A piece of code from Jillegal 3.0 to instrument all non-primitive field assignments in OffHeap object

gistfile1.java

    protected <T> void instrumentNonPrimitiveFieldAssignments(final Class<T> elementType) {
		try {
			Jillegal.init();

			final InstrumentService instrumenterService = InstrumentServiceFactory.getInstrumentService();
			
			final String ownerClassDesc = elementType.getName().replace(".", "/");
			final Set<String> ownerClassDescSet = new HashSet<String>();
			
			ownerClassDescSet.add(ownerClassDesc);
			for (	Class<T> parentClass = (Class<T>) elementType.getSuperclass();
					parentClass != null && !parentClass.equals(Object.class);
					parentClass = (Class<T>) parentClass.getSuperclass()) {
				ownerClassDescSet.add(parentClass.getName().replace(".", "/"));
			}
			
			ClassReader cr = new ClassReader(elementType.getName());
			ClassWriter cw = new ClassWriter(ClassReader.SKIP_DEBUG) {
				@Override
				public MethodVisitor visitMethod(final int access, final String name,
						final String desc, final String signature, final String[] exceptions) {
					MethodVisitor mv =  super.visitMethod(access, name, desc, signature, exceptions);
					return
						new MethodAdapter(mv) {
							public void visitFieldInsn(final int opcode, final String owner,
									final String name, final String desc) {
								if (	opcode == Opcodes.PUTFIELD && 
										ownerClassDescSet.contains(owner) && 
										(desc.startsWith("L") && desc.endsWith(";"))) {
									int fieldOffset = 
											(int) JvmUtil.getUnsafe().
													objectFieldOffset(ReflectionUtil.getField(elementType, name));
									
									// Current operand stack snapshot: ..., <this>, <value_to_set>
									super.visitInsn(Opcode.SWAP); 
									
									// Current operand stack snapshot:
									//		..., <value_to_set>, <this>
									
									super.visitInsn(Opcodes.POP);
									
									// Current operand stack snapshot: 
									// 		..., <value_to_set>
									
									super.visitMethodInsn( // Call "getProperties" method of "java.lang.System" class
											Opcodes.INVOKESTATIC, 
											"java/lang/System",
								            "getProperties", 
								            "()Ljava/util/Properties;");
									
									super.visitLdcInsn(
											DirectMemoryServiceFactory.
												DIRECT_MEMORY_SERVICE_$_SET_OBJECT_FIELD_ACCESSOR); 
									
									super.visitMethodInsn( // Call "get" method of "java/util/Properties" instance
											Opcodes.INVOKEVIRTUAL, 
											"java/util/Properties",
								            "get", 
								            "(Ljava/lang/Object;)Ljava/lang/Object;");
	
									// Current operand stack snapshot: 
									// 		..., <value_to_set>, <DirectMemoryService_setObjectField>
									
									super.visitTypeInsn(
											Opcode.CHECKCAST, 
											PropertyChangeSupport.class.getName().replace(".", "/"));
									
									// Current operand stack snapshot: 
									// 		..., <value_to_set>, <DirectMemoryService_setObjectField>
									
									super.visitInsn(Opcode.SWAP);
									
									// Current operand stack snapshot: 
									// 		..., <DirectMemoryService_setObjectField>, <value_to_set>
									
									super.visitInsn(Opcode.ACONST_NULL);
									
									// Current operand stack snapshot: 
									// 		..., <DirectMemoryService_setObjectField>, <value_to_set>, <null>
	
									super.visitInsn(Opcode.SWAP);
									
									// Current operand stack snapshot: 
									// 		..., <DirectMemoryService_setObjectField>, <null>, <value_to_set> 
									
									super.visitLdcInsn(fieldOffset); // offset of field
									
									// Current operand stack snapshot:
									// 		..., <DirectMemoryService_setObjectField>, <null>, <value_to_set>, <field_offset>
									
									super.visitInsn(Opcode.SWAP);
									
									// Current operand stack snapshot:
									// 		..., <DirectMemoryService_setObjectField>, <null>, <field_offset>, <value_to_set>
									
									super.visitVarInsn(Opcodes.ALOAD, 0); // this
									
									// Current operand stack snapshot:
							 		// 		..., <DirectMemoryService_setObjectField>, <null>, <field_offset>, <value_to_set>, <this>
									
									super.visitInsn(Opcode.SWAP);
									
									// Current operand stack snapshot:
							 		// 		..., <DirectMemoryService_setObjectField>, <null>, <field_offset>, <this>, <value_to_set>, 
									
									// Now <field_offset>, <this>, <value_to_set> are the parameters in this order 
									// to be passed for "setObjectField" method "DirectMemoryService" instance
									
									super.visitMethodInsn( // Call "fireIndexedPropertyChange" method of "java/beans/PropertyChangeSupport" instance
											Opcodes.INVOKEVIRTUAL, 
											PropertyChangeSupport.class.getName().replace(".", "/"),
								            "fireIndexedPropertyChange", 
								            "(Ljava/lang/String;ILjava/lang/Object;Ljava/lang/Object;)V");
									
									logger.debug("Instrumenting assignment to field \"" + name  + 
											"\" in class " + elementType.getName());
								}
								else {
									super.visitFieldInsn(opcode, owner, name, desc);
								}
							}
						};
				}
			};
			cr.accept(cw, ClassReader.SKIP_DEBUG);
			
			byte[] instrumentedBytecodes = cw.toByteArray();
			
			instrumenterService.redefineClass(elementType, instrumentedBytecodes);
			
			logger.info("Instrumented class " + elementType.getName() + " for non-primitive field assignment");
		}
		catch (Throwable t) {
			t.printStackTrace();
			logger.error("Error occured while instrumenting non-primitive field assignments for class " + 
							elementType.getName(), t);
		}
	}