Why Spring BeanUtils Fails to Copy Static Inner Classes and How to Fix It

Background

In a recent project we needed to call a third‑party API. The SDK’s Request class had a serialization bug, so we rewrote a Request class with the same fields, but it contained a static inner class and two List fields.

Problem

We used Spring’s BeanUtils.copyProperties to copy the fields from our AddRequest to the SDK’s MixAddRequest. The request failed because a required field inside the Ticket inner class was missing; the copy method did not transfer the inner‑class instance.

Investigation

We created two demo classes ( CopyTest1 and CopyTest2) each with a static inner class and a List of that inner class. After adding Lombok’s @Data and @ToString, the simple fields were copied, but the inner‑class instance remained null.

Root Cause

BeanUtils requires getter and setter methods for every property; without them the copy fails.

If two classes have inner classes with the same name but belong to different outer classes, BeanUtils treats them as different types and does not copy them.

Java generics are erased at runtime, so a List<Order> becomes a raw List. The SDK accepted the list because it only needed the JSON representation.

The parameter order of Spring’s BeanUtils.copyProperties is opposite to Apache’s version, so importing the wrong method can cause silent failures.

Solution

Copy the inner‑class property separately, or write a recursive copy method for beans that contain nested objects. Ensure all inner‑class fields have setters, and remember the correct parameter order when calling BeanUtils.copyProperties.

After adding a second copyProperties call for the inner class, the request succeeded.