Add comments regarding Java 1.8 byte code level for CGLIB

Unfortunately, it is not possible to raise the byte code level beyond
Java 1.8 for classes generated using CGLIB due to the fact that CGLIB
generates STATICHOOK methods which set static final fields outside the
initializer method <clinit> (i.e., a static initialization block).

Attempting to raise the level to Java 17 (or even Java 9) results in
exceptions like the following.

Caused by: java.lang.IllegalAccessError: Update to static final field
  org.example.MyBean$$SpringCGLIB$$0.CGLIB$THREAD_CALLBACKS attempted from
  a different method (CGLIB$STATICHOOK1) than the initializer method <clinit>
    at org.example.MyBean$$SpringCGLIB$$0.CGLIB$STATICHOOK1(<generated>)
    at org.example.MyBean$$SpringCGLIB$$0.<clinit>(<generated>)

This commit therefore introduces inline comments pointing out why we
stay with Java 1.8 byte code level with CGLIB.

See gh-34602

Author: sbrannen

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

@@ -116,6 +116,8 @@ public void generateClass(ClassVisitor v) {
 			Type sourceType = Type.getType(source);
 			Type targetType = Type.getType(target);
 			ClassEmitter ce = new ClassEmitter(v);
+			// Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+			// which set static final fields outside the initializer method <clinit>.
 			ce.begin_class(Constants.V1_8,
 						   Constants.ACC_PUBLIC,
 						   getClassName(),

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

@@ -117,6 +117,8 @@ public void generateClass(ClassVisitor v) throws Exception {
 			types[i] = (Type)props.get(names[i]);
 		}
 		ClassEmitter ce = new ClassEmitter(v);
+		// Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+		// which set static final fields outside the initializer method <clinit>.
 		ce.begin_class(Constants.V1_8,
 					   Constants.ACC_PUBLIC,
 					   getClassName(),

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

@@ -57,6 +57,8 @@ class BeanMapEmitter extends ClassEmitter {
     public BeanMapEmitter(ClassVisitor v, String className, Class type, int require) {
         super(v);
 
+        // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+        // which set static final fields outside the initializer method <clinit>.
         begin_class(Constants.V1_8, Constants.ACC_PUBLIC, className, BEAN_MAP, null, Constants.SOURCE_FILE);
         EmitUtils.null_constructor(this);
         EmitUtils.factory_method(this, NEW_INSTANCE);

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

@@ -56,6 +56,8 @@ public BulkBeanEmitter(ClassVisitor v,
         Method[] setters = new Method[setterNames.length];
         validate(target, getterNames, setterNames, types, getters, setters);
 
+        // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+        // which set static final fields outside the initializer method <clinit>.
         begin_class(Constants.V1_8, Constants.ACC_PUBLIC, className, BULK_BEAN, null, Constants.SOURCE_FILE);
         EmitUtils.null_constructor(this);
         generateGet(target, getters);

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

@@ -89,6 +89,8 @@ public Object create() {
 		public void generateClass(ClassVisitor v) {
 			Type targetType = Type.getType(target);
 			ClassEmitter ce = new ClassEmitter(v);
+			// Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+			// which set static final fields outside the initializer method <clinit>.
 			ce.begin_class(Constants.V1_8,
 						   Constants.ACC_PUBLIC,
 						   getClassName(),

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

@@ -261,6 +261,8 @@ public void generateClass(ClassVisitor v) {
 			}
 
 			Type[] parameterTypes = TypeUtils.getTypes(newInstance.getParameterTypes());
+			// Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+			// which set static final fields outside the initializer method <clinit>.
 			ce.begin_class(Constants.V1_8,
 					Constants.ACC_PUBLIC,
 					getClassName(),

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

@@ -678,6 +678,8 @@ public void generateClass(ClassVisitor v) throws Exception {
 
 		ClassEmitter e = new ClassEmitter(v);
 		if (currentData == null) {
+			// Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+			// which set static final fields outside the initializer method <clinit>.
 			e.begin_class(Constants.V1_8,
 					Constants.ACC_PUBLIC,
 					getClassName(),
@@ -688,6 +690,8 @@ public void generateClass(ClassVisitor v) throws Exception {
 					Constants.SOURCE_FILE);
 		}
 		else {
+			// Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+			// which set static final fields outside the initializer method <clinit>.
 			e.begin_class(Constants.V1_8,
 					Constants.ACC_PUBLIC,
 					getClassName(),

