Make ApplicationContextServerWebExchangeMatcher and subclasses thread-safe

Fixes gh-14161

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/security/reactive/EndpointRequest.java

@@ -125,7 +125,7 @@ public final class EndpointRequest {
 
 		private final boolean includeLinks;
 
-		private ServerWebExchangeMatcher delegate;
+		private volatile ServerWebExchangeMatcher delegate;
 
 		private EndpointServerWebExchangeMatcher(boolean includeLinks) {
 			this(Collections.emptyList(), Collections.emptyList(), includeLinks);
@@ -246,7 +246,7 @@ public final class EndpointRequest {
 	public static final class LinksServerWebExchangeMatcher
 			extends ApplicationContextServerWebExchangeMatcher<WebEndpointProperties> {
 
-		private ServerWebExchangeMatcher delegate;
+		private volatile ServerWebExchangeMatcher delegate;
 
 		private LinksServerWebExchangeMatcher() {
 			super(WebEndpointProperties.class);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/security/reactive/ApplicationContextServerWebExchangeMatcher.java

@@ -70,8 +70,9 @@ public abstract class ApplicationContextServerWebExchangeMatcher<C>
 		if (this.context == null) {
 			synchronized (this.contextLock) {
 				if (this.context == null) {
-					this.context = createContext(exchange);
-					initialized(this.context);
+					Supplier<C> createdContext = createContext(exchange);
+					initialized(createdContext);
+					this.context = createdContext;
 				}
 			}
 		}