Design Pattern walkthrough

1. Proxy Pattern
- Proxy controls and manages access to the real subject.
- Same interface, so proxy can be substituted anywhere the subject can be used

• Remote proxy: act as local represtative (RMI)
• Virtual proxy: expensive to create, lazy initialization (Hibernate)
• Transaction proxy: wire transactional support (Spring)
• Security: protect

2. Command Pattern
- Encapsulate method invocation: store them, pass them around and invoke them when you need them
- It decouples the requester of an action from the oject that actually performs the action

• Order a meal: order slip
• Job queue: ControllerUpCmd, ApAddCmd
• Device import: commands passed around and executed in context (e.g., session establish, error handling), acts like closure, function pointer. It is like Template method pattern, but instead of subclassing, the method is passed as command objects here

3.Strategy Pattern
- Defines a family algorithms, encapsulates each one and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it
- Encapsulates the behaviour, take what varies and "encapsulate" it so it won't affect the rest of your code

4.Observer Pattern
- Defines a one-to-many dependency between objects so that when one object (subject) changes state, all of its dependents (observers) are notified and updated automatically
- Subjects and observers are *loosely* coupled
- A key part in the familiar Model View Controller (MVC) pattern

 

Convention over Configuration (CoC)

Maven, Grail are examples of applying CoC. So do many modern framework. CoC essentially means a developer only needs to specify unconvertional aspects of the application. Only when the desired behavior "deviates" from the implemented convention is explict configuraiton required.

Comet & asychronous HTTP

Comet [1] is web application model to achieve "server push" or "reverse ajax".

Servlet 3.0 provides the asynchronlus support which allows the servelet request to be suspended and resumed, i.e., non-blocking, to avoid thread starvation.

HTTP persistent connection, also called HTTP keep-alive, is the idea of using a single TCP connection to send/recieve mulitple HTTP request/response. It is also controversial as that it may tie up server side thread for too long. And there is a related idea about HTTP pipelining in which multiple HTTP requests are sent on a single TCP connection without waitting for the corresponding responses.

Proxy server can be use to deal with cross-domains issue for hidden iframe and XMLHttpRequest, making them appears to originate from the same domain.

Passwork complexity check

String pattern = "^.*(?=.*[A-Za-z])(?=.*[0-9])(?=.*[\\W]).*$";

Zero-width positive lookahead assertion (?= some_expression) is used to make sure the password uses a mixture of letters, numbers and special characters.