Support @AuthenticationPrincipal on interfaces

Closes gh-16177

Author: kse-music

core/src/main/java/org/springframework/security/core/annotation/UniqueSecurityAnnotationScanner.java

@@ -18,9 +18,11 @@
 
 import java.lang.annotation.Annotation;
 import java.lang.reflect.AnnotatedElement;
+import java.lang.reflect.Executable;
 import java.lang.reflect.Method;
 import java.lang.reflect.Parameter;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
@@ -29,6 +31,7 @@
 import java.util.concurrent.ConcurrentHashMap;
 
 import org.springframework.core.MethodClassKey;
+import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotationConfigurationException;
 import org.springframework.core.annotation.MergedAnnotation;
 import org.springframework.core.annotation.MergedAnnotations;
@@ -103,11 +106,75 @@ final class UniqueSecurityAnnotationScanner<A extends Annotation> extends Abstra
 		this.types = types;
 	}
 
+	private List<MergedAnnotation<A>> findParameterOnInterface(Method method, Class<?> superOrIfc, Parameter current) {
+		List<MergedAnnotation<A>> directAnnotations = Collections.emptyList();
+		for (Method candidate : superOrIfc.getMethods()) {
+			if (isOverrideFor(method, candidate)) {
+				for (Parameter parameter : candidate.getParameters()) {
+					if (parameter.getName().equals(current.getName())) {
+						directAnnotations = findDirectAnnotations(parameter);
+						if (!directAnnotations.isEmpty()) {
+							return directAnnotations;
+						}
+					}
+				}
+			}
+		}
+		return directAnnotations;
+	}
+
+	private List<MergedAnnotation<A>> findParameterAnnotations(Parameter current) {
+		List<MergedAnnotation<A>> directAnnotations = findDirectAnnotations(current);
+		if (directAnnotations.isEmpty()) {
+			Executable executable = current.getDeclaringExecutable();
+			if (executable instanceof Method method) {
+				Class<?> clazz = method.getDeclaringClass();
+				while (clazz != null) {
+					for (Class<?> ifc : clazz.getInterfaces()) {
+						directAnnotations = findParameterOnInterface(method, ifc, current);
+						if (!directAnnotations.isEmpty()) {
+							return directAnnotations;
+						}
+					}
+					clazz = clazz.getSuperclass();
+					if (clazz == Object.class) {
+						clazz = null;
+					}
+					if (clazz != null) {
+						directAnnotations = findParameterOnInterface(method, clazz, current);
+						if (!directAnnotations.isEmpty()) {
+							return directAnnotations;
+						}
+					}
+				}
+			}
+		}
+		return directAnnotations;
+	}
+
+	private boolean isOverrideFor(Method method, Method candidate) {
+		if (!candidate.getName().equals(method.getName())
+				|| candidate.getParameterCount() != method.getParameterCount()) {
+			return false;
+		}
+		Class<?>[] paramTypes = method.getParameterTypes();
+		if (Arrays.equals(candidate.getParameterTypes(), paramTypes)) {
+			return true;
+		}
+		for (int i = 0; i < paramTypes.length; i++) {
+			if (paramTypes[i] != ResolvableType.forMethodParameter(candidate, i, method.getDeclaringClass())
+				.resolve()) {
+				return false;
+			}
+		}
+		return true;
+	}
+
 	@Override
 	MergedAnnotation<A> merge(AnnotatedElement element, Class<?> targetClass) {
 		if (element instanceof Parameter parameter) {
 			return this.uniqueParameterAnnotationCache.computeIfAbsent(parameter, (p) -> {
-				List<MergedAnnotation<A>> annotations = findDirectAnnotations(p);
+				List<MergedAnnotation<A>> annotations = findParameterAnnotations(p);
 				return requireUnique(p, annotations);
 			});
 		}

web/src/test/java/org/springframework/security/web/method/annotation/AuthenticationPrincipalArgumentResolverTests.java

@@ -214,6 +214,14 @@ public void resolveArgumentCustomMetaAnnotationTpl() throws Exception {
 			.isEqualTo(this.expectedPrincipal);
 	}
 
+	@Test
+	public void resolveArgumentAnnotationFromInterface() {
+		CustomUserPrincipal principal = new CustomUserPrincipal();
+		setAuthenticationPrincipal(principal);
+		assertThat(this.resolver.supportsParameter(getMethodParameter("getUserByInterface", CustomUserPrincipal.class)))
+			.isTrue();
+	}
+
 	private MethodParameter showUserNoAnnotation() {
 		return getMethodParameter("showUserNoAnnotation", String.class);
 	}
@@ -312,7 +320,18 @@ private void setAuthenticationPrincipal(Object principal) {
 
 	}
 
-	public static class TestController {
+	interface UserApi {
+
+		String getUserByInterface(@AuthenticationPrincipal CustomUserPrincipal user);
+
+	}
+
+	public static class TestController implements UserApi {
+
+		@Override
+		public String getUserByInterface(CustomUserPrincipal user) {
+			return "";
+		}
 
 		public void showUserNoAnnotation(String user) {
 		}