Comments on: How to write a Singleton (3/5)

By: vibram five fingers kso shoes

vibram five fingers kso shoes — Sat, 03 Jul 2010 04:00:12 +0000

Women's vibram five fingers kso shoes Gray Pink Shoes can keep your feet in the freest condition,just as if your were barefooted. vibram five fingers kso shoes is made of the best and proper materials,so you don't need to doubt of its comfort with so thin sole and upper. Gray match with a dreamlike and romantic color,pink vibram five fingers kso shoes is very consistent with the style of young girls.In this busy and tense times,people need relax without any fetter.

By: Ben Manes

Ben Manes — Wed, 05 Aug 2009 17:06:14 +0000

@Mohamed
Sure, the following relies on the inner class being loaded separately. On load it creates the parent’s instance. This relies on classloading being a serial process, so synchronization is not needed. (Forgive formatting if it shows up poorly!)

public class Singleton {
private Singleton() {}

public Singleton getInstance() {
return Holder.INSTANCE;
}

private static final class Holder {
static final Singleton INSTANCE = new Singleton();
}
}

By: Mohamed El-Beltagy

Mohamed El-Beltagy — Wed, 05 Aug 2009 10:14:13 +0000

The synchronization part is a popular trick that many developers, as I’ve seen, fall into.
But I do agree with Ran as well: “if the singleton object can be instantiated multiple times without penalty, you could drop the synchronize altogether”

@Ben: it’s very sad that you did not mention what you mean by using a static inner class as the instance holder. I’m one of the people who would like to know it.

By: Ignacio Coloma

Ignacio Coloma — Wed, 05 Aug 2009 09:32:11 +0000

There are already a zillion articles dealing with singletons and lazy initialization. For the sake of S/N ratio on the Internet, please add something that is not already there:

http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

Five articles talking about singleton initialization seems like a bit too much.

By: Ben Manes

Ben Manes — Wed, 05 Aug 2009 07:37:33 +0000

Ran’s optimizations aren’t available on older JVMs, like the Sun JDK5. I’ve seen noticible performance problems due to lazy singleton initialization. The locking can be avoided by using a static inner class as the instance holder, which relies on class-loading. Its fast, simple, and sadly not well known.

By: Ran Biron

Ran Biron — Tue, 04 Aug 2009 17:55:07 +0000

You forgot to mention that even if instantiation is slow, after instantiation the monitor would almost always be locked on the “fast track” (optimistic no-locking with fallback) instead of the “slow track” (preemptive locking).

Also, if the singleton object can be instantiated multiple times without penalty, you could drop the synchronize altogether.