Java - Singleton Pattern

TheSingleton Pattern
…because we all use it but…

Singleton
Pattern
1 & 1 instance
universal access
prevents/controls
concurrency

Singleton
Pattern
logging
database
access
class
loaders
factories

BasicSingleton
no new
utility
method
static
method
static
instance

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;
}
}

BasicSingleton
// Same as before, can handle pre-conditions through static init. block
private static Singleton INSTANCE;
static {
try {
INSTANCE = new Singleton();
} catch (Exception ex) {
throw new RuntimeException("A damn error occured!", ex);
}
}
// Same as before
}
return INSTANCE;
}
}

Broken Idioms
Instance not initialized, no sync
// Seems right, at first, but what about concurrency ?
private static Singleton INSTANCE = null;
// Nothing
}
if (INSTANCE == null) {
}
return INSTANCE;
}
}

Broken Idioms
Synchronized getInstance()
// Concurrency issue fixed, but very slow
// Nothing
}
public static synchronized Singleton getInstance() {
}
return INSTANCE;
}
}

Broken Idioms
Double checked locking
// Does not work this way, broken due to compiler re-ordering
private Singleton() { }
synchronized (Singleton.class) {
}
}
}
return INSTANCE;
}
}

Basic
Singleton 2
The Holder, proposed by Bill Pugh
//No language construct, relies on JVM memory model properties, very lazy
// Nothing
}
// Loaded on the 1st exec of Singleton.getInstance(), not before
private static class SingletonHolder {
public static final Singleton INSTANCE = new Singleton();
}
return SingletonHolder.INSTANCE;
}
}

EnumSingleton
have
methods
can
implements
interface
static
instances
free
thread safe*
* only if
there is no
state
serializable

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() {
// …
}
}

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));
}

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);
}

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;
}
}
}

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

Singletons
Use enums
Use « Lazy Holders »
Beware of Serialization
Singleton are Armful
Don’t use them

A double fix
Fixing double checked locking
// Does not work this way, broken due to compiler re-ordering
}
}
}
return INSTANCE;
}
}

A double fix
Fixing double checked locking
//The right word, at the right time, in the right place can make a difference
private static volatile Singleton INSTANCE = null;
}
}
}
return INSTANCE;
}
}

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/

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

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

Java - Singleton Pattern

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