Implementing the Singleton pattern

Has anyone implemented the singleton pattern in Forte? A singleton is simply ensuring that there is one unique instance of a class. In other OO languages I would create a class method which would use a class variable to hold onto the unique instance. Since forte doesn't have either of these (class methods or variables), I'm not sure how to do it. I thought of using named objects, but it seems like a heavy implementation. Any ideas.

An SO with its shared=TRUE and anchored=TRUE?
Venkat J Kodumudi
Price Waterhouse LLP
Internet: [email protected]
Internet2: [email protected]
-----Original Message-----
From: [email protected] [SMTP:[email protected]]
Sent: Monday, February 02, 1998 1:09 PM
To: Venkat Kodumudi
Subject: Implementing the Singleton pattern
To: [email protected] @ Internet
cc:
From: [email protected] @ Internet
Date: 02/02/98 12:36:02 PM
Subject: Implementing the Singleton pattern
Has anyone implemented the singleton pattern in Forte? A singleton is
simply
ensuring that there is one unique instance of a class. In other OO
languages I
would create a class method which would use a class variable to hold
onto the
unique instance. Since forte doesn't have either of these (class
methods or
variables), I'm not sure how to do it. I thought of using named
objects, but it
seems like a heavy implementation. Any ideas.

Similar Messages

Serializing a class that implements the Singleton pattern

Hello,
I am relatively new to Java and especially to serialization so the answer to this question might be obvious, but I could not make it work event though I have read the documentation and the article "Using XML Encoder" that was linked from the documentation.
I have a class that implements the singleton pattern. It's definition is as follows:
public class JCOption implements Serializable {
private int x = 1;
private static JCOption option = new JCOption();
private JCOption() {}
public static JCOption getOption() { return option; }
public int getX() { return x; }
public void setX(int x) { this.x = x; }
public static void main(String args[]) throws IOException {
    JCOption opt = JCOption.getOption();
    opt.setX(10);
    XMLEncoder encoder = new XMLEncoder(new BufferedOutputStream(new FileOutputStream("Test.xml")));
    encoder.setPersistenceDelegate(opt.getClass(), new JCOptionPersistenceDelegate());
    encoder.writeObject(opt);
    encoder.close();
}Since this class does not fully comply to the JavaBeans conventions by not having a public no-argument constructor, I have create a class JCOptionPersistenceDelegate that extends the PersistenceDelegate. The implementation of the instantiate method is as follows:
protected Expression instantiate(Object oldInstance, Encoder out) {
       Expression expression = new Expression(oldInstance, oldInstance.getClass(), "getOption", new Object[]{});
        return expression;
}The problem is that the resulting XML file only contains the following lines:
    <java version="1.5.0_06" class="java.beans.XMLDecoder">
        <object class="JCOption" property="option"/>
    </java> so there is no trace of the property x.
Thank you in advance for your answers.

How about this:
import java.beans.DefaultPersistenceDelegate;
import java.beans.Encoder;
import java.beans.Expression;
import java.beans.Statement;
import java.beans.XMLEncoder;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
public class JCOption {
    private int x = 1;
    private static JCOption option = new JCOption();
    private JCOption() {}
    public static JCOption getOption() { return option; }
    public int getX() { return x; }
    public void setX(int x) { this.x = x; }
    public static void main(String args[]) throws IOException {
      JCOption opt = JCOption.getOption();
      opt.setX(10);
      ByteArrayOutputStream os = new ByteArrayOutputStream();
      XMLEncoder encoder = new XMLEncoder( os );
      encoder.setPersistenceDelegate( opt.getClass(), new JCOptionPersistenceDelegate() );
      encoder.writeObject(opt);
      encoder.close();
      System.out.println( os.toString() );
class JCOptionPersistenceDelegate extends DefaultPersistenceDelegate {
    protected Expression instantiate(Object oldInstance, Encoder out) {
        return new Expression(
                oldInstance,
                oldInstance.getClass(),
                "getOption",
                new Object[]{} );
    protected void initialize( Class<?> type, Object oldInstance, Object newInstance, Encoder out ) {
        super.initialize( type, oldInstance, newInstance, out );
        JCOption q = (JCOption)oldInstance;
        out.writeStatement( new Statement( oldInstance, "setX", new Object[] { q.getX() } ) );
}   Output:
<?xml version="1.0" encoding="UTF-8"?>
<java version="1.5.0_06" class="java.beans.XMLDecoder">
<object class="JCOption" property="option">
<void property="x">
   <int>10</int>
</void>
</object>
</java>

How we can limit the number of concurrent calls to a WCF service without use the Singleton pattern or without do the change in BizTalk Configuration file?

