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Nullifying static objects

Posted on 2001-08-02
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Last Modified: 2011-09-20
We have designed some classes as to use singleton objects as we r using often these objects in application, but if we want to nullify it in some circumstances we couldn't do that. Even though we assigned null still it is in the memory. Is there any other way to destroy these objects or any other alternate to use instead of singleton objects?

Also whatever objects created in a static class, is still in the memory( up to close the application ) even the reference or the scope of that object gone. How to tackle this problem?
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Question by:ksivananth
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Expert Comment

by:Aggarwal
ID: 6347463
The problem is the way u designed the class.

If we use static then it's life time is going to be the application. So, it can't be deleted.

please post ur code ..to get better details !!
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Expert Comment

by:Oliver_Dornauf
ID: 6347518
Even when you nullify an Object it is decission from the jvm when to delete (garbage collection)
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Expert Comment

by:objects
ID: 6347864
It sounds like there are still references to the object somewhere. As long as something is still referencing the static object then it will not be gc'd.

class XYZ
{
   public static XYZ getInstance();

   public void f();
}


XZY local = XYZ.getInstance();   // BAD, maintains ref

XYZ.getInstance().f();    // GOOD, no reference maintained





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Expert Comment

by:objects
ID: 6347871
And there's no way to force the object to be deleted.
This is up to the garbage collectors perogative.
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Expert Comment

by:objects
ID: 6347876
If the issue is that your singleton uses lots of memory then nullify the 'internals' of the singleton object to make it available for gc even if the singleton object itself is still referenced.
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Expert Comment

by:Oliver_Dornauf
ID: 6347882
You can give the jvm a hint to run the gc with System.gc() and than System.runFinalization().

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Author Comment

by:ksivananth
ID: 6351284
Hi everybody,

class XYZ
{
  private XYZ xyz = null ;

  public static XYZ getInstance()
  {
    if( xyz == null )
     xyz = new XYZ() ;

    return xyz ;
  }

  public void f();

  public void close()
  {
    xyz = null ;
  }
}

  In application I am using the following statement to call the method f().

XYZ.getInstance().f();    

Sometimes I want to nullify this obj, So I am calling the close method. After calling the close method, still I am able to use the "xyz" obj without calling the getInstance() method and it didn't throw NullPointerException. So it is still in the memory.

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Author Comment

by:ksivananth
ID: 6351287
>If the issue is that your singleton uses lots of memory >then nullify the 'internals' of the singleton
>object to make it available for gc even if the singleton >object itself is still referenced.

  What do u mean by internals? is it the obj created within XYZ, if it is that also could not nullified.
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Accepted Solution

by:
objects earned 100 total points
ID: 6354394
> still I am able to use the "xyz" obj without calling the
> getInstance() method and it didn't throw NullPointerException.

Then you must be maintaining a reference to it. Otherwise how are accessing xyz without using getInstance().

> What do u mean by internals?

The member variables of XYZ.

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by:Moondancer
ID: 6956270
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by:Moondancer
ID: 7118718
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