The document discusses the layers of abstraction in web application architecture. It describes how web applications typically have four main layers - the presentation layer, service layer, persistence layer, and domain model layer. The domain model layer contains the core business logic and objects of the application. The service layer acts as a facade to expose use cases to the presentation layer and coordinates access to the domain model layer. The persistence layer isolates the code for interacting with data storage.

1. Web applica*on architecture
Ilio Catallo - info@iliocatallo.it

2. Outline
• Layers of abstrac.on
• Domain model
• Service layer
• Persistence layer
• Presenta.on layer

3. Layers of abstrac-on

4. Layers
A layer is a discrete, orthogonal area of concern within an applica4on

5. Applica'ons are made of layers
Applica'ons are broken down into a series of layers, so that each
layer isolates a different type of concern

6. Web applica*ons are made of layers
For instance, the problem of persis&ng data is isolated in the
persistence layer

7. Web app. layers
Typically, Web applica-ons have four layers of abstrac-on
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

8. Communica)on between layers
Communica)on between layers is from top to bo5om
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

9. Execu&on flow through layers
Layers can help conceptualize the flow through an applica5on
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

10. Layers are not physically isolated
Even if layers are conceptually isolated, they are not necessarily
physically isolated
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

11. Flexibility & testability
Isola&ng concerns (e.g., data persistence) into separate layers
increases flexibility and testability

12. Flexibility
Flexibility is increased because implementa0ons of each layer can
vary independently
public class SQLMovieRepository implements MovieRepository {...}
public class CSVMovieRepository implements MovieRepository {...}
public class RESTMovieRepository implements MovieRepository {...}

13. Testability
Testability is increased because the low-coupling between areas
make it easier to test each part
public class MovieServiceTest {
@Test
public void moviesDirectedByTest() {
...
}
}

14. How to design a layer?

15. Designing layers
When designing a layer, we first try to iden3fy a clear API for it
public interface FlightService {
List<SpecialDeal> getSpecialDeals();
List<Flight> findFlights(String queryString);
}

16. Designing layers
We then expose such an API through a Java interface
public interface FlightService {
List<SpecialDeal> getSpecialDeals();
List<Flight> findFlights(String queryString);
}

17. Designing layers
The interface acts as the layer's contract
• It enforces correct layer usage
• It hides implementa6on details

18. Interfaces as layer contracts
Interfaces are the key enablers to build applica/ons with layers

19. Domain model

20. Domain model
The domain model is the most important layer in the applica/on,
as it contains the business logic of the applica/on
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║┌────────────────────────────────┐
║ n ║│ │
║ ║│ Service layer │
║ m ║│ │
║ o ║└────────────────────────────────┘
║ d ║┌────────────────────────────────┐
║ e ║│ │
║ l ║│ Persistence layer │
║ ║│ │
╚══════╝└────────────────────────────────┘

21. Domain model
In other words, the domain model is the code representa,on of
the business problem we are trying to solve
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║┌────────────────────────────────┐
║ n ║│ │
║ ║│ Service layer │
║ m ║│ │
║ o ║└────────────────────────────────┘
║ d ║┌────────────────────────────────┐
║ e ║│ │
║ l ║│ Persistence layer │
║ ║│ │
╚══════╝└────────────────────────────────┘

22. Domain objects
The domain model consists of a network of interconnected domain
objects
+-------+ +-------+
| | | |
| A +------> B |
| | | |
+---^---+ +---^---+
| |
| |
+---+---+ |
| | |
| C +----------+
| |
+-------+

23. Concepts
Each class in the domain model corresponds to a concept from the
applica2on problem domain
+-------+ +-------+
| | | |
| A +------> B |
| | | |
+---^---+ +---^---+
| |
| |
+---+---+ |
| | |
| C +----------+
| |
+-------+

24. Problem domain concepts
That is, domain objects are specific to the applica4on’s problem
domain
+-------+ +-------+
| | | |
| A +------> B |
| | | |
+---^---+ +---^---+
| |
| |
+---+---+ |
| | |
| C +----------+
| |
+-------+

25. Airline travel Web applica/on
For instance, concepts for an Airline travel app may include:
• Airport
• Flight
• FlightLeg
• SpecialDeal

26. Nouns of the applica0on
A popular technique to determine the domain model is to use the
nouns in use case descrip3ons as domain objects
+-------+ +-------+
| | | |
| A +------> B |
| | | |
+---^---+ +---^---+
| |
| |
+---+---+ |
| | |
| C +----------+
| |
+-------+

