This document discusses approaches to managing architecture for Java and Spring applications. It covers topics like granularity of modules, layers and slices; using packages and code analysis tools to enforce architecture; a plain Java approach using packages and visibility; and the Hera plugin system for localizing change and enabling flexibility. Hera is a small plugin registry that allows selecting plugins based on criteria and callbacks to extend functionality in a modular way.

1. Whoops!
Where did my
architecture go?
Approaches to architecture management for
Java and Spring applications
Oliver Gierke

2. Oliver Gierke
SpringSource Engineer
Spring Data
ogierke@vmware.com
olivergierke
www.olivergierke.de

3. Background
5 years of consulting
Lots of code reviews
Eoin Woods‘ talk on InfoQ

4. Roadmap
Architecture 101
A plain Java based approach
Hera

5. The 3 C' of
Architecture
Constraints
Complexity
Change
© Uwe Friedrichsen

6. The 2 C's of
Architecture
Complexity
Change

7. Architecture 101

8. Know your
dependencies

9. Granularity
Modules
Layers
Vertical slices
Subsystems

10. Granularity
Java ARchive
Package
Class

11. Of layers
and slices…

12. Slice A Slice B Slice C
Layer 1
Layer 2
Layer 3

13. Layers
Well understood
Known to developers
Less important to business

14. Slices
Hardly understood
New to developers
Key for business req

15. Slice A Slice B Slice C
Layer 1
Layer 2
Layer 3

16. " How to implement
an architecture
inside a codebase?

17. Architecture
VS.
Codebase

18. " How to implement
an architecture
inside a codebase?

19. " How to enforce
an architecture
inside a codebase?

20. Code analysis
JDepend
Sonar

21. Demo

22. Sonargraph
Formerly known as SonarJ

23. Demo

24. A plain Java
based approach

25. " How far can we get
with plain Java
means only?

26. Slice A Slice B Slice C
Layer 1
Layer 2
Layer 3

27. Packages

28. ….${layer}.${slice}
VS.
….${slice}.${layer}

29. …domain.core
…service.core
…repository.core

30. …core.domain
…core.service
…core.repository

31. Layers first
Leaks slice internals
Lower layers visible to everyone

32. " Start with less
packages and the
least visibility
possible…

33. Slice A Slice B Slice C
Layer 1
Layer 2
Layer 3

34. Slice A Slice B Slice C
Layer 1
Layer 2
Layer 3

35. Slices first
Start with package per slice
Expose interfaces and domain types
Keep implementations private

36. Slices first
Encapsulates business module
Internals understood anyway

37. Subsystems

38. Background
Risk mgmt. at German public bank
Quite a few other SpringSource clients

39. Systems
grow big!

40. Remember
the 2 C's?

41. Complexity
& Change

42. " How to localize
change?

43. " Separate frequent
from infrequent
change!

44. Granularity
Class
Package
Java ARchive

45. Host
SPI SPI SPI
Plugin Plugin

46. Context
No OSGi
Spring based
Build-time composition
Don‘t touch the host system

47. App
Plugin Plugin
Host

48. " How to make the
host aware of the
plugins?

49. " How to dynamically
collect Spring beans
of a given type?

50. classpath*:META-INF/
spring/plugin-context.xml

51. Host
SPI SPI SPI
Plugin Plugin

52. classpath*:META-INF/
spring/plugin-context.xml
SPI SPI SPI
META-INF/ META-INF/
spring/plugin- spring/plugin-
context.xml context.xml

53. @Component
public class MyComponentImpl implements TransferService {
private List<MyPlugin> plugins;
@Autowired
public MyComponentImpl(List<MyPlugin> plugins) {
this.plugins = plugins;
}
…
}
public interface MyPlugin {
void doSomething();
}

54. Demo

55. XML?
Back in the days

56. (XML?)
Back in the days
Probably not a big deal anymore

57. Easy access?

58. @Component
public class MyComponentImpl implements TransferService {
private List<MyPlugin> plugins;
@Autowired
public MyComponentImpl(List<MyPlugin> plugins) {
this.plugins = plugins;
}
…
}
public interface MyPlugin {
void doSomething();
}

59. @Component
public class MyComponentImpl implements TransferService {
// Constructor omitted
public Result myMethod(SomeParameter parameter) {
// Select the first one to match to invoke?
// Select multiple ones to invoke?
// Select and fallback to one if none found?
// Select and throw an exception if none found?
}
}

60. Hera

61. " The smallest plugin
system ever!

62. Plugins
Selection criterion
Callback method
Let the implementation decide

63. Registry
Equipped with plugins
Common access patterns

64. public interface Plugin<T> {
public boolean supports(T delimiter );
}
public interface PluginRegistry<S extends Plugin<T>, T> {
T getPluginFor(S delimiter);
T getPluginFor(S delimiter, T default);
<E extends Exception> T getPluginFor(S del, E e) throws E;
List<T> getPluginsFor(S delimiter);
…
}

65. @Component
public class MyComponentImpl implements TransferService {
private PluginRegistry<MyPlugin, String> plugins;
@Autowired
public MyComponentImpl(
PluginRegistry<MyPlugin, String> plugins) {
this.plugins = plugins;
}
}
public interface MyPlugin extends Plugin<String> {
void doSomething();
}

66. @Component
public class MyComponentImpl implements TransferService {
private final MyPlugin defaultPlugin = new
MyDefaultPlugin();
public Result myMethod(String parameter) {
// Select the first one to match to invoke?
… = plugins.getPluginFor(parameter);
// Select multiple ones to invoke?
… = plugins.getPluginsFor(parameter);
// Select and fallback to one if none found?
… = plugins.getPluginFor(parameter, defaultPlugin);
// Select and throw an exception if none found?
… = plugins.getPluginsFor(parameter, new
RuntimeException());
}
}

67. OrderAware
PluginRegistry
Respects @Order/Ordered
OAPR.reverse()

68. Bells‘n‘whistles
FactoryBean
Spring namespace
Lazy-eval

69. Hera
http://hera.synyx.org
Apache 2.0
Soon to be on GitHub

70. Spring Integration2

71. <bean class="….FirstSamplePlugin" />
<bean class="….SecondSamplePlugin" />
<int:channel id="input" />
<int:channel id="output" />
<int-hera:dynamic-service-activator
input-channel="input"
outputChannel="output"
plugin-type= "….MyPlugin"
method= "myBusinessMethod"
delimiter="payload"
invocation-arguments= "payload" />

72. Demo

73. Take-aways
Know your dependencies
On every granularity
Start as strict as possible
Get lenient where necessary

74. Thanks & credits
Eoin Woods - Talk @ InfoQ
Uwe Friedrichsen - Slides @ Slideshare

75. Resources
Spring Data JPA @ GitHub
Sonargraph
Hera