Singleton pattern

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Singleton pattern

Singleton pattern

2005-10-04 - By Chris Harrison

Back

Reply: 1 2 3 4 5 6 7 8

This works, but is not optimised. You can optimise the method to do lazy
instantiation like so:

class Singleton
{
private static Singleton instance = null;
private static Object mutex = new Object();

public Singleton()
{
}

public static Singleton getInstance()
{
if (instance == null)
{
synchonized(mutex)
{
instance = new Singleton();
}
}
return instance;
}
}

Its also tempting to do double check locking, but thats been proven to not work:
http://www-128 (See http://www-128.ora-code.com).ibm.com/developerworks/java/library/j-dcl.html

ch.

-- --Original Message-- --
From: An interest list for Sun Java Center J2EE Pattern Catalog [ mailto
:J2EEPATTERNS-INTEREST@(protected)]On Behalf Of Harish
Sent: Tuesday, 4 October 2005 3:17 PM
To: J2EEPATTERNS-INTEREST@(protected)
Subject: Re: Singleton pattern

Hi,
The code works fine for multithreaded access to the getInstance() method.
However, when you analyze it you realize that synchronization is required only
for the first invocation of the method. Subsequent invocations do not require
synchronization because the first invocation is the only invocation that
executes the code at //2,( see below) which is the only line that requires
synchronization. All other invocations determine that instance is non-null and
return it. Multiple threads can safely execute concurrently on all invocations
except the first. However, because the method is synchronized, you pay the cost
of synchronization for every invocation of the method, even though it is only
required on the first invocation.
Another way is to Forgo synchronization and use a static field.
class Singleton
{
private static Singleton instance = new Singleton();
private Singleton()
{
//...
}
public static Singleton getInstance()
{
return instance;
}
}
The code in Listing 10 does not use synchronization and ensures that the
Singleton object is not created until a call is made to the static getInstance(
) method. This is a good alternative if your objective is to eliminate
synchronization
Thanks
Harish

-- -- Original Message -- --
From: "SUBSCRIBE EJB-INTEREST anonymous" <ahtwog@(protected)>
To: <J2EEPATTERNS-INTEREST@(protected)>
Sent: Tuesday, October 04, 2005 8:06 AM
Subject: Singleton pattern

> Hi,
> Is the below singleton pattern thread safe.
> public class Singleton
> {
> private static Singleton singleton ;
> public synchronized static Singleton getInstance() throws
> {
> if(singleton ==null)
> {
> singleton = new Singleton (); ////// >>> 2
> }
>
> return singleton ;
> }
>
> private Singleton () {
> //perform member variable initialization here
> }
>
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<HTML><HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859 (See http://iso-8859.ora-code.com)-1">
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<META content="MSHTML 6.00.2900.2627" name=GENERATOR></HEAD>
<BODY>
This works, but is not optimised.  You can optimise the
method to do lazy instantiation like so: class
Singleton {         private static
Singleton instance = null;         private
static Object mutex = new
Object();         public
Singleton()        
{        
}         public static Singleton
getInstance()        
{        
        if (instance ==
null)        
       
{        
       
       
synchonized(mutex)        
       
       
{        
       
       
        instance = new
Singleton();        
       
       
}        
       
}        
        return
instance;         } } Its
also tempting to do double check locking, but thats been proven to not
work: <A href="http://www-128 (See http://www-128.ora-code.com).ibm.com/developerworks/java/library/j-dcl.html"
target=_blank>http://www-128 (See http://www-128.ora-code.com).ibm.com/developerworks/java/library/j-dcl.html</A>
 
ch.
 -- --Original Message-- -- From: An interest list for Sun
Java Center J2EE Pattern Catalog [<A
href="mailto:J2EEPATTERNS-INTEREST@(protected)">mailto:J2EEPATTERNS-INTEREST
@(protected)</A>]On
Behalf Of Harish Sent: Tuesday, 4 October 2005 3:17 PM To:
J2EEPATTERNS-INTEREST@(protected) Subject: Re: Singleton
pattern Hi, The code  works fine for multithreaded access to
the getInstance() method. However, when you analyze it you realize that
synchronization is required only for the first invocation of the method.
Subsequent invocations do not require synchronization because the first
invocation is the only invocation that executes the code at //2,( see below)
which is the only line that requires synchronization. All other invocations
determine that instance is non-null and return it. Multiple threads can safely
execute concurrently on all invocations except the first. However, because the
method is synchronized, you pay the cost of synchronization for every invocation
of the method, even though it is only required on the first
invocation. Another way is to Forgo synchronization and use a static 
field. class Singleton { private static Singleton instance = new
Singleton();   private Singleton()   {    
//...   }   public static Singleton getInstance()  
{     return instance;   } } The code in Listing
10 does not use synchronization and ensures that the Singleton object is not
created until a call is made to the static getInstance() method. This is a good
alternative if your objective is to eliminate
synchronization Thanks Harish -- -- Original Message -- -- From:
"SUBSCRIBE EJB-INTEREST anonymous" <ahtwog@(protected)> To:
<J2EEPATTERNS-INTEREST@(protected)> Sent: Tuesday, October 04, 2005
8:06 AM Subject: Singleton pattern > Hi, > Is the below
singleton pattern thread safe. > public class Singleton > { >
private static Singleton singleton ; > public synchronized static
Singleton getInstance() throws > { >  if(singleton
==null) >  { >    singleton = new Singleton
();           //////
>>> 2 >  } > >  return singleton ; >
} > > private Singleton ()  { >  //perform member
variable initialization here > } > >
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/A> >
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