Any adverse effects from singleton classes?

We're trying to decide whether it is a good/bad/indifferent idea to use
singleton classes in various places in an application (servlets, session
EJBs, BMP entity EJBs), in a clustered environment. Our app designers
have a number of different potential uses in mind, such as storing some
rarely changed data, and localised caches. We're happy with the
restriction that they obviously wouldn't be replicated. However, we're
not sure about what other implications there might be, especially when
the thing is scaled up. Questions that spring to mind include:
- any inbuilt synchronisation that WL may do (if the singleton had to
dive off to a database to get some further info we wouldn't necessarily
want to have other callers to it synchronise at that point)
- the effect on the JVM if we tie up some of its virtual memory
- any other considerations
Note that they are considering using this technique in both the web app
and the ejbs (which will reside in different clusters).
Are there any guidelines available for the use of singletons,
particularly from a performance point of view?
Thanks
Chris

For caching in clustering, read only entity beans are best bet.
You must not bind a cach ( HashMap or soemthing) object in JNDI tree in
clustering. This could lead to mess as this binding is not cluster aware. You
might see conflict messages in your log file or your local bidning could fail
in one or other server depending upon timing.
Other than that as long as you don't have synchronization in singleton classes
that leads to deadlocks, there should not be a problem.
Viresh Garg
Principal Developer Relations Emgineer
BEA Systems
Larry Presswood wrote:
Well duno if this helps but their new Jolt Pool Manager is a singleton
class.
Also you could use JNDI as a data cache but be careful as it replicates
across
the cluster.
Chris Palmer wrote:
We're trying to decide whether it is a good/bad/indifferent idea to use
singleton classes in various places in an application (servlets, session
EJBs, BMP entity EJBs), in a clustered environment. Our app designers
have a number of different potential uses in mind, such as storing some
rarely changed data, and localised caches. We're happy with the
restriction that they obviously wouldn't be replicated. However, we're
not sure about what other implications there might be, especially when
the thing is scaled up. Questions that spring to mind include:
- any inbuilt synchronisation that WL may do (if the singleton had to
dive off to a database to get some further info we wouldn't necessarily
want to have other callers to it synchronise at that point)
- the effect on the JVM if we tie up some of its virtual memory
- any other considerations
Note that they are considering using this technique in both the web app
and the ejbs (which will reside in different clusters).
Are there any guidelines available for the use of singletons,
particularly from a performance point of view?
Thanks
Chris

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Can not access the Instance Data of a Singleton class from MBean

I am working against the deadline and i am sweating now. From past few days i have been working on a problem and now its the time to shout out.
I have an application (let's call it "APP") and i have a "PerformanceStatistics" MBean written for APP. I also have a Singleton Data class (let's call it "SDATA") which provides some data for the MBean to access and calculate some application runtime stuff. Thus during the application startup and then in the application lifecysle, i will be adding data to the SDATA instance.So, this SDATA instance always has the data.
Now, the problem is that i am not able to access any of the data or data structures from the PerformanceStatistics MBean. if i check the data structures when i am adding the data, all the structures contains data. But when i call this singleton instance from the MBean, am kind of having the empty data.
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I tried all sorts of DATA class being final and all methods being synchronized, static, ect.,, just to make sure. But no luck till now.
Another unfortunate thing is that, i some times get different "ServicePerformanceData " instances (i.e. when i print the ServicePerformanceData.getInstance() they are different at different times). Not sure whats happening. I am running this application in WebLogic server and using the JConsole.
Please see the detailed problem at @ http://stackoverflow.com/questions/1151117/can-not-access-the-instance-data-of-a-singleton-class-from-mbean
I see related problems but no real solutions. Appreciate if anyone can throw in ideas.
http://www.velocityreviews.com/forums/t135852-rmi-singletons-and-multiple-classloaders-in-weblogic.html
http://www.theserverside.com/discussions/thread.tss?thread_id=12194
http://www.jguru.com/faq/view.jsp?EID=1051835
Thanks,
Krishna

I am working against the deadline and i am sweating now. From past few days i have been working on a problem and now its the time to shout out.
I have an application (let's call it "APP") and i have a "PerformanceStatistics" MBean written for APP. I also have a Singleton Data class (let's call it "SDATA") which provides some data for the MBean to access and calculate some application runtime stuff. Thus during the application startup and then in the application lifecysle, i will be adding data to the SDATA instance.So, this SDATA instance always has the data.
Now, the problem is that i am not able to access any of the data or data structures from the PerformanceStatistics MBean. if i check the data structures when i am adding the data, all the structures contains data. But when i call this singleton instance from the MBean, am kind of having the empty data.
Can anyone explain or have hints on what's happening ? Any help will be appreciated.
I tried all sorts of DATA class being final and all methods being synchronized, static, ect.,, just to make sure. But no luck till now.
Another unfortunate thing is that, i some times get different "ServicePerformanceData " instances (i.e. when i print the ServicePerformanceData.getInstance() they are different at different times). Not sure whats happening. I am running this application in WebLogic server and using the JConsole.
Please see the detailed problem at @ http://stackoverflow.com/questions/1151117/can-not-access-the-instance-data-of-a-singleton-class-from-mbean
I see related problems but no real solutions. Appreciate if anyone can throw in ideas.
http://www.velocityreviews.com/forums/t135852-rmi-singletons-and-multiple-classloaders-in-weblogic.html
http://www.theserverside.com/discussions/thread.tss?thread_id=12194
http://www.jguru.com/faq/view.jsp?EID=1051835
Thanks,
Krishna

