Risk of static singleton classes being unloaded by the ClassLoader?

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David M. Karr - 22 Aug 2003 20:18 GMT

When implementing a Singleton pattern, I sometimes consider whether to
use the classic "getInstance()" concept, or simply implement a "static
utility" class, with all static methods. In most cases, the static
utility class fulfills all of my needs.

However, I've been told (which I find hard to believe) that a class
with all static methods has a risk of being unloaded by the
ClassLoader, supposedly because you end up with no permanent
references to any part of the class.

Is there any truth to this?

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A. Bolmarcich - 22 Aug 2003 22:19 GMT

> When implementing a Singleton pattern, I sometimes consider whether to
> use the classic "getInstance()" concept, or simply implement a "static
[quoted text clipped - 7 lines]
>
> Is there any truth to this?

It was true with the JDK 1.0. The JDK 1.1 java command had a -noclassgc
option that prevented classes from being gargage collected. Starting
with JDK 1.2 a class may not be garbage collected until after the
ClassLoader that loaded the class may be. Starting with JDK 1.2
the java command has a -Xnoclassgc option that disables class garbage
collection.

No matter what JVM you use, you can prevent a class from being garbage
collected by keeping a reference to the class in another class or object
that will not be garbage collected.

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Stephen Kellett - 23 Aug 2003 00:37 GMT

>> When implementing a Singleton pattern, I sometimes consider whether to
>> use the classic "getInstance()" concept, or simply implement a "static
[quoted text clipped - 7 lines]
>>
>> Is there any truth to this?

Even if the class is unloaded, at the point at which a call is made to a
static method, the class will be loaded. If you want to avoid any such
problems, always ensure that you have one reference in use.

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Stephen Kellett
Object Media Limited http://www.objmedia.demon.co.uk
RSI Information: http://www.objmedia.demon.co.uk/rsi.html

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Risk of static singleton classes being unloaded by the ClassLoader?

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