27. Nouns of the applica0on
In other words, the nouns that are used when describing the
problem domain suggest domain objects
+-------+ +-------+
| | | |
| A +------> B |
| | | |
+---^---+ +---^---+
| |
| |
+---+---+ |
| | |
| C +----------+
| |
+-------+

28. Airport class
public class Airport {
private String name;
private String airportCode;
public Airport(String name,
String airportCode) {...}
// other methods
}

29. FlightLeg class
public class FlightLeg {
private Airport departFrom;
private Date departOn;
private Airport arriveAt;
private Date arriveOn;
public FlightLeg(Airport departFrom,
Date departOn,
Airport arriveAt,
Date arriveOn) {...}
// other methods
}

30. Flight class
public class Flight {
private List<FlightLeg> legs;
private BigDecimal totalCost;
public Flight(List<FlightLeg> legs,
BigDecimal totalCost) {...}
public boolean isNonStop() {...}
public long getTotalTravelTime() {...}
// other methods
}

31. Airline travel Web applica/on
Hence, the resul-ng domain model is as follows:
+---------+ +---------+ +----------+
| |////| | | | |
| |////+-----> +------> |
| |////| | | | |
+---------+ +---------+ +----------+
Flight FlightLeg Airport

32. Domain model ≠ set of rela0onships
It is important to note that business logic does not mean just a set
of rela,onships between objects
+---------+ +---------+ +----------+
| |////| | | | |
| |////+-----> +------> |
| |////| | | | |
+---------+ +---------+ +----------+
Flight FlightLeg Airport

33. Domain objects have state and behavior
As well as storing data, a domain model object should implement
the business logic that operates on the data
+---------+ +---------+ +----------+
| |////| | | | |
| |////+-----> +------> |
| |////| | | | |
+---------+ +---------+ +----------+
Flight FlightLeg Airport

34. Domain objects have state and behavior
That is, domain objects should have both state and behavior
+---------+
| |////|
| |////|
+---------+
Flight

35. Assign behavior to domain objects
Calculated values and derived answers should be provided by the
object you are talking with
+---------+
| |////|
| |////|
+---------+
Flight

36. Assign behavior to domain objects
Examples:
• Calculated value: Flight.getTotalTravelTime()
• Derived answer: Flight.isNonStop()

37. Object-oriented features
Pu#ng the business logic inside the domain objects makes it
possible to harness the full power of OOP

38. Object-oriented features
Think about how to take advantage of object-oriented features to
help solve the business problem
• Inheritance
• Polymorphism

39. Anemic domain model
A domain model that contains only state and rela1onships (but no
behavior) is usually called an anemic domain model

40. Pervasivity
The domain model is a layer that largely permeates the other layers
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║┌────────────────────────────────┐
║ n ║│ │
║ ║│ Service layer │
║ m ║│ │
║ o ║└────────────────────────────────┘
║ d ║┌────────────────────────────────┐
║ e ║│ │
║ l ║│ Persistence layer │
║ ║│ │
╚══════╝└────────────────────────────────┘

41. Ver$cality
The domain model is a ver$cal layer, as all the other layers have
dependencies on the domain model
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║┌────────────────────────────────┐
║ n ║│ │
║ ║│ Service layer │
║ m ║│ │
║ o ║└────────────────────────────────┘
║ d ║┌────────────────────────────────┐
║ e ║│ │
║ l ║│ Persistence layer │
║ ║│ │
╚══════╝└────────────────────────────────┘

42. Centrality
That is, even though each layer is responsible for its own concern,
all the layers live to serve the domain model
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║┌────────────────────────────────┐
║ n ║│ │
║ ║│ Service layer │
║ m ║│ │
║ o ║└────────────────────────────────┘
║ d ║┌────────────────────────────────┐
║ e ║│ │
║ l ║│ Persistence layer │
║ ║│ │
╚══════╝└────────────────────────────────┘

43. Independence
However, the domain model has no dependency on any other layer
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║┌────────────────────────────────┐
║ n ║│ │
║ ║│ Service layer │
║ m ║│ │
║ o ║└────────────────────────────────┘
║ d ║┌────────────────────────────────┐
║ e ║│ │
║ l ║│ Persistence layer │
║ ║│ │
╚══════╝└────────────────────────────────┘