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Disable your antivirus and firewall
Connect the phone cable to a USB port directly on the computer, not a hub
Before updating right click on the name of the phone in iTunes and choose "Backup"
When you are given the choice choose "Download only", not "Download and Update"
After the download completes successfully click the Update button to install it.
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Place of Singleton class in OO Design

Hi
Can any body make me clear on, why we make use of Singleton Class and how it is related to Object Oriented world.
Thanks,
Prasanth

Personally, I�ve never seen a case where singletons
are being badly used. In this forum, I�ve always been
reading a lot of people saying that Singleton is
anti-pattern, Singleton is evil, Singleton is
abusively implemented, etc, etc. It�s very easy to
say those things, perhaps because of some authorities
who endorse what you say, but nobody has said WHY
EXACTLY Singletons are bad.I thought I did say exactly that in this thread.
Let me try again.
Some people believe that simply by using Singletons that their designs follow OO principals. In effect it is quite possible to create an entire application or most of an application using procedural programming by using singletons.
In terms of OO it is considered a bad design when the design is procedural.
>
If it is anti-pattern, then it would be better if it
were not used. But if it is in fact used and it is
considered good sometimes, then we can conclude that
it is not an anti-pattern. This is just logic.
Incorrect. Everything can be both abused and used correctly.
However at some point the incorrect usage and the complications from that outweigh the benefits of the correct usage. That might be a subjective or objective determination.
Examples of this.
1. In two shops where I worked that used unix and C++ exclusively the use of operator overloading and multiple inheritence was completely banned. The majority of developers (I never found one that disagreed) believed that the usage of those two would be detrimental to the code that was being created.
2. The creators of java decided that pointers should not be allowed.
Now it could be that that all of those people are wrong. Or it could be that many of them were going on ancedotal evidence learned either first hand, second hand (or many hands) and yet that evidence was wrong.
Myself I suspect it would be very expensive to determine objectively many thing that technological decisions are made upon. So one is left with whatever one can find.
I�ve already googleg for some information, but what I
found are just lots of sites that repeat what a lot
of people here say, more or less.
I think people have to have a good explanation for
what they say.Not necessarily. Most larger technological decisions are often made with only subjective knowledge. Many people insist that unix is more stable than windows. Or that java/C++/C# (pick one) is better than java/C++/C3 (pick another) with seldom any more reason than just because they like one or the other.
Finally you might note that I haven't seen the abuse that is being reported here. I do however recognize the possibility that it could be occurring.

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Hi,
I'm new in JSF & in this forum so maybe this question is already aswer.
My problem is that I made a jsf component to deal with a tree menu. The resources in the leaf nodes of the menu are stored in a action calling pattern (i.e.: #{myBean.myMethod}) & I'm looking for a way to bind & execute the call from the class of the component that treats the events.
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How to make Singleton Class secure

I have a Singleton class which I instantiate using reflection by a call to a public getInstance method that return the object from a private constructor. I have to use reflection as I dont have any other options since my class instantiation is decided on runtime. How will I secure other form instantiating the same class again by using reflection on my private constructor? Any ideas?

How much do these different implementations of your singleton have in common? What I'm getting at is, is there some common code you could extract out of them all, and make that a singleton in it's own right? Needing to do this sort of hacking is often a sign you're fudging something together, that could be solved more elegantly. We use the Singleton pattern generally when the state of the object needs to exist exactly once. Can you extract that from the disparate objects?
The singleton classes that I load do not have anything in common. They are all similar but unique, and independant in their own respect. I will have to decide only on what singleton object I have to load in runtime, based on certain criterion. This is done by reflection, by a call to the public getInstance() method of the respective singleton classes, since I am deciding on runtime. My problem is that can I prevent others from accessing my private constructor from outside the class using reflection, which enables them to create another duplicate object. Can I restrict them to use only my getInstance() method instead?? I know this is hacking to access the private members, but I want to know whether there is any way where I can restrict this hacking???In the above code I dont want these statements to work at any case
java.lang.reflect.Constructor[] c = cl.getDeclaredConstructors();
c[0].setAccessible(true);
A anotherA = (A) c[0].newInstance(new Object[]{"Duplicate"});

TestStand create different object for singleton class

Hi all,
we have a singleton class which has some functions used to do testing a harware.
Our main Exe will create an object for that singleton class(which opens Com1 port and communicate with hardware). so the Exe will do basic communication test with hardware it is working well. We are using Teststand operator interface to do various testing by using sequence files. Main exe will use teststand usercontrols to execute tests when the user clicks Testbutton. after that, teststand try to create an object for that singleton but it returns new object not the existing one which is created by EXE. So it throwing me exeception "Com1 port access denied." (since we created object for signleton class @ very first in EXE)
My question is Since that teststand runs in a separate Appdoamin will the singltonclass create separate object for different appdomain? if so is there any solution to reslove this?
Hope i clearly explained my probs.
Thanks in advance
Srini