How can we send only one message to a WCF service at a time? How we can limit the number of concurrent calls to a WCF service without use the Singleton pattern or without do the change in BizTalk Configuration file? Can we do it by Host throttling?

Hi Pawan,
You need to use WCF-Custom adapter and add the ServiceThrottlingBehavior service behavior to a WCF-Custom Locations.
ServiceThrottlingBehavior.MaxConcurrentCalls - Gets or sets a value that specifies the maximum number of messages actively processing across a ServiceHost. The MaxConcurrentCalls property specifies the maximum number of messages actively
processing across a ServiceHost object. Each channel can have one pending message that does not count against the value of MaxConcurrentCalls until WCF begins to process it.
Follow MSDN-
http://msdn.microsoft.com/en-us/library/ee377035%28BTS.10%29.aspx
http://msdn.microsoft.com/en-us/library/system.servicemodel.description.servicethrottlingbehavior.maxconcurrentcalls.aspx
I hope this helps.
Rachit
If this answers your question please mark it accordingly. If this post is helpful, please vote as helpful by clicking the upward arrow mark next to my reply.

Trying to implement the Builder pattern with inheritance

This is just a bit long, but don't worry, it's very understandable.
I'm applying the Builder pattern found in Effective Java (J. Bloch). The pattern is avaiable right here :
http://books.google.fr/books?id=ka2VUBqHiWkC&lpg=PA15&ots=yXGmIjr3M2&dq=nutritionfacts%20builder%20java&pg=PA14
My issue is due to the fact that I have to implement that pattern on an abstract class and its extensions. I have declared a Builder inside the base class, and the extensions specify their own extension of the base's Builder.
The abstract base class is roughly this :
public abstract class Effect extends Trigger implements Cloneable {
    protected Ability parent_ability;
    protected Targetable target;
    protected EffectBinder binder;
    protected Effect(){
    protected Effect(EffectBuilder parBuilder){
        parent_ability = parBuilder.parent_ability;
        target = parBuilder.target;
        binder = parBuilder.binder;
    public static class EffectBuilder {
        protected Ability parent_ability;
        protected Targetable target;
        protected EffectBinder binder;
        protected EffectBuilder() {}
        public EffectBuilder(Ability parParentAbility) {
            parent_ability = parParentAbility;
        public EffectBuilder target(Targetable parTarget)
        { target = parTarget; return this; }
        public EffectBuilder binder(EffectBinder parBinder)
        { binder = parBinder ; return this; }
    // etc.
}And the following is one of its implementation :
public class GainGoldEffect extends Effect {
    private int gold_gain;
    public GainGoldEffect(GainGoldEffectBuilder parBuilder) {
        super(parBuilder);
        gold_gain = parBuilder.gold_gain;
    public class GainGoldEffectBuilder extends EffectBuilder {
        private int gold_gain;
        public GainGoldEffectBuilder(int parGoldGain, Ability parParentAbility) {
            this.gold_gain = parGoldGain;
            super.parent_ability = parParentAbility;
        public GainGoldEffectBuilder goldGain(int parGoldGain)
        { gold_gain = parGoldGain; return this; }
        public GainGoldEffect build() {
            return new GainGoldEffect(this);
    // etc.
}Effect requires 1 parameter to be correctly instantiated (parent_ability), and 2 others that are optional (target and binder). Implementing the Builder Pattern means that I won't have to rewrite specific construcors that cover all the combination of parameters of the Effect base class, plus their own parameter as an extension. I expect the gain to be quite huge, as there will be at least a hundred of Effects in this API.
But... in the case of these 2 classes, when I'm trying to create the a GoldGainEffect like this :
new GainGoldEffect.GainGoldEffectBuilder(1 , locAbility).goldGain(5);the compiler says "GainGoldEffect is not an enclosing class". Is there something wrong with the way I'm trying to extend the base Builder ?
I need your help to understand this and find a solution.
Thank you for reading.

