Default resolution of non-annotated Principal argument

Closes gh-25981

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/PrincipalMethodArgumentResolver.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2002-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.web.servlet.mvc.method.annotation;
+
+import java.security.Principal;
+
+import javax.servlet.http.HttpServletRequest;
+
+import org.springframework.core.MethodParameter;
+import org.springframework.lang.Nullable;
+import org.springframework.web.bind.support.WebDataBinderFactory;
+import org.springframework.web.context.request.NativeWebRequest;
+import org.springframework.web.method.support.HandlerMethodArgumentResolver;
+import org.springframework.web.method.support.ModelAndViewContainer;
+
+/**
+ * Resolves an argument of type {@link Principal}, similar to
+ * {@link ServletRequestMethodArgumentResolver} but irrespective of whether the
+ * argument is annotated or not. This is doen to enable custom argument
+ * resolution of a {@link Principal} argument (with custom annotation).
+ *
+ * @author Rossen Stoyanchev
+ * @since 5.3.1
+ */
+public class PrincipalMethodArgumentResolver implements HandlerMethodArgumentResolver {
+
+
+	@Override
+	public boolean supportsParameter(MethodParameter parameter) {
+		Class<?> paramType = parameter.getParameterType();
+		return Principal.class.isAssignableFrom(paramType);
+	}
+
+	@Override
+	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
+			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
+
+		Class<?> paramType = parameter.getParameterType();
+
+		HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
+		if (request == null) {
+			throw new IllegalStateException(
+					"Current request is not of type [HttpServletRequest]: " + webRequest);
+		}
+
+		Principal principal = request.getUserPrincipal();
+		if (principal != null && !paramType.isInstance(principal)) {
+			throw new IllegalStateException(
+					"Current user principal is not of type [" + paramType.getName() + "]: " + principal);
+		}
+
+		return principal;
+	}
+
+}

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter.java

@@ -715,6 +715,7 @@ public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
 		}
 
 		// Catch-all
+		resolvers.add(new PrincipalMethodArgumentResolver());
 		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
 
 		return resolvers;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ServletRequestMethodArgumentResolver.java

@@ -50,7 +50,9 @@ import org.springframework.web.servlet.support.RequestContextUtils;
  * <li>{@link MultipartRequest}
  * <li>{@link HttpSession}
  * <li>{@link PushBuilder} (as of Spring 5.0 on Servlet 4.0)
- * <li>{@link Principal}
+ * <li>{@link Principal} but only if not annotated in order to allow custom
+ * resolvers to resolve it, and the falling back on
+ * {@link PrincipalMethodArgumentResolver}.
  * <li>{@link InputStream}
  * <li>{@link Reader}
  * <li>{@link HttpMethod} (as of Spring 4.0)