Ensure ErrorControllers work when using AOP

Add a BeanFactoryPostProcessor to set PRESERVE_TARGET_CLASS_ATTRIBUTE
to true on all ErrorController bean definitions. Without this attribute
AOP advice on @Controllers causes ErrorController beans to be created
as JDK proxies (since they implement a single valid looking interface)
and therefore not get found by Spring MVC.

Fixes gh-4236

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorMvcAutoConfiguration.java

@@ -24,8 +24,12 @@ import javax.servlet.Servlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
+import org.springframework.aop.framework.autoproxy.AutoProxyUtils;
+import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Value;
+import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
+import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.boot.autoconfigure.AutoConfigureBefore;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionOutcome;
@@ -102,6 +106,11 @@ public class ErrorMvcAutoConfiguration
 				new ErrorPage(this.properties.getServletPrefix() + this.errorPath));
 	}
 
+	@Bean
+	public static PreserveErrorControllerTargetClassPostProcessor preserveErrorControllerTargetClassPostProcessor() {
+		return new PreserveErrorControllerTargetClassPostProcessor();
+	}
+
 	@Configuration
 	@ConditionalOnProperty(prefix = "error.whitelabel", name = "enabled", matchIfMissing = true)
 	@Conditional(ErrorTemplateMissingCondition.class)
@@ -225,4 +234,29 @@ public class ErrorMvcAutoConfiguration
 
 	}
 
+	/**
+	 * {@link BeanFactoryPostProcessor} to ensure that the target class of ErrorController
+	 * MVC beans are preserved when using AOP.
+	 */
+	static class PreserveErrorControllerTargetClassPostProcessor
+			implements BeanFactoryPostProcessor {
+
+		@Override
+		public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)
+				throws BeansException {
+			String[] errorControllerBeans = beanFactory
+					.getBeanNamesForType(ErrorController.class, false, false);
+			for (String errorControllerBean : errorControllerBeans) {
+				try {
+					beanFactory.getBeanDefinition(errorControllerBean).setAttribute(
+							AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);
+				}
+				catch (Throwable ex) {
+					// Ignore
+				}
+			}
+		}
+
+	}
+
 }

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/BasicErrorControllerDirectMockMvcTests.java

@@ -24,6 +24,10 @@ import java.lang.annotation.Target;
 
 import javax.servlet.ServletException;
 
+import org.aspectj.lang.ProceedingJoinPoint;
+import org.aspectj.lang.annotation.Around;
+import org.aspectj.lang.annotation.Aspect;
+import org.aspectj.lang.annotation.Pointcut;
 import org.junit.After;
 import org.junit.Rule;
 import org.junit.Test;
@@ -34,6 +38,7 @@ import org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfigurati
 import org.springframework.boot.builder.SpringApplicationBuilder;
 import org.springframework.boot.test.ApplicationContextTestUtils;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.EnableAspectJAutoProxy;
 import org.springframework.context.annotation.Import;
 import org.springframework.http.MediaType;
 import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
@@ -101,11 +106,21 @@ public class BasicErrorControllerDirectMockMvcTests {
 		setup((ConfigurableWebApplicationContext) new SpringApplication(
 				WebMvcIncludedConfiguration.class).run("--server.port=0",
 						"--error.whitelabel.enabled=false"));
-
-		thrown.expect(ServletException.class);
+		this.thrown.expect(ServletException.class);
 		this.mockMvc.perform(get("/error").accept(MediaType.TEXT_HTML));
 	}
 
+	@Test
+	public void errorControllerWithAop() throws Exception {
+		setup((ConfigurableWebApplicationContext) new SpringApplication(
+				WithAopConfiguration.class).run("--server.port=0"));
+		MvcResult response = this.mockMvc
+				.perform(get("/error").accept(MediaType.TEXT_HTML))
+				.andExpect(status().isOk()).andReturn();
+		String content = response.getResponse().getContentAsString();
+		assertTrue("Wrong content: " + content, content.contains("status=999"));
+	}
+
 	@Target(ElementType.TYPE)
 	@Retention(RetentionPolicy.RUNTIME)
 	@Documented
@@ -115,6 +130,7 @@ public class BasicErrorControllerDirectMockMvcTests {
 			HttpMessageConvertersAutoConfiguration.class, ErrorMvcAutoConfiguration.class,
 			PropertyPlaceholderAutoConfiguration.class })
 	protected static @interface MinimalWebConfiguration {
+
 	}
 
 	@Configuration
@@ -127,6 +143,7 @@ public class BasicErrorControllerDirectMockMvcTests {
 	@MinimalWebConfiguration
 	@EnableWebMvc
 	protected static class WebMvcIncludedConfiguration {
+
 		// For manual testing
 		public static void main(String[] args) {
 			SpringApplication.run(WebMvcIncludedConfiguration.class, args);
@@ -137,6 +154,7 @@ public class BasicErrorControllerDirectMockMvcTests {
 	@Configuration
 	@MinimalWebConfiguration
 	protected static class VanillaConfiguration {
+
 		// For manual testing
 		public static void main(String[] args) {
 			SpringApplication.run(VanillaConfiguration.class, args);
@@ -147,11 +165,30 @@ public class BasicErrorControllerDirectMockMvcTests {
 	@Configuration
 	@MinimalWebConfiguration
 	protected static class ChildConfiguration {
+
 		// For manual testing
 		public static void main(String[] args) {
 			new SpringApplicationBuilder(ParentConfiguration.class)
 					.child(ChildConfiguration.class).run(args);
 		}
+
+	}
+
+	@Configuration
+	@EnableAspectJAutoProxy(proxyTargetClass = false)
+	@MinimalWebConfiguration
+	@Aspect
+	protected static class WithAopConfiguration {
+
+		@Pointcut("within(@org.springframework.stereotype.Controller *)")
+		private void controllerPointCut() {
+		};
+
+		@Around("controllerPointCut()")
+		public Object mvcAdvice(ProceedingJoinPoint pjp) throws Throwable {
+			return pjp.proceed();
+		}
+
 	}
 
 }