This instruction puts value into a nonstatic field of the object on the top of the stack. field-spec is composed of two parts, a classname and field name (f.e. foo/bar/ClassName/fieldName). descriptor is the Java type descriptor of the field.
This instruction puts value into a static field of the object on the top of the stack. field-spec is composed of two parts, a classname and field name (f.e. foo/bar/ClassName/fieldName). descriptor is the Java type descriptor of the field.