Java - Singleton Pattern
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The document discusses the Singleton design pattern in Java, detailing various implementations, their advantages, and pitfalls, particularly regarding concurrency and serialization. It highlights broken idioms associated with singletons, such as improper synchronization and double-checked locking issues. The document advises against the use of singletons due to their potential to create hard-coded dependencies and global state issues, suggesting alternatives like inversion of control (IoC).
![Serializable
Singleton
Part 1
public class Elvis implements Serializable {
private static final Elvis INSTANCE = new Elvis();
private static final long serialVersionUID = 42L;
private volatile String[] songs = {"Hound Dog", "Heartbreak Hotel"};
public static Elvis getInstance() { return INSTANCE; }
private Elvis() { }
public void leaveTheBuilding() { }
public void setFavouriteSongs(String[] songs) {
this.songs = songs.clone();
}
public void printFavorites() {
System.out.println(Arrays.toString(songs));
}](https://image.slidesharecdn.com/2014-01-30-charlescasadei-singletonpattern-140708030833-phpapp02/85/Java-Singleton-Pattern-20-320.jpg)
![Serializable
Singleton
Part 3
private Object writeReplace() {
return new ElvisSerializableForm(songs);
}
private static class ElvisSerializableForm implements Serializable {
private final String[] songs;
public ElvisSerializableForm(String[] favoriteSongs) {
this.songs = favoriteSongs;
}
publicObject readResolve() {
Elvis.INSTANCE.setFavouriteSongs(songs);
return Elvis.INSTANCE;
}
}
}](https://image.slidesharecdn.com/2014-01-30-charlescasadei-singletonpattern-140708030833-phpapp02/85/Java-Singleton-Pattern-22-320.jpg)
Java - Singleton Pattern
- 1. TheSingleton Pattern …because we all use it but…
- 3. Singleton Pattern 1 & 1 instance universal access prevents/controls concurrency
- 7. BasicSingleton // Minimal Singleton (or is it ?), can’t handle pre-conditions public class Singleton { private static final Singleton INSTANCE = new Singleton(); private Singleton() { if (INSTANCE != null) { throw new IllegalStateException("Woops, the matrix is broken"); } } public static Singleton getInstance() { return INSTANCE; } }
- 8. BasicSingleton // Same as before, can handle pre-conditions through static init. block public class Singleton { private static Singleton INSTANCE; static { try { INSTANCE = new Singleton(); } catch (Exception ex) { throw new RuntimeException("A damn error occured!", ex); } } private Singleton() { // Same as before } public static Singleton getInstance() { return INSTANCE; } }
- 10. Broken Idioms Instance not initialized, no sync // Seems right, at first, but what about concurrency ? public class Singleton { private static Singleton INSTANCE = null; private Singleton() { // Nothing } public static Singleton getInstance() { if (INSTANCE == null) { INSTANCE = new Singleton(); } return INSTANCE; } }
- 11. Broken Idioms Synchronized getInstance() // Concurrency issue fixed, but very slow public class Singleton { private static Singleton INSTANCE = null; private Singleton() { // Nothing } public static synchronized Singleton getInstance() { if (INSTANCE == null) { INSTANCE = new Singleton(); } return INSTANCE; } }
- 12. Broken Idioms Double checked locking // Does not work this way, broken due to compiler re-ordering public class Singleton { private static Singleton INSTANCE = null; private Singleton() { } public static Singleton getInstance() { if (INSTANCE == null) { synchronized (Singleton.class) { if (INSTANCE == null) { INSTANCE = new Singleton(); } } } return INSTANCE; } }
- 14. Basic Singleton 2 The Holder, proposed by Bill Pugh //No language construct, relies on JVM memory model properties, very lazy public class Singleton { private Singleton() { // Nothing } // Loaded on the 1st exec of Singleton.getInstance(), not before private static class SingletonHolder { public static final Singleton INSTANCE = new Singleton(); } public static Singleton getInstance() { return SingletonHolder.INSTANCE; } }
- 15. EnumSingleton
