Make GeneratorStrategy.generate unreachable on native

This change provides also more information to the user
about the missing generated class when that happens.

Closes gh-29521

spring-core/src/main/java/org/springframework/aot/nativex/feature/PreComputeFieldFeature.java

@@ -34,6 +34,7 @@ class PreComputeFieldFeature implements Feature {
 
 	private static Pattern[] patterns = {
 			Pattern.compile(Pattern.quote("org.springframework.core.NativeDetector#imageCode")),
+			Pattern.compile(Pattern.quote("org.springframework.cglib.core.AbstractClassGenerator#imageCode")),
 			Pattern.compile(Pattern.quote("org.springframework.") + ".*#.*Present"),
 			Pattern.compile(Pattern.quote("org.springframework.") + ".*#.*PRESENT"),
 			Pattern.compile(Pattern.quote("reactor.") + ".*#.*Available"),

spring-core/src/main/java/org/springframework/cglib/core/AbstractClassGenerator.java

@@ -43,6 +43,9 @@ abstract public class AbstractClassGenerator<T> implements ClassGenerator {
 	private static final boolean DEFAULT_USE_CACHE =
 			Boolean.parseBoolean(System.getProperty("cglib.useCache", "true"));
 
+	// See https://github.com/oracle/graal/blob/master/sdk/src/org.graalvm.nativeimage/src/org/graalvm/nativeimage/ImageInfo.java
+	private static final boolean imageCode = (System.getProperty("org.graalvm.nativeimage.imagecode") != null);
+
 
 	private GeneratorStrategy strategy = DefaultGeneratorStrategy.INSTANCE;
 
@@ -359,6 +362,12 @@ abstract public class AbstractClassGenerator<T> implements ClassGenerator {
 					// ignore
 				}
 			}
+			// SPRING PATCH BEGIN
+			if (imageCode) {
+				throw new UnsupportedOperationException("CGLIB runtime enhancement not supported on native image. " +
+						"Make sure to include a pre-generated class on the classpath instead: " + getClassName());
+			}
+			// SPRING PATCH END
 			byte[] b = strategy.generate(this);
 			String className = ClassNameReader.getClassName(new ClassReader(b));
 			ProtectionDomain protectionDomain = getProtectionDomain();