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What is the best way to manage property sets to apply to EJB, and easily be able to vary them between machines/environments (e.g. DEV, TEST, PROD)? For example, is there a way to configure your EJB properties on the App Server (which guarantees you can vary them by machine/environment).
Specifically:
1) I have a Singleton EJB which needs certain properties set (environment) specific. Is there annotation(s) which are used to tell the EJB Container where to look up those properties and will automatically apply them to the bean?
2) What is the best way to manage different property sets, i.e. dev, test, prod, so that the J2EE app is portable between servers, and you can seamlessly manage the properties specific to each server?
If there are any good documentation links - let me know. I've Googled around and seen nothing directly to the points above.
I use a singleton helper class "PropertiesHelper" which has a Properties member and reads from an xml configuration file upon the first property access attempt. The helper encapsulates the entire set of configuration settings and prevents them from being read from disk more than once.
The file location is specified in my java opts and read in by PropertiesHelper using System.getProperty()
As for system properties annotations, I don't believe Java supports this natively, but if you're so inclined you may want to look at some AOP/Dependency Injection frameworks like Google Guice which are better at this "cross-cutting".
So.. I can't understand why should I even use the Singleton pattern in ActionScript 3. Can anyone explain me this? Maybe I just don't understand the purpose of it. I mean how it differs from other patterns? How it works?
I checked the PureMVC source and it's full of Singletons. Why are they using them in the View, Module, Controller?
I have next to no practical experience with PureMVC so I can't argue for or against their use of Singletons. Hence, I'll try to keep my answer more generic.
A singleton is a type of object that can only be instantiated once and is globally accessible.
Typically, this kind of pattern is used in order to have easy access to services of some kind, perhaps a service facade used to retrieve data from a server or an application model that holds information about settings or such.
The singleton pattern is by many considered to be an anti-pattern for a number of reasons, a few of which are mentioned below:
They carry state, making certain tasks such as unit testing virtually impossible.
They inherently violate the Single Responsibility Principle.
They promote tight coupling between classes due to them being globally accessible.
I won't list all of the reasons why a singleton may be an anti pattern, there are plenty of resources on the subject.
The singleton pattern restricts the instantiation of an object to only one instance. Sometimes in systems this pattern is used so an object that controls parts of the system can't be just created at-will. If you have some object that manages settings, for example, you would want something that changes settings to only modify that one object, and not create a new one.
After reading the nice answers in this question, I watched the screencasts by Justin Etheredge. It all seems very nice, with a minimum of setup you get DI right from your code.
Now the question that creeps up to me is: why would you want to use a DI framework that doesn't use configuration files? Isn't that the whole point of using a DI infrastructure so that you can alter the behaviour (the "strategy", so to speak) after building/releasing/whatever the code?
Can anyone give me a good use case that validates using a non-configured DI like Ninject?
I don't think you want a DI-framework without configuration. I think you want a DI-framework with the configuration you need.
I'll take spring as an example. Back in the "old days" we used to put everything in XML files to make everything configurable.
When switching to fully annotated regime you basically define which component roles yor application contains. So a given
service may for instance have one implementation which is for "regular runtime" where there is another implementation that belongs
in the "Stubbed" version of the application. Furthermore, when wiring for integration tests you may be using a third implementation.
When looking at the problem this way you quickly realize that most applications only contain a very limited set of component roles
in the runtime - these are the things that actually cause different versions of a component to be used. And usually a given implementation of a component is always bound to this role; it is really the reason-of-existence of that implementation.
So if you let the "configuration" simply specify which component roles you require, you can get away without much more configuration at all.
Of course, there's always going to be exceptions, but then you just handle the exceptions instead.
I'm on a path with krosenvold, here, only with less text: Within most applications, you have a exactly one implementation per required "service". We simply don't write applications where each object needs 10 or more implementations of each service. So it would make sense to have a simple way say "this is the default implementation, 99% of all objects using this service will be happy with it".
In tests, you usually use a specific mockup, so no need for any config there either (since you do the wiring manually).
This is what convention-over-configuration is all about. Most of the time, the configuration is simply a dump repeating of something that the DI framework should know already :)
In my apps, I use the class object as the key to look up implementations and the "key" happens to be the default implementation. If my DI framework can't find an override in the config, it will just try to instantiate the key. With over 1000 "services", I need four overrides. That would be a lot of useless XML to write.