- 16. EnumSingleton have methods can implements interface static instances free thread safe* * only if there is no state serializable
- 17. EnumSingleton Proposed by Joshua Bloch // OK in most cases, can be hacked by malicious serialized form public enum Elvis implements Singer,Zombie { ELVIS; @Override public void singASong() { // … } @Override public void eatBrains() { // … } }
- 20. Serializable Singleton Part 1 public class Elvis implements Serializable { private static final Elvis INSTANCE = new Elvis(); private static final long serialVersionUID = 42L; private volatile String[] songs = {"Hound Dog", "Heartbreak Hotel"}; public static Elvis getInstance() { return INSTANCE; } private Elvis() { } public void leaveTheBuilding() { } public void setFavouriteSongs(String[] songs) { this.songs = songs.clone(); } public void printFavorites() { System.out.println(Arrays.toString(songs)); }
- 21. Serializable Singleton Part 2 // this should never be invoked private Object readResolve() { return INSTANCE; } private void readObject(ObjectInputStream ex) throws IOException { throw new InvalidObjectException("Cannot load another Elvis"); } private Object writeReplace() { return new ElvisSerializableForm(songs); }
- 22. Serializable Singleton Part 3 private Object writeReplace() { return new ElvisSerializableForm(songs); } private static class ElvisSerializableForm implements Serializable { private final String[] songs; public ElvisSerializableForm(String[] favoriteSongs) { this.songs = favoriteSongs; } publicObject readResolve() { Elvis.INSTANCE.setFavouriteSongs(songs); return Elvis.INSTANCE; } } }
- 24. Singletons are Armful global state hard to test hard coded dependencies Similar to Village of the Damned design anti- pattern Might be OK if there is NO STATE use IoC for factories
- 25. Singletons
- 26. Singletons Use enums Use « Lazy Holders » Beware of Serialization Singleton are Armful Don’t use them
- 27. Singletons
- 28. Quizz Fixing the broken ones
- 29. A double fix Fixing double checked locking // Does not work this way, broken due to compiler re-ordering public class Singleton { private static Singleton INSTANCE = null; private Singleton() { } public static Singleton getInstance() { if (INSTANCE == null) { synchronized (Singleton.class) { if (INSTANCE == null) { INSTANCE = new Singleton(); } } } return INSTANCE; } }
- 30. A double fix Fixing double checked locking //The right word, at the right time, in the right place can make a difference public class Singleton { private static volatile Singleton INSTANCE = null; private Singleton() { } public static Singleton getInstance() { if (INSTANCE == null) { synchronized (Singleton.class) { if (INSTANCE == null) { INSTANCE = new Singleton(); } } } return INSTANCE; } }
- 31. Biblio
- 32. Biblio Bloch, Joshua. EffectiveJava,Second Edition. O’Reilly, 2008 Croisier, Olivier. De la bonne implémentation du Singleton en Java. 2008. http://thecodersbreakfast.net/index.php?post/2008/02/25/26-de- la-bonne-implementation-du-singleton-en-java Hevery, Miško. Root Cause of Singletons, 2008. http://misko.hevery.com/2008/08/25/root-cause-of-singletons/ Hevery, Miško. Singleton are Pathlogical Liars, 2008. http://misko.hevery.com/2008/08/17/singletons-are-pathological- liars/
- 33. Biblio Hevery, Miško. Where Have All the Singletons Gone ?, 2008. http://misko.hevery.com/2008/08/21/where-have-all-the- singletons-gone/ Kabutz, Heinz. The Java Specialists’ Newsletter #163, 2008. http://www.javaspecialists.eu/archive/Issue163.html Manson, Jeremy. Double Checked Locking, 2008. http://jeremymanson.blogspot.fr/2008/05/double-checked- locking.html Pugh, Bill. The « Double-Checked Locking is Broken » Declaration. http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleChecke dLocking.html
- 34. Biblio Pugh, Bill. The Java Memory Model, 2004. http://www.cs.umd.edu/~pugh/java/memoryModel/ Pugh, Bill. JSR 133 (Java Memory Model) FAQ, 2004. http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133- faq.html Wikipedia, Initialization-on-demand holder idiom. http://en.wikipedia.org/wiki/Initialization-on- demand_holder_idiom Wikipedia, Singleton pattern. http://en.wikipedia.org/wiki/Singleton_pattern