Servlet Authentication Architecture

At the heart of Spring Security’s authentication model is the SecurityContextHolder. It contains the SecurityContext.

The SecurityContextHolder is where Spring Security stores the details of who is authenticated. Spring Security does not care how the SecurityContextHolder is populated. If it contains a value, it is used as the currently authenticated user.

The simplest way to indicate a user is authenticated is to set the SecurityContextHolder directly:

To obtain information about the authenticated principal, access the SecurityContextHolder.

By default, SecurityContextHolder uses a ThreadLocal to store these details, which means that the SecurityContext is always available to methods in the same thread, even if the SecurityContext is not explicitly passed around as an argument to those methods. Using a ThreadLocal in this way is quite safe if you take care to clear the thread after the present principal’s request is processed. Spring Security’s FilterChainProxy ensures that the SecurityContext is always cleared.

Some applications are not entirely suitable for using a ThreadLocal, because of the specific way they work with threads. For example, a Swing client might want all threads in a Java Virtual Machine to use the same security context. You can configure SecurityContextHolder with a strategy on startup to specify how you would like the context to be stored. For a standalone application, you would use the SecurityContextHolder.MODE_GLOBAL strategy. Other applications might want to have threads spawned by the secure thread also assume the same security identity. You can achieve this by using SecurityContextHolder.MODE_INHERITABLETHREADLOCAL. You can change the mode from the default SecurityContextHolder.MODE_THREADLOCAL in two ways. The first is to set a system property. The second is to call a static method on SecurityContextHolder. Most applications need not change from the default. However, if you do, take a look at the JavaDoc for SecurityContextHolder to learn more.

The SecurityContext is obtained from the SecurityContextHolder. The SecurityContext contains an Authentication object.

The Authentication interface serves two main purposes within Spring Security:

The Authentication contains:

GrantedAuthority instances are high-level permissions that the user is granted. Two examples are roles and scopes.

You can obtain GrantedAuthority instances from the Authentication.getAuthorities() method. This method provides a Collection of GrantedAuthority objects. A GrantedAuthority is, not surprisingly, an authority that is granted to the principal. Such authorities are usually “roles”, such as ROLE_ADMINISTRATOR or ROLE_HR_SUPERVISOR. These roles are later configured for web authorization, method authorization, and domain object authorization. Other parts of Spring Security interpret these authorities and expect them to be present. When using username/password based authentication GrantedAuthority instances are usually loaded by the UserDetailsService.

Usually, the GrantedAuthority objects are application-wide permissions. They are not specific to a given domain object. Thus, you would not likely have a GrantedAuthority to represent a permission to Employee object number 54, because if there are thousands of such authorities you would quickly run out of memory (or, at the very least, cause the application to take a long time to authenticate a user). Of course, Spring Security is expressly designed to handle this common requirement, but you should instead use the project’s domain object security capabilities for this purpose.

AuthenticationManager is the API that defines how Spring Security’s Filters perform authentication. The Authentication that is returned is then set on the SecurityContextHolder by the controller (that is, by Spring Security’s Filters instances) that invoked the AuthenticationManager. If you are not integrating with Spring Security’s Filters instances, you can set the SecurityContextHolder directly and are not required to use an AuthenticationManager.

While the implementation of AuthenticationManager could be anything, the most common implementation is ProviderManager.

ProviderManager is the most commonly used implementation of AuthenticationManager. ProviderManager delegates to a List of AuthenticationProvider instances. Each AuthenticationProvider has an opportunity to indicate that authentication should be successful, fail, or indicate it cannot make a decision and allow a downstream AuthenticationProvider to decide. If none of the configured AuthenticationProvider instances can authenticate, authentication fails with a ProviderNotFoundException, which is a special AuthenticationException that indicates that the ProviderManager was not configured to support the type of Authentication that was passed into it.

In practice each AuthenticationProvider knows how to perform a specific type of authentication. For example, one AuthenticationProvider might be able to validate a username/password, while another might be able to authenticate a SAML assertion. This lets each AuthenticationProvider do a very specific type of authentication while supporting multiple types of authentication and expose only a single AuthenticationManager bean.

ProviderManager also allows configuring an optional parent AuthenticationManager, which is consulted in the event that no AuthenticationProvider can perform authentication. The parent can be any type of AuthenticationManager, but it is often an instance of ProviderManager.

In fact, multiple ProviderManager instances might share the same parent AuthenticationManager. This is somewhat common in scenarios where there are multiple SecurityFilterChain instances that have some authentication in common (the shared parent AuthenticationManager), but also different authentication mechanisms (the different ProviderManager instances).

By default, ProviderManager tries to clear any sensitive credentials information from the Authentication object that is returned by a successful authentication request. This prevents information, such as passwords, being retained longer than necessary in the HttpSession.

This may cause issues when you use a cache of user objects, for example, to improve performance in a stateless application. If the Authentication contains a reference to an object in the cache (such as a UserDetails instance) and this has its credentials removed, it is no longer possible to authenticate against the cached value. You need to take this into account if you use a cache. An obvious solution is to first make a copy of the object, either in the cache implementation or in the AuthenticationProvider that creates the returned Authentication object. Alternatively, you can disable the eraseCredentialsAfterAuthentication property on ProviderManager. See the Javadoc for the ProviderManager class.

You can inject multiple AuthenticationProvider instances into ProviderManager. Each AuthenticationProvider performs a specific type of authentication. For example, DaoAuthenticationProvider supports username/password-based authentication, while JwtAuthenticationProvider supports authenticating a JWT token.

AuthenticationEntryPoint is used to send an HTTP response that requests credentials from a client.

Sometimes, a client proactively includes credentials (such as a username and password) to request a resource. In these cases, Spring Security does not need to provide an HTTP response that requests credentials from the client, since they are already included.

In other cases, a client makes an unauthenticated request to a resource that they are not authorized to access. In this case, an implementation of AuthenticationEntryPoint is used to request credentials from the client. The AuthenticationEntryPoint implementation might perform a redirect to a log in page, respond with an WWW-Authenticate header, or take other action.

AbstractAuthenticationProcessingFilter is used as a base Filter for authenticating a user’s credentials. Before the credentials can be authenticated, Spring Security typically requests the credentials by using AuthenticationEntryPoint.

Next, the AbstractAuthenticationProcessingFilter can authenticate any authentication requests that are submitted to it.

When the user submits their credentials, the AbstractAuthenticationProcessingFilter creates an Authentication from the HttpServletRequest to be authenticated. The type of Authentication created depends on the subclass of AbstractAuthenticationProcessingFilter. For example, UsernamePasswordAuthenticationFilter creates a UsernamePasswordAuthenticationToken from a username and password that are submitted in the HttpServletRequest.

Next, the Authentication is passed into the AuthenticationManager to be authenticated.

If authentication fails, then Failure.

If authentication is successful, then Success.