Summary

If a bean class contains multiple getter/setter methods with a same name but different types, the binding mechanism does not sort these methods properly which may lead to a non-deterministic/erroneous behaviour.

Affects v2.7.5 (current main).

Details

Let's have following bean classes:

static class Child extends Parent<ChildProperty> {
    @Override
    public ChildProperty getProperty() {
        return null;
    }
}

abstract static class Parent<T extends ParentProperty> {
    abstract public T getProperty();
}

static class ChildProperty extends ParentProperty {
}

abstract static class ParentProperty {
}

When compiled, the Child class does have two getter methods. One from the Parent class with the ParentProperty return type, another from it's own implementation with the ChildPropertyReturnType.

$ javap Child.class
Compiled from "Child.java"
class Child extends Parent<ChildProperty> {
  Child();
  public ChildProperty getProperty();
  public ParentProperty getProperty();
}

The binding mechanism in the JavaBeanBinder.Bean.addProperties() method, reads all declared methods via reflection and then tries to sort those methods by its name. Reason of sorting is non-determinism of the Class.getDeclaredMethods() reflection method. This issue was previously noted in #24068

https://github.com/spring-projects/spring-boot/blob/v2.7.5/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/JavaBeanBinder.java#L130-L143

The problems is: Sorting is solely based on method name which does not (fully) remove the aforementioned non-determinism.

Additional details

• I've encountered this issue in Kotlin, where constructs like abstract var property: T are slightly more common.
• Here is a test app FieldsOrderingTest.txt (please rename the txt extension to java)
• Maybe unexpectedly Java compiler does not flag the public ParentProperty getProperty() on the Child class as abstract. The following code returns array of false values:

  Object[] abstracts = Arrays.stream(Child.class.getDeclaredMethods())
          .map(Method::getModifiers)
          .map(Modifier::isAbstract)
          .toArray();
  // abstracts = {Object[2]@710}
  // 0 = {Boolean@712} false
  // 1 = {Boolean@712} false

  This means the JavaBeanBinder.Bean.isCandidate() method returns true for both getProperty() methods in the Child class.