With dependency injection unit tests become very simple to set up, because you can inject mocks instead of real objects in your object under test. You don't need configuration for that, just create and injects the mocks in the unit test code.
I received this comment on my blog, from Nate Kohari:
Glad you're considering using Ninject!
Ninject takes the stance that the
configuration of your DI framework is
actually part of your application, and
shouldn't be publicly configurable. If
you want certain bindings to be
configurable, you can easily make your
Ninject modules read your app.config.
Having your bindings in code saves you
from the verbosity of XML, and gives
you type-safety, refactorability, and
intellisense.
you don't even need to use a DI framework to apply the dependency injection pattern. you can simply make use of static factory methods for creating your objects, if you don't need configurability apart from recompiling code.
so it all depends on how configurable you want your application to be. if you want it to be configurable/pluggable without code recompilation, you'll want something you can configure via text or xml files.
I'll second the use of DI for testing. I only really consider using DI at the moment for testing, as our application doesn't require any configuration-based flexibility - it's also far too large to consider at the moment.
DI tends to lead to cleaner, more separated design - and that gives advantages all round.
If you want to change the behavior after a release build, then you will need a DI framework that supports external configurations, yes.
But I can think of other scenarios in which this configuration isn't necessary: for example control the injection of the components in your business logic. Or use a DI framework to make unit testing easier.
You should read about PRISM in .NET (it's best practices to do composite applications in .NET). In these best practices each module "Expose" their implementation type inside a shared container. This way each module has clear responsabilities over "who provide the implementation for this interface". I think it will be clear enough when you will understand how PRISM work.
When you use inversion of control you are helping to make your class do as little as possible. Let's say you have some windows service that waits for files and then performs a series of processes on the file. One of the processes is to convert it to ZIP it then Email it.
public class ZipProcessor : IFileProcessor
{
IZipService ZipService;
IEmailService EmailService;
public void Process(string fileName)
{
ZipService.Zip(fileName, Path.ChangeFileExtension(fileName, ".zip"));
EmailService.SendEmailTo(................);
}
}
Why would this class need to actually do the zipping and the emailing when you could have dedicated classes to do this for you? Obviously you wouldn't, but that's only a lead up to my point :-)
In addition to not implementing the Zip and email why should the class know which class implements the service? If you pass interfaces to the constructor of this processor then it never needs to create an instance of a specific class, it is given everything it needs to do the job.
Using a D.I.C. you can configure which classes implement certain interfaces and then just get it to create an instance for you, it will inject the dependencies into the class.
var processor = Container.Resolve<ZipProcessor>();
So now not only have you cleanly separated the class's functionality from shared functionality, but you have also prevented the consumer/provider from having any explicit knowledge of each other. This makes reading code easier to understand because there are less factors to consider at the same time.
Finally, when unit testing you can pass in mocked dependencies. When you test your ZipProcessor your mocked services will merely assert that the class attempted to send an email rather than it really trying to send one.
//Mock the ZIP
var mockZipService = MockRepository.GenerateMock<IZipService>();
mockZipService.Expect(x => x.Zip("Hello.xml", "Hello.zip"));
//Mock the email send
var mockEmailService = MockRepository.GenerateMock<IEmailService>();
mockEmailService.Expect(x => x.SendEmailTo(.................);
//Test the processor
var testSubject = new ZipProcessor(mockZipService, mockEmailService);
testSubject.Process("Hello.xml");
//Assert it used the services in the correct way
mockZipService.VerifyAlLExpectations();
mockEmailService.VerifyAllExceptions();
So in short. You would want to do it to
01: Prevent consumers from knowing explicitly which provider implements the services it needs, which means there's less to understand at once when you read code.
02: Make unit testing easier.
Pete
Here is a problem I've struggled with ever since I first started learning object-oriented programming: how should one implement a logger in "proper" OOP code?
By this, I mean an object that has a method that we want every other object in the code to be able to access; this method would output to console/file/whatever, which we would use for logging--hence, this object would be the logger object.
We don't want to establish the logger object as a global variable, because global variables are bad, right? But we also don't want to have the pass the logger object in the parameters of every single method we call in every single object.
In college, when I brought this up to the professor, he couldn't actually give me an answer. I realize that there are actually packages (for say, Java) that might implement this functionality. What I am ultimately looking for, though, is the knowledge of how to properly and in the OOP way implement this myself.
