Document Migration to SecurityContextHolderFilter

Closes gh-12098

docs/modules/ROOT/pages/migration.adoc

@@ -13,6 +13,21 @@ endif::[]
 
 == Servlet
 
+=== Explicit Save SecurityContextRepository
+
+In Spring Security 5, the default behavior is for the xref:servlet/authentication/architecture.adoc#servlet-authentication-securitycontext[`SecurityContext`] to automatically be saved to the xref:servlet/authentication/persistence.adoc#securitycontextrepository[`SecurityContextRepository`] using the xref:servlet/authentication/persistence.adoc#securitycontextpersistencefilter[`SecurityContextPersistenceFilter`].
+Saving must be done just prior to the `HttpServletResponse` being committed and just before `SecurityContextPersistenceFilter`.
+Unfortunately, automatic persistence of the `SecurityContext` can surprise users when it is done prior to the request completing (i.e. just prior to committing the `HttpServletResponse`).
+It also is complex to keep track of the state to determine if a save is necessary causing unnecessary writes to the `SecurityContextRepository` (i.e. `HttpSession`) at times.
+
+In Spring Security 6, the default behavior is that the xref:servlet/authentication/persistence.adoc#securitycontextholderfilter[`SecurityContextHolderFilter`] will only read the `SecurityContext` from  `SecurityContextRepository` and populate it in the `SecurityContextHolder`.
+Users now must explicitly save the `SecurityContext` with the `SecurityContextRepository` if they want the `SecurityContext` to persist between requests.
+This removes ambiguity and improves performance by only requiring writing to the `SecurityContextRepository` (i.e. `HttpSession`) when it is necessary.
+
+To opt into the new Spring Security 6 default, the following configuration can be used.
+
+include::partial$servlet/architecture/security-context-explicit.adoc[]
+
 [[requestcache-query-optimization]]
 === Optimize Querying of `RequestCache`
 

docs/modules/ROOT/pages/servlet/authentication/persistence.adoc

@@ -144,29 +144,8 @@ image::{figures}/securitycontextholderfilter.png[]
 <1> Before running the rest of the application, `SecurityContextHolderFilter` loads the `SecurityContext` from the `SecurityContextRepository` and sets it on the `SecurityContextHolder`.
 <2> Next, the application is ran.
 
-Unlike, xref:servlet/authentication/persistence.adoc#securitycontextpersistencefilter[`SecurityContextPersisteneFilter`], `SecurityContextHolderFilter` only loads the `SecurityContext` it does not save the `SecurityContext`.
+Unlike, xref:servlet/authentication/persistence.adoc#securitycontextpersistencefilter[`SecurityContextPersistenceFilter`], `SecurityContextHolderFilter` only loads the `SecurityContext` it does not save the `SecurityContext`.
 This means that when using `SecurityContextHolderFilter`, it is required that the `SecurityContext` is explicitly saved.
 
-.Explicit Saving of SecurityContext
-====
-.Java
-[source,java,role="primary"]
-----
-public SecurityFilterChain filterChain(HttpSecurity http) {
-	http
-		// ...
-		.securityContext((securityContext) -> securityContext
-			.requireExplicitSave(true)
-		);
-	return http.build();
-}
-----
 
-.XML
+include::partial$servlet/architecture/security-context-explicit.adoc[]
-[source,xml,role="secondary"]
-----
-<http security-context-explicit-save="true">
-	<!-- ... -->
-</http>
-----
-====

docs/modules/ROOT/partials/servlet/architecture/security-context-explicit.adoc

@@ -0,0 +1,76 @@
+.Explicit Saving of SecurityContext
+====
+.Java
+[source,java,role="primary"]
+----
+public SecurityFilterChain filterChain(HttpSecurity http) {
+	http
+		// ...
+		.securityContext((securityContext) -> securityContext
+			.requireExplicitSave(true)
+		);
+	return http.build();
+}
+----
+
+.Kotlin
+[source,kotlin,role="secondary"]
+----
+@Bean
+open fun springSecurity(http: HttpSecurity): SecurityFilterChain {
+    http {
+        securityContext {
+            requireExplicitSave = true
+        }
+    }
+    return http.build()
+}
+----
+
+.XML
+[source,xml,role="secondary"]
+----
+<http security-context-explicit-save="true">
+	<!-- ... -->
+</http>
+----
+====
+
+
+Upon using the configuration, it is important that any code that sets the `SecurityContextHolder` with a `SecurityContext` also saves the `SecurityContext` to the `SecurityContextRepository` if it should be persisted between requests.
+
+For example, the following code:
+
+.Setting `SecurityContextHolder` with `SecurityContextPersistenceFilter`
+====
+.Java
+[source,java,role="primary"]
+----
+SecurityContextHolder.setContext(securityContext);
+----
+
+.Kotlin
+[source,kotlin,role="secondary"]
+----
+SecurityContextHolder.setContext(securityContext)
+----
+====
+
+should be replaced with
+
+.Setting `SecurityContextHolder` with `SecurityContextHolderFilter`
+====
+.Java
+[source,java,role="primary"]
+----
+SecurityContextHolder.setContext(securityContext);
+securityContextRepository.saveContext(securityContext, httpServletRequest, httpServletResponse);
+----
+
+.Kotlin
+[source,kotlin,role="secondary"]
+----
+SecurityContextHolder.setContext(securityContext)
+securityContextRepository.saveContext(securityContext, httpServletRequest, httpServletResponse)
+----
+====