This document discusses design patterns and the singleton pattern. It provides an overview of what design patterns are, the different types of design patterns (creational, behavioral, structural), and how design patterns provide solutions to common programming problems. It then focuses on the singleton pattern, explaining that it ensures a class only has one instance and provides a global access point. The document discusses different implementations of the singleton pattern and considerations around thread safety and lazy initialization. It provides examples of when and where the singleton pattern should be used, such as for shared application data or expensive resources.

1. DESIGN PATTERNS
Kaushal Shah

2. WHAT ARE DESIGN PATTERNS
 Solutions to common application design problems
 Solve problems associated with Object creation and interaction
 Evolved over a long period of time and provide best solutions to recurring problems
 Provide boilerplates that can be applied to real life

3. TYPES OF DESIGN PATTERNS
 Creational Patterns
 Behavioral Patterns
 Structural Patterns

4. ENTERPRISE JAVA PATTERNS
 J2EE often contains excessive ‘plumbing’ code like JNDI lookup, XML configuration,
try catch blocks that acquire and release JDBC resources
 Maintaining such code was a drain of resources
 Enterprise Java Patterns solve these problems
 Some of these patterns evolved from the ‘classic’ patterns, others were new and
addressed the flaws of J2EE

5. DESIGN PATTERNS V/S ENTERPRISE PATTERNS
 Enterprise patterns target enterprise software and it’s problems which differ from
problems of desktop applications
 In Java EE 7 most design patterns are embedded in the platform, ready to be used
 Most of the patterns can be implemented via annotations without XML
 Enterprise software development requires discipline and coordination between
developers.
 Design patterns are a good way to approach this problem

6. IMPORTANCE OF DESIGN PATTERNS
 Gives the developers a common ‘vocabulary’
 Code becomes more readable and clean
 Refactoring the code is very simple
 Tested and sound solutions
Software can be made without design patterns, but it’s a lot harder.

7. Anti-Patterns
• Design patterns represent
collected wisdom
• Does not mean we need to
use them all the time
• Over using design patterns
tends to overcomplicate the
system
• May lead to poor
performance

8. MULTI TIER
ARCHITECTURE

9. MULTI TIER ARCHITECTURE
Java EE architecture is separated into tiers:
 Client/Presentation Tier: Usually browsers that connects to Java EE via HTTP
 EIS Tier: Databases or any resource that provides data
 Middle Tier: Consists of web container and EJB container

10. MIDDLE TIER
BUSINESS LAYER:
 Executes business logic
 Involves data retrieved from the EIS Tier
WEB LAYER:
 Manages interactions between Client and Business layer
 Receives and processes request from client for a resource
 Maintains user’s states in a session

11. CONTEXT AND DEPENDENCY INJECTION
 Decouples relation between a component and it’s dependencies
 Injects dependencies into an Object rather than object creating dependency by
using ‘new’ keyword
 Can easily swap out dependency for another compatible object

12. INTERCEPTORS
 Used in Java EE to address ‘cross cutting concerns’
 Common concerns like Logging, Security that are not related to the business logic

13. SINGLETON
PATTERN

14. WHAT IS A SINGLETON ?
 Ensures a class only has one instance
 Provides a global point of access to it
 Falls under Creational design patterns
 Spring uses singleton when creating beans

15. IMPLEMENTATIONS

16. THREAD SAFETY

17. PRE-LOADING THE CLASS

18. LAZY INITIALIZATION

19. DOUBLE CHECKING

20. ENUM IMPLEMENTATION

21. SINGLETON BEANS

22. SINGLETONS AT STARTUP

23. STARTUP ORDER

24. WHEN AND WHERE TO USE SINGLETON
 Access shared data across the whole application domain
 To load and cache expensive resources only once
 To create an application logger
 To manage objects within a class implementing Factory pattern
 To create a Façade object
Heavy use of Singleton sign of misuse

25. THANK YOU !!