44. Domain objects do not need injec/on
In general, the domain model will not need dependencies injected
+---------+ +---------+ +----------+
| |////| | | | |
| |////+-----> +------> |
| |////| | | | |
+---------+ +---------+ +----------+
Flight FlightLeg Airport

45. Service Layer

46. Service Layer
The service layer exposes the use cases of the applica1on
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │╔════════════════════════════════╗
│ n │║ ║
│ │║ Service layer ║
│ m │║ ║
│ o │╚════════════════════════════════╝
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

47. Airline travel Web applica/on
FlightService implements two applica.on use cases
public interface FlightService {
List<SpecialDeal> getSpecialDeals();
List<Flight> findFlights(String queryString);
}

48. Coordinator
The service layer coordinates calls to the domain objects
+------------------------------------------+
| |
+--+--+ |
| | |
| S | +-------+ +-------+ |
| e | | | | | |
| r +-------> A +------> B | |
| v | | | | | |
| i | +---+---+ +---^---+ |
| c | | | |
| e | | | |
| | | +---+---+ |
| l | +----------> | |
| a | | C | |
| y +----------------------> | |
| e | +-------+ |
| r | |
| | |
+--+--+ |
| |
+------------------------------------------+

49. Single entry point
The service layer is the single entry point for the business logic
+------------------------------------------+
| |
+--+--+ |
| | |
| S | +-------+ +-------+ |
| e | | | | | |
| r +-------> A +------> B | |
| v | | | | | |
| i | +---+---+ +---^---+ |
| c | | | |
| e | | | |
| | | +---+---+ |
| l | +----------> | |
| a | | C | |
| y +----------------------> | |
| e | +-------+ |
| r | |
| | |
+--+--+ |
| |
+------------------------------------------+

50. Façade pa(ern
That is, the service layer acts as a façade
+------------------------------------------+
| |
+--+--+ |
| | |
| S | +-------+ +-------+ |
| e | | | | | |
| r +-------> A +------> B | |
| v | | | | | |
| i | +---+---+ +---^---+ |
| c | | | |
| e | | | |
| | | +---+---+ |
| l | +----------> | |
| a | | C | |
| y +----------------------> | |
| e | +-------+ |
| r | |
| | |
+--+--+ |
| |
+------------------------------------------+

51. Exposing the service layer
The business logic can then be exposed in different ways
• Web applica+ons
• Web service
• Desktop applica+on

52. Exposing the service layer
This thanks to the fact that, no ma0er the means, it is always the
same business logic to be executed

53. Transac'onality
Ideally, each applica-on’s use case should represent a single
transac'onal unit of work that either succeeds or fails

54. Transac'onality
Since the service layer is the single entry-point, we can apply
security and transac+onal requirements
@Transactional
public class FlightServiceImpl implements FlightService {
...
}

55. Coarse grained & stateless
Each method in the service layer should be coarse grained and
stateless
public interface FlightService {
List<SpecialDeal> getSpecialDeals();
List<Flight> findFlights(String queryString);
}

56. Coarse grained
Coarse grained: a single method call will accomplish the en1re use
case
public interface FlightService {
List<SpecialDeal> getSpecialDeals();
...
}

57. Stateless
Stateless: calls to a service method do not create any state on the
object implemen4ng the service interface
public interface FlightService {
List<SpecialDeal> getSpecialDeals();
...
}

58. Service layer
The service layer should never have a dependency on the
presenta3on layer
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │╔════════════════════════════════╗
│ n │║ ║
│ │║ Service layer ║
│ m │║ ║
│ o │╚════════════════════════════════╝
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

59. Service layer
However, the service layer depends on the persistence layer
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │╔════════════════════════════════╗
│ n │║ ║
│ │║ Service layer ║
│ m │║ ║
│ o │╚════════════════════════════════╝
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

60. Persistence layer

61. Persistence layer
The persistence layer is responsible for interfacing with the
underlying persistence mechanism
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │╔════════════════════════════════╗
│ e │║ ║
│ l │║ Persistence layer ║
│ │║ ║
└──────┘╚════════════════════════════════╝

62. Persistence layer
The persistence layer knows how to store and retrieve domain
objects from the data store
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │╔════════════════════════════════╗
│ e │║ ║
│ l │║ Persistence layer ║
│ │║ ║
└──────┘╚════════════════════════════════╝

63. Persistence layer
It does so in such a way that the service layer does not know which
underlying data store is used
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │╔════════════════════════════════╗
│ e │║ ║
│ l │║ Persistence layer ║
│ │║ ║
└──────┘╚════════════════════════════════╝

