Singleton pattern

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Singleton pattern

Singleton pattern

2005-10-04 - By Harish

Back

Reply: 1 2 3 4 5 6 7 8

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
> }
>
> ====================================================================
> Companion Site: http://www.corej2eepatterns.com
> J2EE BluePrints: http://java.sun.com/blueprints/corej2eepatterns
> List Archive: http://archives.java.sun.com/archives/j2eepatterns-interest.html
> Unsubscribing: email "signoff J2EEPATTERNS-INTEREST" to listserv@(protected)
>

====================================================================
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Unsubscribing: email "signoff J2EEPATTERNS-INTEREST" to listserv@(protected)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=iso-8859 (See http://iso-8859.ora-code.com)-1">
<META content="MSHTML 6.00.2800.1505" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY>
<DIV>
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 <CODE>Singleton</CODE> object is not created until a call is made to the
<CODE>static getInstance()</CODE> method. This is a good alternative if your
objective is to eliminate synchronization
Thanks
Harish
 </DIV>
<DIV>-- -- Original Message -- -- 
<DIV>From: "SUBSCRIBE EJB-INTEREST
anonymous" <<A href="mailto:ahtwog@(protected)">ahtwog@(protected)</A>></DIV>
<DIV>To: <<A
href="mailto:J2EEPATTERNS-INTEREST@(protected)">J2EEPATTERNS-INTEREST@(protected)</A>></DIV>
<DIV>Sent: Tuesday, October 04, 2005 8:06
AM</DIV>
<DIV>Subject: Singleton
pattern</DIV></DIV>
<DIV> </DIV>> 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 > } > >
==================================================================== >
Companion Site: <A href="http://www.corej2eepatterns.com">http://www.corej2eepatterns.com</A> > J2EE BluePrints: <A
href="http://java.sun.com/blueprints/corej2eepatterns">http://java.sun.com/blueprints/corej2eepatterns</A> > List Archive: <A
href="http://archives.java.sun.com/archives/j2eepatterns-interest.html">http://archives.java.sun.com/archives/j2eepatterns-interest.html<
/A> > Unsubscribing: email "signoff
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