Callbacks methods are the way of managing life cycle of an instance. Callback methods are generally used by containers. The methods are called at specific time during the lifetime of an instance. For example in servlet destroy() method is called by the servlet container that indicates that the servlet is being taken out of service. This type of methods are generally called by the container, the developer does not need to call these methods explicitly. Most of the languages specifies the callback method by passing the address of the subroutine to the system to the request is calling back from, But java performs the same thing by using interfaces. Java does not allow passing the address of subroutine but allows passing an instance of a class that implements the standard interface. For this purpose anonymous classes are mainly used as they support a better compact definition of the class that is required as a new class.
One more example of callback methods is ejbCreate method that is used by an ejb container to create a ejb bean - the developer does not call it explicitly in code- the container calls it- although the developer can override it- or put stuff in it...
Callback methods have totally different meaning regarding the context of security. In order to login securely call the server which then calls us back. This makes it harder for you to spoof being someone else. This may be done over phone lines or over the Internet. It is the same scheme a jealous husband might use to ensure his wife is really home as she claims.
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