Annotations are used with the @ character in front of them and can be attached to packages, classes, interfaces, fields, methods, method parameters, generic type declaration and use, and, finally, to annotations. Annotations can be used almost everywhere and they are used to describe some program meta information. For example, the @RestController annotation does not directly alter the behavior of the OrderController class. The behavior of the class is described by the Java code that is inside. The annotation helps Spring to understand what the class is and how it can and should be used. When Spring scans all the packages and classes to discover the different Spring beans, it sees the annotation on the class and takes it into account. There can be other annotations on the class that Spring does not understand. They may be used by some other framework or program code. Spring ignores them as any well-behaving framework. For example, as we will see later, we have in our code base...