The GainGoldEffectBuilder class must be static.
Otherwise a Builder would require a GainGoldEffect object to exist, which is backwards.

Why the singleton pattern used rather than static things?

class Single
    private Single() {}
    private void print() { System.out.println("Foo~"); }
    private static Single one = new Single();
    public static void print() { one.print(); }
class Stat
    public static void print() { System.out.println("Foo~"); }
}What's the different?? What is the benefit of the singleton one?

There is no practical difference - because, in fact, that is not the singleton pattern!
A singleton would look like:
class Singleton {
    private Singleton() {}
    public void print() { System.out.println("Foo~"); }
    private static Singleton instance = new Singleton();
    public static Singleton getInstance() { return instance; }
}The benefit of this pattern is, that your client code doesn't have to know that there is only one instance (getInstance could return another instance for every call) or of which actual type it is (getInstance could return an instance of any Subclass of Singleton).
See http://c2.com/cgi/wiki?SingletonPattern for more information on the pattern.

Trying to implement the Observer pattern with a notify() method

OK, I'm not implementing the observer exactly rigidly but I'm doing something thereabouts inline with the pattern. I have an abstract class which provides some common functionality for the observers and is also used for type checking elsewhere. Each observer must inherit from this class. Now; the abstract class has one abstract method notify() as per the generic observer.
I get an error that notify() cannot be overridden as it's final. I understand what all that means but I'm curious where it's implemented that's actually affecting my own code? Is notify() a standard method in all objects in Java?
package zoopackage;
public abstract class ZooObserver
     protected Cage cage;
     public void setCage(Cage myCage)
          this.cage = myCage;
     abstract public void notify();
}And...
package zoopackage.observers;
import zoopackage.*;
public class VisualObserver extends ZooObserver
     public void notify()
}

Is notify() a standard method in all objects in Java?Yes, as are notifyAll() and wait(). You'll also notice in the API that there already is an Observer interface and an Observable object. That's probably sufficient to implement what you want to do.
Brian

"requires unreachable" warning emitted when using static variables and the singleton pattern

I'm implementing code contracts on an existing code base and I've come across this situation in a few places. Wherever we have additional logic with code contracts where a singleton instance is instantiated, all code contracts emit a "reference use
unreached", or "requires unreachable" warning.
The code below demonstrates this.
static string instance = null;
static string InstanceGetter()
if (instance == null)
instance = "initialized";
AdditionalLogic(instance); // requires unreachable warning on this line
return instance;
static void AdditionalLogic(string str)
Contract.Requires(str != null);
Console.WriteLine(str);

Would the class get unloaded even if the Singleton object has some valid/reachable references pointing to it?
On a different note, will all the objects/instances of a class get destroyed if the class gets unloaded.That's the other way round, really: instances cannot be garbage-collected as long as they are strongly reachable (at least one reference pointing to them), and classes cannot be unloaded as long as such an instance exists.
To support tschodt's point, see the JVM specifications: http://java.sun.com/docs/books/jvms/second_edition/html/Concepts.doc.html#32202
*2.17.8 Unloading of Classes and Interfaces*
A class or interface may be unloaded if and only if its class loader is unreachable. The bootstrap class loader is always reachable; as a result, system classes may never be unloaded.
Will the same case (Custom ClassLoader getting unloaded) occur if I write the singleton using the code wherein we lazily initialize the Singleton (and actually create a Singleton instance rather than it being a Class/static variable).You should check the meaning of this vocabulary ("object", "instance", "reference"): http://download.oracle.com/javase/tutorial/java/javaOO/summaryclasses.html
The difference between the lazy vs eager initializations is the time when an instance is created. Once it is created, it will not be garbage collected as long as it is reachable.
Now I'm wondering, whether being referenced by a static attribute of a class guarantees being reachabe. I'm afraid not (see the JLS definition of reachable: http://java.sun.com/docs/books/jls/third_edition/html/execution.html#44762). That is, again, unless the class has been loaded by the system class loader.