64. Persistence concerns
That is, the persistence layer isolates the applica0on from changes
in the persistence mechanisms
┌──────┐┌────────────────────────────────┐
│ D ││ │
│ o ││ Presentation layer │
│ m ││ │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │╔════════════════════════════════╗
│ e │║ ║
│ l │║ Persistence layer ║
│ │║ ║
└──────┘╚════════════════════════════════╝

65. Airline travel Web applica/on
public interface AccountRepository {
Account findByUsername(String username);
Account findById(long id);
Account save(Account account);
}

66. Airline travel Web applica/on
public interface AccountRepository {
Optional<Account> findByUsername(String username);
Optional<Account> findById(long id);
Account save(Account account);
}

67. The service layer is a coordina0on layer
The service layer coordinates the persistence layer and the
domain model
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║╔════════════════════════════════╗
║ n ║║ ║
║ ║║ Service layer ║
║ m ║║ ║
║ o ║╚════════════════════════════════╝
║ d ║╔════════════════════════════════╗
║ e ║║ ║
║ l ║║ Persistence layer ║
║ ║║ ║
╚══════╝╚════════════════════════════════╝

68. Persis&ng domain objects
The service layer ensures that the appropriate domain objects are
loaded and persisted in the use case
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║╔════════════════════════════════╗
║ n ║║ ║
║ ║║ Service layer ║
║ m ║║ ║
║ o ║╚════════════════════════════════╝
║ d ║╔════════════════════════════════╗
║ e ║║ ║
║ l ║║ Persistence layer ║
║ ║║ ║
╚══════╝╚════════════════════════════════╝

69. Domain objects are unaware
The domain objects are un unaware of being persisted
╔══════╗┌────────────────────────────────┐
║ D ║│ │
║ o ║│ Presentation layer │
║ m ║│ │
║ a ║└────────────────────────────────┘
║ i ║╔════════════════════════════════╗
║ n ║║ ║
║ ║║ Service layer ║
║ m ║║ ║
║ o ║╚════════════════════════════════╝
║ d ║╔════════════════════════════════╗
║ e ║║ ║
║ l ║║ Persistence layer ║
║ ║║ ║
╚══════╝╚════════════════════════════════╝

70. Presenta(on layer

71. Presenta(on layer
We can further detail the architecture by spli3ng the presenta5on
layer into a Web layer and a user interface layer
┌──────┐┌────────────────────────────────┐
│ ││ User interface layer │
│ D │└────────────────────────────────┘
│ o │┌────────────────────────────────┐
│ m ││ Web layer │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

72. Web layer
The Web layer has two responsibili-es
• It acts as the glue between the service layer and HTTP
• It drives the user through the correct pages in the order

73. HTTP oddi(es
The HTTP world is populated with strange creatures
• Request parameters
• Headers
• Cookies

74. Web logic ≠ business logic
These aspects are not business logic specific, and thus should be
kept isolated from the service layer

75. Web logic ≠ business logic
It's on the Web layer to hide the details of the Web from the
business logic
┌──────┐┌────────────────────────────────┐
│ ││ User interface layer │
│ D │└────────────────────────────────┘
│ o │┌────────────────────────────────┐
│ m ││ Web layer │
│ a │└────────────────────────────────┘
│ i │╔════════════════════════════════╗
│ n │║ ║ HTTP request?
│ │║ Service layer ║ I don't know what
│ m │║ ║ you are talking
│ o │╚════════════════════════════════╝ about...
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

76. The Web layer is an integra0on layer
The Web layer converts the incoming HTTP requests to something
that can be handled by the service layer...
┌──────┐┌────────────────────────────────┐
│ ││ User interface layer │
│ D │└────────────────────────────────┘
│ o │╔════════════════════════════════╗ Don't worry! I've
│ m │║ Web layer ║ got you covered
│ a │╚════════════════════════════════╝
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

77. The Web layer is an integra0on layer
...and then transforms the result from the service layer into a
response for the user interface
┌──────┐┌────────────────────────────────┐
│ ││ User interface layer │
│ D │└────────────────────────────────┘
│ o │╔════════════════════════════════╗ Here's the data
│ m │║ Web layer ║ you need to
│ a │╚════════════════════════════════╝ render the UI!
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

