Using class variable after injecting it via Spring throws NullPointerException

Question

If I have a class variable foo, injected via Spring and I use foo to initialize another class variable like below, why does it throw a NullPointerException? However, if I perform the initialization in a method call of the class, it works fine.

class Baz {

   @In
   Object foo;

   Object bar = foo.someMethod();

}

Answer 1

Because you are using foo before Spring runs. Take a look at Java class initialization to understand it.