redirectToInterceptPage(intermediatePage);
----

NOTE: Since both _restartResponseAtSignInPage_ and _redirectToInterceptPage_ internally throw an exception, the code placed after them will not be executed.

