I am implementing a singleton class in Java to make sure no more than one instance of the class is created.
A:
Singletons are not threadsafe per default. The common way to create a singleton is to have a factory method that handles creation of the one instance. That way you can control instance creation.
Brian Rasmussen
2009-09-30 07:44:01
+2
A:
public class Singleton {
public static final Singleton INSTANCE = new Singleton();
private Singleton() { ... }
}
Using a static instance variable is preferred to the internal "Holder" class. If you like, you can also make the static member private and provide a method for accessing it, but all that does is add another method call to the stack.
Tim Bender
2009-09-30 08:05:38
It rather depends what you're trying to achieve as to what is preferred - the holder gives you a lazy singleton, the simpler version doesn't give you lazy loading.
Pete Kirkham
2009-09-30 08:09:40
Yes, it does. Java doesn't load classes until they're first used.
Michael Borgwardt
2009-09-30 08:11:47
Not entirely true. It loads them when they're first referenced, e.g. in the method signature of another class.
skaffman
2009-09-30 08:48:54
Accessor methods are normally inlined by the VM. Do not choose public field over accessor to get rid of a "method call on the stack" - in your code, it will look like a method call, but in reality, it'll be the same thing.
gustafc
2009-09-30 08:56:59
A:
What about lazy instanciation: getInstance() returns the singleton or create it if it is the first call.
public class MySingleton
{
private static MySingleton instance;
private MySingleton()
{
// construct object . . .
}
// For lazy initialization
public static synchronized MySingleton getInstance()
{
if (instance==null)
{
instance = new MySingleton();
}
return instance;
}
// Remainder of class definition . . .
}
enguerran
2009-09-30 08:19:29
Lazy instantiation is almost never necessary, and if it is, Java loads classes lazily, so creating the instance in the declaration actually *is* lazy instantiation - without synchronization overhead (in very rare cases that's not lazy enough and skaffman's answer is better).
Michael Borgwardt
2009-09-30 08:26:09
Also note this implementation synchronizes every attempt to obtain a reference to the instance. Since most code will do something like:Singleton.getInstance.doSomething();Singleton.getInstance.doSomethingElse();The overall effect would be additional synchronization severely limiting concurrency with no thread safety guarantees in terms of member access/mutation.
Tim Bender
2009-09-30 08:49:36
Double-checked locking in java is broken http://www.javaworld.com/jw-02-2001/jw-0209-double.htmlIn short, this way of guaranteeing that this class is instantiated only once may result in hard-to-find concurrency issues. Never use this.
Buhb
2009-09-30 09:29:34
DCL is non-broken as of Java 5. Check out http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html, scroll down to "Under the new Java Memory Model".
gustafc
2009-09-30 21:10:49
+10
A:
The best way to make a singleton? Use an enum.
public enum Singleton {
INSTANCE;
public void foo(){ ... }
}
// Usage:
Singleton.INSTANCE.foo();
You get lots of help from the VM not only to avoid double instantiation, but it also helps you avoid deserialization corruption.
gustafc
2009-09-30 08:51:08
A:
If you care about concurrency, you won't be using shared global state.
jrockway
2009-09-30 08:52:35
If the singleton holds the accumulation of results from a singular parallelized problem why not? A singleton epitomizes global state, and as such it would be a fair usage scenario.
_ande_turner_
2009-09-30 09:13:32
There are several use cases for classes represented with only one instance that is shared between processes. A typical well designed webapp using spring will probably have a bunch of classes instantiated in singleton scope, shared between worker threads.
Buhb
2009-09-30 09:32:06
+4
A:
Perhaps the best way is to use an enum with a single instance. This has the added benefit of being serializable and guaranteeing singleton-ness against serialization and reflection, which no "straightforward" Singleton implementation does (private? I have reflection, I scoff derisively at your access modifiers!). It's also very simple to implement:
public enum Singleton {
INSTANCE;
// fields and methods go here
}
Michael Borgwardt
2009-09-30 09:14:31