我老土，<<Core J2EE Patterns>>这两天才开读。

在表现层中，作者写到
“The typing and constraints information is abstracted out of the model state and into a generic framework. This separates the validation of the model from the application logic in which the model is being used, thus reducing their coupling.

Model vali Form Data Validation Framework dation is performed by comparing the metadata and constraints to the model state. The metadata and constraints about the model are typically accessible from some sort of simple data store, such as a properties file. A benefit of this approach is that the system becomes more generic, because it factors the state typing and constraint information out of the application logic.

An example is to have a component or subsystem that encapsulates validation logic, such as deciding whether a string is empty, whether a certain number is within a valid range, whether a string is formatted in a particular way, and so on. When various disparate application components want to validate different aspects of a model, each component does not write its own validation code. Rather, the centralized validation mechanism is used. The centralized validation mechanism will typically be configured either programmatically, through some sort of factory, or declaratively, using configuration files.

Thus, the validation mechanism is more generic, focusing on the model state and its requirements, independent of the other parts of the application. A drawback to using this strategy is the potential reduction in efficiency and performance. Also, more generic solutions, although often powerful, are sometimes less easily understood and maintained.