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, then it is used as the currently authenticated user.

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

If you wish to obtain information about the authenticated principal, you can do so by accessing the SecurityContextHolder.

By default the 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 care is taken 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 aren’t 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. SecurityContextHolder can be configured 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. This is achieved 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 won’t need to change from the default, but 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 serves two main purposes within Spring Security:

The Authentication contains:

GrantedAuthoritys are high level permissions the user is granted. A few examples are roles or scopes.

GrantedAuthoritys can be obtained 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 on configured for web authorization, method authorization and domain object authorization. Other parts of Spring Security are capable of interpreting these authorities, and expect them to be present. When using username/password based authentication GrantedAuthoritys 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 wouldn’t 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’d 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 (i.e. Spring Security’s Filterss) that invoked the AuthenticationManager. If you are not integrating with Spring Security’s Filterss 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 AuthenticationProviders. 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 AuthenticationProviders can authenticate, then authentication will fail with a ProviderNotFoundException which is a special AuthenticationException that indicates 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 allows each AuthenticationProvider to do a very specific type of authentication, while supporting multiple types of authentication and only exposing 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 will attempt to clear any sensitive credentials information from the Authentication object which is returned by a successful authentication request. This prevents information like passwords being retained longer than necessary in the HttpSession.

This may cause issues when you are using 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, then it will no longer be possible to authenticate against the cached value. You need to take this into account if you are using a cache. An obvious solution is to make a copy of the object first, either in the cache implementation or in the AuthenticationProvider which creates the returned Authentication object. Alternatively, you can disable the eraseCredentialsAfterAuthentication property on ProviderManager. See the Javadoc for more information.

Multiple AuthenticationProviders can be injected 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 will proactively include credentials such as a username/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 will make 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, etc.

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 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.