78. Exposing the use cases
The Web layer is an adapter that connects to the service layer and
exposes it to the end user

79. Exposing the use cases
That is, the Web layer exposes the use cases to the Web

80. Naviga&on logic
Apart serving as an adapter, the Web layer is also in charge of
handling the naviga&on logic

81. Naviga&on logic
That is, the Web layer drives the user through the correct pages in
the correct order

82. TODO Web applica-on
GET /todo
+----------------------------+
| |
| |
+-----v-----+ +-----+------+
| | | |
GET /todo | List of | GET /todo/:id | Individual |
+-----------> TODOS +----------------> TODO |
| | | |
+--+-------^+ +------------+
| |
| |
| |
| |
+-------+
POST /todo

83. Naviga&on logic
The naviga*on is bound to the Web layer only. There is no
naviga*on logic in the service layer or in the domain model

84. Naviga&on logic
By guiding the user through the site, the Web layer determines the
user experience

85. Thin layer
The Web layer is a thin layer
• It is just a bridge between the Web and the service layer
• It does not contain any business logic

86. Web layer’s dependencies
The Web layer depends on the service layer, to which it delegates
the actual processing
┌──────┐┌────────────────────────────────┐
│ ││ User interface layer │
│ D │└────────────────────────────────┘
│ o │╔════════════════════════════════╗
│ m │║ Web layer ║
│ a │╚════════════════════════════════╝
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

87. Web layer’s dependencies
The Web layer depends on the domain model, in that informa5on
from the Web could be converted to domain objects
┌──────┐┌────────────────────────────────┐
│ ││ User interface layer │
│ D │└────────────────────────────────┘
│ o │╔════════════════════════════════╗
│ m │║ Web layer ║
│ a │╚════════════════════════════════╝
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

88. User Interface layer

89. User Interface layer
The user interface layer is responsible for presen&ng the
applica3on to the end user
┌──────┐╔════════════════════════════════╗
│ │║ User interface layer ║
│ D │╚════════════════════════════════╝
│ o │┌────────────────────────────────┐
│ m ││ Web layer │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

90. User Interface layer
The user interface layer renders the response generated by the
Web layer into the form requested by the user’s client
┌──────┐╔════════════════════════════════╗
│ │║ User interface layer ║
│ D │╚════════════════════════════════╝
│ o │┌────────────────────────────────┐
│ m ││ Web layer │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

91. User Interface layer
For instance:
• A Web browser will probable request a HTML page
• A Web service may want an XML (or JSON) document
• Another client could request a PDF document

92. Many view technologies are available
There exists a plethora of of view technologies
• JSP pages
• JSF pages
• Handlebars

93. Many view technologies are available
However, the naviga/on logic and the need of calling the service
layer does not change with the specific view technology

94. Many view technologies are available
That is why the user interface and the web layer are split
┌──────┐╔════════════════════════════════╗
│ │║ User interface layer ║
│ D │╚════════════════════════════════╝
│ o │┌────────────────────────────────┐
│ m ││ Web layer │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

95. User interface layer’s dependency
The user interface layer may depend on the domain model
┌──────┐╔════════════════════════════════╗
│ │║ User interface layer ║
│ D │╚════════════════════════════════╝
│ o │┌────────────────────────────────┐
│ m ││ Web layer │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

96. User interface layer’s dependency
Indeed, it is o+en convenient to expose and render domain objects
┌──────┐╔════════════════════════════════╗
│ │║ User interface layer ║
│ D │╚════════════════════════════════╝
│ o │┌────────────────────────────────┐
│ m ││ Web layer │
│ a │└────────────────────────────────┘
│ i │┌────────────────────────────────┐
│ n ││ │
│ ││ Service layer │
│ m ││ │
│ o │└────────────────────────────────┘
│ d │┌────────────────────────────────┐
│ e ││ │
│ l ││ Persistence layer │
│ ││ │
└──────┘└────────────────────────────────┘

97. User interface layer’s dependency
This is useful when we start to use forms in our applica2on, as it
makes possible to objec*fy form fields directly into domain
objects

98. References

99. References
• S. Ladd, K. Donald, Expert Spring MVC and Web flows, Apress
PublicaBons
• M. Deinum, K. Serneels, Pro Spring MVC: with Web flows,
Apress PublicaBons
• Chris Richardson, POJOs in AcBon, Manning PublicaBons

100. References
• Mar%n Fowler, Anemic Domain Model
• StackOverflow, What Does it Mean to “Program to an Interface”?