The static field example is from the book.
-Adrian
--- On Sat, 7/3/10, Jacques Le Roux <jacques.le.r...@les7arts.com> wrote:
> Thanks Adrian,
>
> Are they examples from the books or your own?
>
> Jacques
>
> From: "Adrian Crum" <adrian.c...@yahoo.com>
> > As the article and the book (Java Concurrency In
> Practice) both point out - the motivation for using DCL (to
> overcome slow speed)
> > is less of an issue (or a non-issue) with newer JVMs.
> >
> > I am not a Java expert. But enough Java experts have
> stressed the need to eliminate this anti-pattern that I am
> willing to take
> > their word for it. I said that in preparation for
> repeating what Adam said earlier - there are enough
> potential problems with DCL
> > that it should not be used. I agree that there might
> be some DCL code in OFBiz that hasn't caused any problems so
> far, and it
> > might be tempting to think "if it isn't broke, don't
> fix it" - but personally, I prefer to remove the potential
> problem.
> >
> > I have seen DCL used in two circumstances in OFBiz: to
> lazy load a static field (singleton), and to get elements
> from a Map.
> >
> > 1. Lazy load a static field
> > ---------------------------
> >
> > // This is bad, don't do this
> > public class MyClass {
> > private static Resource resource;
> >
> > public static Resource getInstance() {
> > if (resource == null {
> > synchronized (MyClass.class) {
> > if (resource == null) {
> > resource = new
> Resource();
> > }
> > }
> > }
> > return resource;
> > }
> > }
> >
> > The easiest fix in this example is to make
> getInstance() synchronized:
> >
> > public synchronized static Resource getInstance() {
> > if (resource == null {
> > resource = new Resource();
> > }
> > return resource;
> > }
> >
> > The lazy load has been preserved, but at a cost of
> synchronizing every call to getInstance().
> >
> > It would be better to eliminate the lazy load
> altogether:
> >
> > public class MyClass {
> > private static Resource resource = new
> Resource();
> >
> > public static Resource getInstance() {
> > return resource;
> > }
> > }
> >
> > 2. Get elements from a Map
> > --------------------------
> >
> > // This is bad, don't do this
> > public class MyClass {
> > private static Map<String, Resource>
> resourceMap = FastMap.newInstance();
> >
> > public static Resource getResource(String
> resourceName) {
> > Resource resource =
> resourceMap.get(resourceName);
> > if (resource == null {
> > synchronized (MyClass.class) {
> > resource =
> resourceMap.get(resourceName);
> > if (resource == null) {
> > resource = new
> Resource();
> >
> resourceMap.put(resourceName, resource);
> > }
> > }
> > }
> > return resource;
> > }
> > }
> >
> > Again, the easiest fix is to make the getResource
> method synchronized:
> >
> > public synchronized static Resource getResource(String
> resourceName) {
> > Resource resource =
> resourceMap.get(resourceName);
> > if (resource == null {
> > resource = new Resource();
> > resourceMap.put(resourceName, resource);
> > }
> > return resource;
> > }
> >
> > This fix comes at a cost of synchronizing every call
> to getResource. If the cost of creating a new instance of
> Resource is low, we
> > can skip synchronization altogether:
> >
> > public static Resource getResource(String
> resourceName) {
> > Resource resource =
> resourceMap.get(resourceName);
> > if (resource == null {
> > resource = new Resource();
> > resourceMap.put(resourceName, resource);
> > resource =
> resourceMap.get(resourceName);
> > }
> > return resource;
> > }
> >
> > Using this method, there is a chance that more than
> one thread will create a Resource instance, but we are not
> concerned about
> > that - we just do a second
> resourceMap.get(resourceName) method call to make sure we
> have the Resource instance that "won."
> >
> > -Adrian
> >
> > --- On Sat, 7/3/10, Jacques Le Roux <jacques.le.r...@les7arts.com>
> wrote:
> >
> >> From: Jacques Le Roux <jacques.le.r...@les7arts.com>
> >> Subject: Re: svn commit: r959875 -
> /ofbiz/trunk/framework/widget/src/org/ofbiz/widget/ModelWidgetAction.java
> >> To: dev@ofbiz.apache.org
> >> Date: Saturday, July 3, 2010, 4:41 AM
> >> Very good article indeed. What do we
> >> learn more in the book about DCL?
> >> If there is nothing more in the book, I suppose we
> prefer
> >> to use the static variable solution in OFBiz?
> >>
> >> Thanks
> >>
> >> Jacques
> >>
> >> From: "Adrian Crum" <adrian.c...@yahoo.com>
> >> > --- On Fri, 7/2/10, Scott Gray <scott.g...@hotwaxmedia.com>
> >> wrote:
> >> >> The book would have done well to use a
> more
> >> complicated
> >> >> example of why DCL shouldn't be used
> though. This
> >> >> article seems to do a better better job
> of it: http://www.ibm.com/developerworks/java/library/j-dcl.html
> >> >
> >> > That was the article I found a while back -
> before I
> >> got the book.
> >>
> >>
> >
> >
> >
> >
>
>
>
