A discussion on Singleton and DI/IoC patterns.

1. codecentric AG
Dusan Zamurovic
A pattern discussion

2. codecentric AG
 Singleton pattern
 things we sometimes forget
 Dependency injection / Inversion of Control
 Paired with Singleton
 Example without third party library
TOPICS

3. codecentric AG
 What Singleton pattern does?
 Implementation examples
 Wrong implementations
 Implementations that look okay
 Right implementations
Singleton

4. codecentric AG
Singleton 01
public class Singleton01 {
// TODO Make this class final?
private static Singleton01 instance = null;
private Singleton01() {
// Private constructor prevents instantiation,
// but also prevents sub-classing.
}
public static Singleton01 getInstance() {
if (instance == null) {
instance = new Singleton01();
}
return instance;
}
}

5. codecentric AG
Singleton 02
private static Singleton02 instance = null;
public static Singleton02 getInstance() {
if (instance == null) {
simulateMultiThreadEnvironment();
instance = new Singleton02();
}
return instance;
}
// Not synchronized, not thread safe.

6. codecentric AG
Singleton 03
private static Singleton03 instance = null;
public synchronized static Singleton03 getInstance() {
if (instance == null) {
simulateMultiThreadEnvironment();
instance = new Singleton03();
}
return instance;
}
// Synchronized, thread safe.

7. codecentric AG
Singleton 04
private static Singleton04 instance = null;
public static Singleton04 getInstance() {
if (instance == null) {
synchronized (Singleton04.class) {
simulateMultiThreadEnvironment();
instance = new Singleton04();
}
}
return instance;
}
// Synchronized, performance optimization, not thread safe.

8. codecentric AG
Singleton 05
private static Singleton05 instance = null;
public static Singleton05 getInstance() {
if (instance == null) {
synchronized (Singleton05.class) {
if (instance == null) {
instance = new Singleton05();
}
}
}
return instance;
}
// Double-checked locking – doesn’t work prior to Java 1.5.

9. codecentric AG
Singleton 06
private static final Singleton06 INSTANCE = new Singleton06();
public static Singleton06 getInstance() {
return INSTANCE;
}
// Eager instantiating of the class. This works.
// What about serialization/deserialization?

10. codecentric AG
Singleton 06 Serializable
public class Singleton06 implements Serializable {
private static final Singleton06 INSTANCE = new Singleton06();
public static Singleton06 getInstance() {
return INSTANCE;
}
}
protected Object readResolve() {
return getInstance();
}

11. codecentric AG
Singleton 07
public enum Singleton07 {
INSTANCE;
}
// What about serialization/deserialization?
// What about reflection?

12. codecentric AG
 What is DI / IoC?
 Implementation example
 No third party library
Dependency Injection / Inversion of Control

13. codecentric AG
Dependency Injection / Inversion of Control
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Component {
}
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Inject {
}
// @Component is used to mark a class – a thing to inject.
// @Inject is used to mark a field – a place to inject.

14. codecentric AG
Dependency Injection / Inversion of Control
@Component
public class Controller {
@Inject
private ManageCapable manager;
…
}
@Component
public class Manager implements ManageCapable {
@Inject
private Repository repository;
…
}

15. codecentric AG
Dependency Injection / Inversion of Control
@Component
public class Repository {
public Resourcable save(Resourcable aResource) {
// Here we "persist" the given resource
// and return it like we really did it.
return aResource;
}
}

16. codecentric AG
Dependency Injection / Inversion of Control
public class PackageScanner {
…
}
public class ClassScanner {
…
}
public class DependencyInjectionManager {
…
}
// https://github.com/dzamurovic/meetup_singleton.git

17. codecentric AG
Dependency Injection / Inversion of Control
public synchronized void run() throws Exception {
if (initialized) {
return;
}
// Find classes annotated with @Component.
List<Class> components = packageScanner.findAnnotatedClasses(
"rs.codecentric.meetup.diioc.example", Component.class);
// For each component...
for (Class component : components) {
Class definingClass = classScanner.getDefiningClass(component);
// ... check if its instance already exists...
if (!graph.containsKey(definingClass)) {
// ... and instantiate it, if it doesn't.
String className = component.getName();
graph.put(definingClass, Class.forName(component.getName()).newInstance());
}
}
}

18. codecentric AG
Dependency Injection / Inversion of Control
// For each object that is created...
for (Iterator<Class> classIterator = graph.keySet().iterator();
classIterator.hasNext();) {
Class definingClass = classIterator.next();
Object object = graph.get(definingClass);
// ... find dependencies it needs, ...
for (Field f : classScanner.findAnnotatedFields(object.getClass(),
Inject.class)) {
if (!graph.containsKey(f.getType())) {
// ... throw an error if a dependency cannot be satisfied, ...
}
// ... or set a value of the dependency.
Object dependency = graph.get(f.getType());
f.set(object, dependency);
}
}
initialized = true;
}

19. codecentric AG
Dependency Injection / Inversion of Control
public static void main(String args[]) throws Exception {
final DependencyInjectionManager diManager =
DependencyInjectionManager.getInstance();
Thread t1 = new DependencyInjectionThread("DI-thread-0", diManager);
Thread t2 = …
t1.start();
t2.st…
t1.join();
t2.j…
diManager.describeDependencyGraph();
}

20. codecentric AG
QUESTIONS?
https://github.com/dzamurovic/meetup_singleton.git
dusan.zamurovic@codecentric.de
@ezamur
@CodingSerbia
http://www.meetup.com/Coding-Serbia/
www.codecentric.de
www.codecentric.rs
https://blog.codecentric.de
6/27/2014 20