Web Service Implementation Heavily Based on The Singleton Pattern

Dear all
I have recently joined a project that is 2/3 through development of a medium sized application providing synchronous and asynchronous implementations for a number of Use Cases exposed as Web Services. The design is one that I have not come across before and am keen to understand.
Web Service calls enter the application through a singleton, which in other respects is a POJO. This class manages a number of instance variable references to other classes responsible for implementing each Web Service method, including data access using Hibernate. Since the top level singleton class is, well, a singleton, I understand this to mean that all of the other classes maintained as instance variables will themselves only ever have one object instantiated (assuming of course that no other thread of execution instantiates them except our singleton WS entry point). To my way of thinking this turns the whole of the Web Service implementation into a kind of singleton application. This strikes me as a procedural programming idiom that happens to be implemented using Java, since Objects with state are not being used. I have not yet asked the question "why?" of my new colleagues but I would like to ask you what the consequences of this design are likely to be. Particularly considering that this application will be expected to handle concurrent servicing of Web Service requests - and so be thread-safe. Can it be thread safe? Are there likely performance implications? Forgive my ignorance is what I describe known as a Singleton Web Service?
Regards
David

Web Service calls enter the application through a
singleton, which in other respects is a POJO. I'm confused about this sentence. Is the application somehow separate from the web service? Is the application acting as a service to this web service? How is the application being hosted? Does it run in a container as well?
This
class manages a number of instance variable
references to other classes responsible for
implementing each Web Service method, including data
access using Hibernate. If all the references are stateless, preferably interfaced-based, you aren't likely to have thread issues.
Since the top level
singleton class is, well, a singleton, I understand
this to mean that all of the other classes maintained
as instance variables will themselves only ever have
one object instantiated (assuming of course that no
other thread of execution instantiates them except
our singleton WS entry point). To my way of thinking
this turns the whole of the Web Service
implementation into a kind of singleton application.You keep using web service and application. Are we just talking about a web service implementation here?
This strikes me as a procedural programming idiom
that happens to be implemented using Java, since
Objects with state are not being used. You are correct, sir. If services are stateless, and they should be, then they aren't exactly rich objects with state and behavior. It's very procedural, indeed.
I have not
yet asked the question "why?" of my new colleagues
but I would like to ask you what the consequences of
this design are likely to be. Particularly
considering that this application will be expected
to handle concurrent servicing of Web Service
requests - and so be thread-safe. Can it be thread
safe? If the web service is indeed stateless, then it should be thread-safe.
Are there likely performance implications?Maybe. Best to measure if those arise to see if the web service is the cause.
Forgive my ignorance is what I describe known as a
Singleton Web Service?I don't know if it's been given a pattern name. If it's a Singleton, then that's true. If it's a service, that applies. If it's deployed on the web, I'd say the label fits.
%

Re: Singleton Pattern

At PerSe Technologies, we have implemented the singleton pattern in our
Enterprise Component Toolkit Framework. We have what we call our "shared
data managers", which are visible to the appropriate architecture layer.
We did not implement this as a service object and do not use a Forte
"Shared" object. Instead, we use mutexes to control data locking ourselves
which allows multiple tasks to execute against the shared data manager
concurrently. This approach ensures us that we have one and only one copy
of a given object at a time. Our approach also gives another benefit in
that it caches frequently accessed data and has improved overall
performance of our applications.
Dustin Breese
[email protected]
Supervising Technical Consultant
PerSe Technologies
From: Mark Addesso <[email protected]>
Date: Mon, 2 Feb 1998 12:36:02 -0500
Subject: Implementing the Singleton pattern
Has anyone implemented the singleton pattern in Forte? A singleton is =
simply ensuring that there is one unique instance of a class. In other =
OO languages I would create a class method which would use a class =
variable to hold onto the unique instance. Since forte doesn't have =
either of these (class methods or variables), I'm not sure how to do it.=
I thought of using named objects, but it seems like a heavy =
implementation. Any ideas.

