Allow AOP proxies to be created using the original ClassLoader

Closes gh-26601

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/AbstractAutoProxyCreator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -46,6 +46,7 @@ import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.config.ConfigurableBeanFactory;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.config.SmartInstantiationAwareBeanPostProcessor;
+import org.springframework.core.SmartClassLoader;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -458,7 +459,11 @@ public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
 			proxyFactory.setPreFiltered(true);
 		}
 
-		return proxyFactory.getProxy(getProxyClassLoader());
+		ClassLoader targetClassLoader = getProxyClassLoader();
+		if (targetClassLoader instanceof SmartClassLoader && targetClassLoader != beanClass.getClassLoader()) {
+			targetClassLoader = ((SmartClassLoader) targetClassLoader).getOriginalClassLoader();
+		}
+		return proxyFactory.getProxy(targetClassLoader);
 	}
 
 	/**

spring-core/src/main/java/org/springframework/core/SmartClassLoader.java

@@ -47,6 +47,28 @@ public interface SmartClassLoader {
 		return false;
 	}
 
+	/**
+	 * Return the original ClassLoader for this SmartClassLoader, or potentially
+	 * the present loader itself if it is self-sufficient.
+	 * <p>The default implementation returns the local ClassLoader reference as-is.
+	 * In case of a reloadable or other selectively overriding ClassLoader which
+	 * commonly deals with unaffected classes from a base application class loader,
+	 * this should get implemented to return the original ClassLoader that the
+	 * present loader got derived from (e.g. through {@code return getParent();}).
+	 * <p>This gets specifically used in Spring's AOP framework to determine the
+	 * class loader for a specific proxy in case the target class has not been
+	 * defined in the present class loader. In case of a reloadable class loader,
+	 * we prefer the base application class loader for proxying general classes
+	 * not defined in the reloadable class loader itself.
+	 * @return the original ClassLoader (the same reference by default)
+	 * @since 5.3.5
+	 * @see ClassLoader#getParent()
+	 * @see org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator
+	 */
+	default ClassLoader getOriginalClassLoader() {
+		return (ClassLoader) this;
+	}
+
 	/**
 	 * Define a custom class (typically a CGLIB proxy class) in this class loader.
 	 * <p>This is a public equivalent of the protected