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

@@ -94,23 +94,25 @@ public Class create() {
     }
 
     @Override
-	protected ClassLoader getDefaultClassLoader() {
+    protected ClassLoader getDefaultClassLoader() {
         return null;
     }
 
     @Override
-	protected Object firstInstance(Class type) {
+    protected Object firstInstance(Class type) {
         return type;
     }
 
     @Override
-	protected Object nextInstance(Object instance) {
+    protected Object nextInstance(Object instance) {
         throw new IllegalStateException("InterfaceMaker does not cache");
     }
 
     @Override
-	public void generateClass(ClassVisitor v) throws Exception {
+    public void generateClass(ClassVisitor v) throws Exception {
         ClassEmitter ce = new ClassEmitter(v);
+        // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+        // which set static final fields outside the initializer method <clinit>.
         ce.begin_class(Constants.V1_8,
                        Constants.ACC_PUBLIC | Constants.ACC_INTERFACE | Constants.ACC_ABSTRACT,
                        getClassName(),

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

@@ -48,6 +48,8 @@ class MixinEmitter extends ClassEmitter {
     public MixinEmitter(ClassVisitor v, String className, Class[] classes, int[] route) {
         super(v);
 
+        // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+        // which set static final fields outside the initializer method <clinit>.
         begin_class(Constants.V1_8,
                     Constants.ACC_PUBLIC,
                     className,

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

@@ -105,6 +105,8 @@ public void generateClass(ClassVisitor v) {
             }
 
             ClassEmitter ce = new ClassEmitter(v);
+            // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+            // which set static final fields outside the initializer method <clinit>.
             ce.begin_class(Constants.V1_8,
                            Constants.ACC_PUBLIC,
                            getClassName(),

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

@@ -75,6 +75,8 @@ public FastClassEmitter(ClassVisitor v, String className, Class type) {
         super(v);
 
         Type base = Type.getType(type);
+        // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+        // which set static final fields outside the initializer method <clinit>.
         begin_class(Constants.V1_8, Constants.ACC_PUBLIC, className, FAST_CLASS, null, Constants.SOURCE_FILE);
 
         // constructor

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

@@ -236,6 +236,8 @@ public void generateClass(ClassVisitor v) throws NoSuchMethodException {
 
             ClassEmitter ce = new ClassEmitter(v);
             CodeEmitter e;
+            // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+            // which set static final fields outside the initializer method <clinit>.
             ce.begin_class(Constants.V1_8,
                            Constants.ACC_PUBLIC,
                            getClassName(),

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

@@ -117,6 +117,8 @@ public void generateClass(ClassVisitor cv) {
             final MethodInfo method = ReflectUtils.getMethodInfo(ReflectUtils.findInterfaceMethod(iface));
 
             ClassEmitter ce = new ClassEmitter(cv);
+            // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+            // which set static final fields outside the initializer method <clinit>.
             ce.begin_class(Constants.V1_8,
                            Constants.ACC_PUBLIC,
                            getClassName(),

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

@@ -38,6 +38,8 @@ class ParallelSorterEmitter extends ClassEmitter {
 
     public ParallelSorterEmitter(ClassVisitor v, String className, Object[] arrays) {
         super(v);
+        // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+        // which set static final fields outside the initializer method <clinit>.
         begin_class(Constants.V1_8, Constants.ACC_PUBLIC, className, PARALLEL_SORTER, null, Constants.SOURCE_FILE);
         EmitUtils.null_constructor(this);
         EmitUtils.factory_method(this, NEW_INSTANCE);

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

@@ -133,6 +133,8 @@ public StringSwitcher create() {
         @Override
         public void generateClass(ClassVisitor v) throws Exception {
             ClassEmitter ce = new ClassEmitter(v);
+            // Byte code level cannot be higher than 1.8 due to STATICHOOK methods
+            // which set static final fields outside the initializer method <clinit>.
             ce.begin_class(Constants.V1_8,
                            Constants.ACC_PUBLIC,
                            getClassName(),