Is the danger that someone might make the Singleton
cloneable really a the most important reason to make a
Singleton non-extensible? I think not.It IS a good reason if cloning or subclassing it means
you can end up with more than one in the same JVM. If
you really implemented Singleton because "There Can
Only BE One", I'd say prohibiting subclassing and
cloning might be important.
I gotta go with the OP on this. That comment is abit queer.
I gotta go with the definition of Singleton on this
and disagree. It's not like it's that difficult.
Don't implement Cloneable or Serializable and make
the constructors private. Done. What's the
problem?
At some point it is just too much.
Are you going to implement a security manager as well? Because unless you do I can create JNI method and create a new object as well.
If they don't understand that the singleton is a singleton then are you going to assume that that is the only problem that they are having?
When I program I make assumptions about those that I work with and those that will come after me. I like to think that they have a minimal level of intelligent. They might not write the best code all the time but they do understand most of the ideas. And given that singleton the most used and probably easiest pattern to understand is it really necessary to be so protective?

Singleton pattern, scheduled receive location and suspended orchestrations

Hello,
In our project we have several orchestrations that are triggered every minute with the Scheduler Adapter (business requirement is near real-time).
Each of these orchestration must have only 1 instance running at the same time, so we have implemented the singleton pattern. We followed a design like this one: https://fehlberg.wordpress.com/2008/06/06/biztalk-singleton-orchestration-design/
But we end up having suspended orchestrations with the error "The instance completed without consuming all of its messages. The instance and its unconsumed messages have been suspended."
I know it's a risk with this pattern. I thought it would happen only when the instance would run in more than 1 min (because the scheduler adapter is sending a file every minute).
But it happens even with some instances running in about 15sec, so there is something I don't understand, here is an example:
Here we can see that the TransferRegion orchestration started at 10:26:00 (which is expected) and failed.
When I open the orchestration debugger, the time is different, the orchestration is shown starting at 10:26:44 (44 sec later):
And the orchestration ended at 10:27:00:453.
The message not consumed that caused the error has been triggered at 10:27:00.
So I understand that the message arrived just after the listen shape but before the orchestration ends, but I don't understand why the orchestration started 44 sec after is was supposed to ...
Any idea? I'm kind of clueless for now ...

So, are these 8 Orchestrations actually processing messages or are they doing some other work every minute or so?
If they're not processing actual messages, maybe a Windows Service or job scheduled by the SQL Agent or Windows Scheduler would be more...appropriate. We all love BizTalk, but some parts of the stack do some things better.
If they are processing messages or doing some genuine BPI type work, calling services, transformations etc., then maybe a Singleton+Scheduled Task is not the best solution.
I'm assuming the actual requirement is the process must run with no less than 1 minute between executions, but if an execution takes >1 min, don't overlap. An internal 1 min timer would probably work very well, but you still have the problem of
activating and deactivating the Orchestration. You'd basically have to build some control infrastructure similar to the EDI Batching Service.
Here's a completely different suggestion:
SQL Table that maintains the state of each process, say Active or Complete.
Poll every 15 seconds a Stored Procedure that tests the status and returns an activation message when that process shows Complete. It would flip it to Active at the same time.
Orchestration runs and the last shape sends a message to change the status to Complete.
Rinse Repeat. No long running Orchestrations, Singletons, Correlations, etc...
Hi John,
We have 8 independent orchestrations. Each of them is triggered by different scheduled receive location.
The orchestrations then process messages (calling stored procedures, mapping and calling web services).
You are right for the requirement. I tried to deactivate the receive location at the beginning of the process and restart it at the end, but I ran into some weird errors on the SSO DB and others like "Could not retrieve transport type data for Receive
Location 'Trigger_xxxxx_SCHEDULE' from config store. The transaction associated with the current connection has completed but has not been disposed. The transaction must be disposed before the connection can be used to execute SQL statements."
Thanks for the new design suggestion, I will see the effort required and if the client approves it ;)

Implementing Singleton Pattern in ESB

How do I go about implementing a singleton pattern within the ESB. Example usage would be for a cache. Prior to making an expensive service call I want to make sure that the data does not already exist within the cache.

