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The persistence layer
DAO
business object
the object-oriented representation of the problem domain
should never depend on any other layer
finer-grained business logic
data transfer objects(DTOs)
domain classes
the classes are identified by deriving them from the nouns in the system
The business logic layer
service classes
coarse-grained business functions
to demarcate transactions
The presentation layer
responsible for the layout and formatting of all information
web front ends
HTML and JavaScript
cross-platform compatibility
ease of deployment
need not have to download an application
scalability
the level of performance and security
rich client
advanced user controls or complex data manipulation
Swing ; Java
WinForms ; .NET
developing two-tier database applications
Oracle Forms
PowerBuilder
Visual Basic
hybrid clients
Adobe Flex
Ajax