Hi Srini,
How are you calling the executable? From a Call Executable step? Or are you using another means of calling it? Also, why is TestStand trying to recreate the object? As long as you have the correct handle to the object I don't think it matters what app domain you are in.
Regards,
jigg
CTA, CLA
teststandhelp.com
~Will work for kudos and/or BBQ~

Singleton class issue

I have a singleton class. A method in this class creates an array of structures. This class is shared across different process. So there is a chance that some one unknowigly calls the method to create the array of structures and hence the member variable may now points to another array of structure.
Is there any way in Java to restict invoking of a function only once.
A static variable with a check will solve this issue, but any other method so that calling of method itself becomes invalid to avoid RT usage?.

Hi,
I think, I understood know.
You want to have a singleton holding a defined number of other objects (your arrays).
This objects (your arrays) are semantically singletons itsself, because they should be constructed only once and than reused.
Assuming I interpreted right, see my comments inline below.
I know that who ever participate in this discussion is
pretty sure about singleton class. Some of the code
what I gave was irretating as Martin pointed out. So
let me try to give more transparent code snippet,Thanks, that helped.
>
My aim :- I need a global object which is going to
hold some arrays(data structures). This should be
shared across various threads. (forget about the
synchronousation part here). All these arrays won't be
of same size, so I need methods for each
datastructures to initialise to its required size.That's a little bit part of your problem, see below.
My wayforward :- Create the global object as
singleton. This object has methods for initialising
different data structures.OK, fine.
What is my doubt :- please see the following code
fragment,
public class Singleton {
private Singleton singleton;
private Singleton() {
public static Singleton getInstance() {
if (singleton == null) {
singleton = new Singleton();
return singleton;
//other methods for the class
private someObjType1 myArray1 = null;
private someObjType2 myArray2 = null;
private someObjType3 myArray3 = null; // etc....This "smells" like an candidate for another data structure, so that you don't have a fixed number of array.
F.E.
// Associate class of array elements (someObjTypeX) as keys with arrays of someObjTypeX
private Map arrayMap = new HashMap();>
public void CreateArray1(int size) {
if (myArray1 == null) {
myArray1 = new someObjType1[size];
}Using the map, you create array should look like
public void CreateArray(Class clazzOfArrayElem, int size)
Object arr = arrayMap.get(clazzOfArrayElem)
if(arr == null)
    arr = java.lang.reflect.Array.newInstance(clazzOfArrayElem, size);
    arrayMap.put(clazzOfArrayElem, arr);
}Additionally the "ugliness" of this method results from the problem, that don't know the size of arrays at compile time.
So it seem to be preferable to use a dynamic structure like a Vector instead of array. Then you don't have the need to pass the size in the accessor method.
Then you can use a "cleaner" accessor method like
public Vector getVector(Class clazzOfArrayElem)
Vector vec = arrayMap.get(clazzOfArrayElem)
if(vec == null)
    vec = new Vector();
    arrayMap.put(clazzOfArrayElem, vec);
return vec;
}If you want to expose an array oriented interface f.e. for type safety you can have an accessor like
public Object[] getAsArray(Class clazzOfArrayElem)
Vector vec = arrayMap.get(clazzOfArrayElem)
if(vec == null)
    vec = new Vector();
    arrayMap.put(clazzOfArrayElem, vec);
arr = java.lang.reflect.Array.newInstance(clazzOfArrayElem, 0);
return (Object[])vec.toArray(arr);
// Real typesafe accessors as needed following
public SomeClass1[] getAsArrayOfSomeClass1()
return (SomeClass1[])getAsArray(SomeClass1.class);
/.....This accessor for array interface have the additional advantage, that they return a copy of the data (only the internal arrays, no the object within the arrays!), so that a least a client using your class doesn't have access to your internal data (with the Vector he has).
>
// similarly for other data structures...
So here CreateArray1 method will work fine because of
the check myArray1 == null.
Actual question :- Can I remove this check and
restrick call to CreateArray1 more than one time ?.No. As I understood you want to have something like a "only-use-once-method". There is no such thing in Java I know of.
But I don't see the need to remove this check. Reference comparison should have no real impact to the performance.
this is going to benifit me for some other cases
also.How?
Hope this helps.
Martin

Any detrimental effects on unhiding iPod_Control folder?

There are many guides on the internet which direct one to set folders to show hidden directories in order for music to be accessed on the iPod from a compy. Here's my issue.
On the computers I use during my classes, we're very restricted. I cannot install iTunes since I don't have the permissions. I can't eject the iPod properly since I don't even have those permissions. However, the more relevant issue is that I can't get to Options for folders, so I can't make hidden directories show.
During work with these computers, I'd love to be able to listen to my music while using my iPod as something akin to a jump drive. Since I can't do that, I'd love to be able to get to my music but I can't even do that since I don't have permissions to access folders directly through the address bar on Internet Explorer.
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Any adverse effects from singleton classes?

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