什么是软件框架

最近在看SSH框架(Spring/Structs/Hibernate),什么是框架？为什么要用框架？框架应该怎么用？(What/Why/How)

From WIKI

In computer programming, a software framework is an abstraction in which software providing generic functionality can be selectively changed by additional user-written code, thus providing application-specific software. A software framework is a universal, reusable software platform to develop applications, products and solutions. Software frameworks include support programs, compilers, code libraries, tool sets, and application programming interfaces (APIs) that bring together all the different components to enable development of a project or solution.

 

Frameworks contain key distinguishing features that separate them from normal libraries:

 

•     inversion of control(控制反转): In a framework, unlike in libraries or normal user applications, the overall program's flow of control is not dictated by the caller, but by the framework.[1]. 
•     default behavior(默认行为): A framework has a default behavior. This default behavior must be some useful behavior and not a series of no-ops.
•     extensibility(扩展性): A framework can be extended by the user usually by selective overriding or specialized by user code to provide specific functionality.
•     non-modifiable framework code(框架代码不可修改): The framework code, in general, is not supposed to be modified, excepting extensibility. Users can extend the framework, but should not modify its code.