Have a look at metalink note 746108.1
cheers
James

Serializing object of class implementing Singleton Pattern

hi ,
if i've a object of a singleton pattern based class and i seriailize that and when i'm desrializing that, i 'll get the object back, but on calling getInstance() method for my singleton class, it will agian call constructor because for the class's static object reference is null, i need to avoid this, for this i've written a setInstance method, which sets the Singleton class's static refernce to the object i desialized, which is fine, but it defeats the singleton pattern
can anybody suggest me some other way of achieving this...

If you are trying to deserialize a Singleton implement the Object readResolve() method.
http://java.sun.com/javase/6/docs/platform/serialization/spec/input.html#5903
~Ryan

Use of Singleton pattern in Distributed environment

Can somebody say why it is not advisable to use Singleton pattern for developing client server applications.

Client-server does not imply distributed environment, IMHO. There are pretty simple C/S architectures where Singleton pattern is not generally a bad thing.
Distributed environments usually have multiple VMs running, while the singleton pattern typically implemented is a per-VM singleton.

Is abap thread safe? Some question in Singleton pattern in ABAP

Hi Grus,
I have a very basic question but really make me headache...
Recently I am learning the design pattern in ABAP and as you know in JAVA there is a key word "Synchronized" to keep thread safe. But in ABAP, I didn't find any key words like that. So does that mean ABAP is always a single thread language? And I found that there is a way looks like "CALL FUNCTION Remotefunction STARTING NEW TASK Taskname DESTINATION dest" to make multi-thread works. As you can see it use the destination, so does that mean actually the function module is always executed in a remote system, and in every system, it is always single thread?
Could you help me on the question? Thanks a lot, grus
And here comes up to my mind another question...It's a little bit mad but I think it may works....What if I set every attribute and method as static in the singleton class...Since as you can see it is already a singleton so every attribute in it should be only one piece. So then I don't even need to implement a get_instance( ) method to return the instance. Just call "class_name=>some_method( )" directly then singleton is achieved...What do you think?
BR,
Steve

Steve,
I've the same question, few days ago I tried to use the singleton in ABAP. In Java programming is possible to use the same reference in two sessions or programs, sharing attributes, methods and all data, but I could not do in ABAP.
In my test I created a program with one global class using the singleton pattern, so I expected that when I run my program and see the reference returned after the get_instance method it should be equal to the same program run in another session, but the ABAP will create a new reference and instantiate again.
So I deduced that the only way to share it between sessions in ABAP is using the ABAP Shared Memory Objects.
I can be wrong, but I think that ABAP use a thread by user session (Each window) and we can't change it.
Best regards.

Singleton pattern class and static class

Hi,
what is difference between the singleton pattern class and static class ?
in singleton pattern, we declare a static member[which hold one value at a time] ,static method[ which return static member value] and a private constructor[not allow to direct instantiation]. My words are -- as a singleton pattern we implement static class .
so can we say a singleton pattern is static class it means both are same or is any difference between these two ??

malcolmmc wrote:
On several occasions I've had to convert a static (never instanceated) class to a singleton type, for example because I realise I need an instance per thread, or I decide to make a program more modular, with more than one configuration.
I've never done the opposite.
So generally I favour the singleton if there any "state" involved, because it's more flexible in unanticipated directions. It also gives extra flexibility when building tests, especially if you program to an interface.Total agreement; if anything it is dead hard to override static method logic with mock code in unit tests (I had to do it by keeping the static methods in place but making them internally use a singleton instance of itself of which the instance variable could be overwritten with a mock implementation for unit testing purposes...).
'Static classes' and/or methods are great for very simple util functions, but I wouldn't use them for anything other than that.

Implementing the Singleton pattern

Similar Messages

Maybe you are looking for