How do app servers inject into private fields?

Question

I saw this question

Inject into private, package or public field or provide a setter?

about how to manually inject into annotated private fields (The way is addi

Answer 1

It's a simple reflection "trick". It relies on the Field.setAccessible() method to force the member to be accessible programmatically:

Set the accessible flag for this object to the indicated boolean value. A value of true indicates that the reflected object should suppress Java language access checking when it is used. A value of false indicates that the reflected object should enforce Java language access checks.

The Reflection API is used to get a handle on the field, setAccessible() is called, and then it can be set by the injection framework.

See an example here.

No magic, no custom VM.