Add Nullable Annotations

Added Nullable to methods that may return a null value

Closes gh-18046

web/src/main/java/org/springframework/security/web/authentication/preauth/RequestAttributeAuthenticationFilter.java

@@ -58,7 +58,7 @@ public class RequestAttributeAuthenticationFilter extends AbstractPreAuthenticat
 	 * missing and {@code exceptionIfVariableMissing} is set to {@code true}.
 	 */
 	@Override
-	protected Object getPreAuthenticatedPrincipal(HttpServletRequest request) {
+	protected @Nullable Object getPreAuthenticatedPrincipal(HttpServletRequest request) {
 		String principal = (String) request.getAttribute(this.principalEnvironmentVariable);
 		if (principal == null && this.exceptionIfVariableMissing) {
 			throw new PreAuthenticatedCredentialsNotFoundException(
@@ -73,7 +73,7 @@ public class RequestAttributeAuthenticationFilter extends AbstractPreAuthenticat
 	 * credentials value. Otherwise a dummy value will be used.
 	 */
 	@Override
-	protected Object getPreAuthenticatedCredentials(HttpServletRequest request) {
+	protected @Nullable Object getPreAuthenticatedCredentials(HttpServletRequest request) {
 		if (this.credentialsEnvironmentVariable != null) {
 			return request.getAttribute(this.credentialsEnvironmentVariable);
 		}

web/src/main/java/org/springframework/security/web/authentication/preauth/RequestHeaderAuthenticationFilter.java

@@ -59,7 +59,7 @@ public class RequestHeaderAuthenticationFilter extends AbstractPreAuthenticatedP
 	 * {@code exceptionIfHeaderMissing} is set to {@code true}.
 	 */
 	@Override
-	protected Object getPreAuthenticatedPrincipal(HttpServletRequest request) {
+	protected @Nullable Object getPreAuthenticatedPrincipal(HttpServletRequest request) {
 		String principal = request.getHeader(this.principalRequestHeader);
 		if (principal == null && this.exceptionIfHeaderMissing) {
 			throw new PreAuthenticatedCredentialsNotFoundException(
@@ -74,7 +74,7 @@ public class RequestHeaderAuthenticationFilter extends AbstractPreAuthenticatedP
 	 * will be used.
 	 */
 	@Override
-	protected Object getPreAuthenticatedCredentials(HttpServletRequest request) {
+	protected @Nullable Object getPreAuthenticatedCredentials(HttpServletRequest request) {
 		if (this.credentialsRequestHeader != null) {
 			return request.getHeader(this.credentialsRequestHeader);
 		}