Implementation View
The implementation view shown here is only a starting point and will be refined in during development phase.
Layers
The layers, packages and its hierarchy are represented using the following diagrams. The package hierarchy starts with standard Java namespace compliant structure and then is divided into two sub-packages for the different layers viz. src – Application (Services, Data Access) and Web Content – Presentation.
Figure 7 - Implementation View
Error handling
Error handling will be implemented by leveraging the Exceptions feature of the Java language. The following guidelines are suggested when dealing with exceptions
| No. | Exception Guideline | Description |
|---|---|---|
| 1 | Exceptions should not create additional package dependencies | Assume that a client class in package A accesses a class in package B. The class in package B should not throw an exception that belongs to package C (which is used by package B). This produces dependencies between package A and C. |
| 2 | Exceptions by package. | If a package’s classes throw any exceptions, the package should have its own top-level checked exception. The package should then define exception subclasses for any exceptions that may be handled differently by clients. Good models for this paradigm can be found in the Java packages java.io, java.sql and javax.naming. |
| 3 | No blind catches of Exception | A class is responsible for knowing what exceptions it may encounter, and it must treat each exception individually. If the handling of many exceptions is identical, it could be extracted into helper methods. |
| 4 | No empty catch-blocks | At the very least, a catch-block should contain an assertion that it should never be reached or a comment stating that it is irrelevant |
| 5 | Write sensible throws clauses | Fewer (<3) the number of exceptions thrown, better it is. Always throw exceptions that make sense to the calling class, if not wrap that exception in another, which more closely captures the error type |
| 6 | Chaining Exceptions | Always chain exceptions so that the root cause of the error is available for logging it into the error/system log file. This is very useful for diagnosing errors in production environment |
| 7 | Use Message Catalogs for easy localization | Use message catalogs for message text of an exception, whose message is directly presented to the end user. This will help the application to be localized or internationalized by just adding another message catalog |
Applications when encountering an exception should always log it to the Application/System log. Lower level components should avoid writing to an error/system log.