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 adding setters
or through a constructor)

But, the point is how do an application server (like glassfish, axis2, jboss, …)
is able to inject into a final private field (without adding setters or constructors
to the user class)?

Quoting the cited question:

public SomeClass {
  @Inject
  private SomeResource resource;
}

Do they use a customized JVM (not the standard one) that allows to access private fields?

Thanks

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.