Document limitations of CGLIB proxy class generation in JPMS module setups

Includes extended exception messages with common hints and explanations.

Closes gh-32671

Author: jhoeller

Diff for: framework-docs/modules/ROOT/pages/core/aop/proxying.adoc

@@ -19,6 +19,10 @@ you can do so. However, you should consider the following issues:
   since the CGLIB proxy instance is created through Objenesis. Only if your JVM does
   not allow for constructor bypassing, you might see double invocations and
   corresponding debug log entries from Spring's AOP support.
+* Your CGLIB proxy usage may face limitations with the JDK 9+ platform module system.
+  As a typical case, you cannot create a CGLIB proxy for a class from the `java.lang`
+  package when deploying on the module path. Such cases require a JVM bootstrap flag
+  `--add-opens=java.base/java.lang=ALL-UNNAMED` which is not available for modules.
 
 To force the use of CGLIB proxies, set the value of the `proxy-target-class` attribute
 of the `<aop:config>` element to true, as follows:

Diff for: spring-context/src/main/java/org/springframework/context/annotation/ConfigurationClassEnhancer.java

@@ -28,6 +28,7 @@
 import org.springframework.aop.scope.ScopedProxyFactoryBean;
 import org.springframework.asm.Opcodes;
 import org.springframework.asm.Type;
+import org.springframework.beans.factory.BeanDefinitionStoreException;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.NoSuchBeanDefinitionException;
@@ -37,6 +38,7 @@
 import org.springframework.beans.factory.support.SimpleInstantiationStrategy;
 import org.springframework.cglib.core.ClassGenerator;
 import org.springframework.cglib.core.ClassLoaderAwareGeneratorStrategy;
+import org.springframework.cglib.core.CodeGenerationException;
 import org.springframework.cglib.core.SpringNamingPolicy;
 import org.springframework.cglib.proxy.Callback;
 import org.springframework.cglib.proxy.CallbackFilter;
@@ -106,12 +108,19 @@ public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader)
 			}
 			return configClass;
 		}
-		Class<?> enhancedClass = createClass(newEnhancer(configClass, classLoader));
-		if (logger.isTraceEnabled()) {
-			logger.trace(String.format("Successfully enhanced %s; enhanced class name is: %s",
-					configClass.getName(), enhancedClass.getName()));
+		try {
+			Class<?> enhancedClass = createClass(newEnhancer(configClass, classLoader));
+			if (logger.isTraceEnabled()) {
+				logger.trace(String.format("Successfully enhanced %s; enhanced class name is: %s",
+						configClass.getName(), enhancedClass.getName()));
+			}
+			return enhancedClass;
+		}
+		catch (CodeGenerationException ex) {
+			throw new BeanDefinitionStoreException("Could not enhance configuration class [" + configClass.getName() +
+					"]. Consider declaring @Configuration(proxyBeanMethods=false) without inter-bean references " +
+					"between @Bean methods on the configuration class, avoiding the need for CGLIB enhancement.", ex);
 		}
-		return enhancedClass;
 	}
 
 	/**

Diff for: spring-core/src/main/java/org/springframework/cglib/core/ReflectUtils.java

@@ -544,7 +544,15 @@ public String getMessage() {
 
 		// No defineClass variant available at all?
 		if (c == null) {
-			throw new CodeGenerationException(t);
+			throw new CodeGenerationException(t) {
+				@Override
+				public String getMessage() {
+					return "No compatible defineClass mechanism detected: " +
+							"JVM should be started with --add-opens=java.base/java.lang=ALL-UNNAMED " +
+							"for ClassLoader.defineClass to be accessible. On the module path, " +
+							"you may not be able to define this CGLIB-generated class at all.";
+				}
+			};
 		}
 
 		// Force static initializers to run.