Re: singleton

From:

Thomas Fritsch <i.dont.like.spam@invalid.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 22 Sep 2006 17:33:59 GMT

Message-ID:

<newscache$ns806j$nm1$1@news.ops.de>

swan2030@gmail.com wrote:

I saw two ways of instantiate singleton.
Somebody can help me compare those two. ( with multi-thread in mind).

1)
private static final MyClass singleton = new MyClass();
public static MyClass getInstance()
{
return singleton;
}

2) Lazy and thread safe
static private class Holder
{
static protected final MyClass instance= new MyCLass();
}

public static MyClass getInstance()
{
return MyClass.Holder.instance;
}

3) Lazy and thread-safe
private static MyClass singleton;
public synchronized static MyClass getInstance()
{
   if (singleton == null)
     singleton = new MyClass();
   return singleton;
}

IMHO all 3 implementations are equivalent with respect to thread-safety.
But they are different with respect to memory/time-consumption.

(1) has the advantage of being simple.
And, by the way, it is thread-safe, too.
It has the disadvantage of creating the MyClass instance, even if it is
never needed. This may be an issue, if the MyClass constructor loads a
lot of classes or does other memory/time-consuming actions.

(2) and (3) have the advantage of creating the MyClass instance lazy,
i.e. as late as possible. This may save some memory/time, if the
getInstance() method is never called.

--
Thomas