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| | | Singleton pattern | Singleton pattern 2005-10-04 - By Dan Marchant
Back
Supposively it doesn't work on most modern JVMs though.
But unless you are doing real-time programming with java you will
probably never notice it breaking. In the webapp environment, use a
servlet to initialize your singletons and this will usually prevent
any double-checked locking issues since the webapp will not be
available until it is initialized there by preventing business logic
to run from user behavior. You can set the servlet to be initialized
on webapp deployment within your web.xml.
Article on why this code example is broken:
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
Bug from sun themselves:
http://bugs.sun.com/bugdatabase/view_bug.do;:YfiG?bug_idc12351
Hope this helps.
Dan
On 10/4/05, bayar <bayar@(protected)> wrote:
> as ibm article says, the true one is;
>
> 1. public static Singleton getInstance()
> 2. {
> 3. if (instance == null)
> 4. {
> 5. synchonized(mutex)
> 6. {
> if (instance == null)
> {
> 7. instance = new Singleton();
> }
> 8. }
> 9. }
> 10. return instance;
> 11. }
> 12. }
>
>
> -- --Original Message-- --
> From: An interest list for Sun Java Center J2EE Pattern Catalog
> [mailto:J2EEPATTERNS-INTEREST@(protected)]On Behalf Of Stefano Greco
> Sent: Tuesday, October 04, 2005 2:45 PM
> To: J2EEPATTERNS-INTEREST@(protected)
> Subject: Re: Singleton pattern
> Importance: High
>
>
> Hi,
> I don't think it's a correct.
>
> in the method
>
> 1. public static Singleton getInstance()
> 2. {
> 3. if (instance == null)
> 4. {
> 5. synchonized(mutex)
> 6. {
> 7. instance = new Singleton();
> 8. }
> 9. }
> 10. return instance;
> 11. }
> 12. }
>
> Two or more threads may valuate the if statement in line 3 and they may have
> all condition to "true"
> then all threads stop on line 5 and "only one" make the instance of the
> Singleton.
>
> But, when the "only one" 's finished and exit from the synchronized lines
> another thread make the instance of the Singleton
> on so all other threads.
>
>
> bye Stefano
>
> >-- Messaggio Originale --
> >Date: Tue, 4 Oct 2005 15:40:39 +1000
> >Reply-To: An interest list for Sun Java Center J2EE Pattern Catalog
> <J2EEPATTERNS-INTEREST@(protected)>
> >From: Chris Harrison <CHarrison@(protected)>
> >Subject: Re: Singleton pattern
> >To: J2EEPATTERNS-INTEREST@(protected)
> >
> >
> >
> >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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