[[servlet-authentication-securitycontextholder]]
= SecurityContextHolder

:figures: images/servlet/authentication/architecture

At the heart of Spring Security's authentication model is the `SecurityContextHolder`.
It contains the <<servlet-authentication-securitycontext>>.

image::{figures}/securitycontextholder.png[]

The `SecurityContextHolder` is where Spring Security stores the details of who is <<authentication,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.

.Setting `SecurityContextHolder`
====
.Java
[source,java,role="primary"]
----
SecurityContext context = SecurityContextHolder.createEmptyContext(); // <1>
Authentication authentication =
    new TestingAuthenticationToken("username", "password", "ROLE_USER"); // <2>
context.setAuthentication(authentication);

SecurityContextHolder.setContext(context); // <3>
----

.Kotlin
[source,kotlin,role="secondary"]
----
val context: SecurityContext = SecurityContextHolder.createEmptyContext() // <1>
val authentication: Authentication = TestingAuthenticationToken("username", "password", "ROLE_USER") // <2>
context.authentication = authentication

SecurityContextHolder.setContext(context) // <3>
----
====

<1> We start by creating an empty `SecurityContext`.
It is important to create a new `SecurityContext` instance instead of using `SecurityContextHolder.getContext().setAuthentication(authentication)` to avoid race conditions across multiple threads.
<2> Next we create a new <<servlet-authentication-authentication,`Authentication`>> object.
Spring Security does not care what type of `Authentication` implementation is set on the `SecurityContext`.
Here we use `TestingAuthenticationToken` because it is very simple.
A more common production scenario is `UsernamePasswordAuthenticationToken(userDetails, password, authorities)`.
<3> Finally, we set the `SecurityContext` on the `SecurityContextHolder`.
Spring Security will use this information for <<servlet-authorization,authorization>>.

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

.Access Currently Authenticated User
====
.Java
[source,java,role="primary"]
----
SecurityContext context = SecurityContextHolder.getContext();
Authentication authentication = context.getAuthentication();
String username = authentication.getName();
Object principal = authentication.getPrincipal();
Collection<? extends GrantedAuthority> authorities = authentication.getAuthorities();
----

.Kotlin
[source,kotlin,role="secondary"]
----
val context = SecurityContextHolder.getContext()
val authentication = context.authentication
val username = authentication.name
val principal = authentication.principal
val authorities = authentication.authorities
----
====

// FIXME: add links to HttpServletRequest.getRemoteUser() and @CurrentSecurityContext @AuthenticationPrincipal

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 <<servlet-filterchainproxy,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.