You do want to establish the logger as a global variable, because global variables are not bad. At least, they aren't inherently bad. A logger is a great example of the proper use of a globally accessible object. Read about the Singleton design pattern if you want more information.
There are some very well thought out solutions. Some involve bypassing OO and using another mechanism (AOP).
Logging doesn't really lend itself too well to OO (which is okay, not everything does). If you have to implement it yourself, I suggest just instantiating "Log" at the top of each class:
private final log=new Log(this);
and all your logging calls are then trivial: log.print("Hey");
Which makes it much easier to use than a singleton.
Have your logger figure out what class you are passing in and use that to annotate the log. Since you then have an instance of log, you can then do things like:
log.addTag("Bill");
And log can add the tag bill to each entry so that you can implement better filtering for your display.
log4j and chainsaw are a perfect out of the box solution though--if you aren't just being academic, use those.
A globally accessible logger is a pain for testing. If you need a "centralized" logging facility create it on program startup and inject it into the classes/methods that need logging.
How do you test methods that use something like this:
public class MyLogger
{
public static void Log(String Message) {}
}
How do you replace it with a mock?
Better:
public interface ILog
{
void Log(String message);
}
public class MyLog : ILog
{
public void Log(String message) {}
}
I've always used the Singleton pattern to implement a logging object.
You could look at the Singleton pattern.
Create the logger as a singleton class and then access it using a static method.
I think you should use AOP (aspect-oriented programming) for this, rather than OOP.
In practice a singleton / global method works fine, in my opinion. Preferably the global thing is just a framework to which you can connect different listeners (observer pattern), e.g. one for console output, one for database output, one for Windows EventLog output, etc.
Beware for overdesign though, I find that in practice a single class with just global methods can work quite nicely.
Or you could use the infrastructure the particular framework you work in offers.
The Enterprise Library Logging Application Block that comes from Microsoft's Pattern & Practices group is a great example of implementing a logging framework in an OOP environment. They have some great documentation on how they have implemented their logging application block and all the source code is available for your own review or modification.
There are other similar implementations: log4net, log4j, log4cxx
They way they have implemented the Enterprise Library Logging Application Block is to have a static Logger class with a number of different methods that actually perform the log operation. If you were looking at patterns this would probably be one of the better uses of the Singleton pattern.
I am all for AOP together with log4*. This really helped us.
Google gave me this article for instance. You can try to search more on that subject.
(IMHO) how 'logging' happens isn't part of your solution design, it's more part of whatever environment you happen to be running in - like System and Calendar in Java.
Your 'good' solution is one that is as loosely coupled to any particular logging implementation as possible so think interfaces. I'd check out the trail here for an example of how Sun tackled it as they probably came up with a pretty good design and laid it all out for you to learn from!
use a static class, it has the least overhead and is accessible from all project types within a simple assembly reference
note that a Singleton is equivalent, but involves unnecessary allocation
if you are using multiple app domains, beware that you may need a proxy object to access the static class from domains other than the main one
also if you have multiple threads you may need to lock around the logging functions to avoid interlacing the output
IMHO logging alone is insufficient, that's why I wrote CALM
good luck!
Maybe inserting Logging in a transparent way would rather belong in the Aspect Oriented Programming idiom. But we're talking OO design here...
The Singleton pattern may be the most useful, in my opinion: you can access the Logging service from any context through a public, static method of a LoggingService class.
Though this may seem a lot like a global variable, it is not: it's properly encapsulated within the singleton class, and not everyone has access to it. This enables you to change the way logging is handled even at runtime, but protects the working of the logging from 'vilain' code.
In the system I work on, we create a number of Logging 'singletons', in order to be able to distinguish messages from different subsystems. These can be switched on/off at runtime, filters can be defined, writing to file is possible... you name it.
I've solved this in the past by adding an instance of a logging class to the base class(es) (or interface, if the language supports that) for the classes that need to access logging. When you log something, the logger looks at the current call stack and determines the invoking code from that, setting the proper metadata about the logging statement (source method, line of code if available, class that logged, etc.) This way a minimal number of classes have loggers, and the loggers don't need to be specifically configured with the metadata that can be determined automatically.
This does add considerable overhead, so it is not necessarily a wise choice for production logging, but aspects of the logger can be disabled conditionally if you design it in such a way.
Realistically, I use commons-logging most of the time (I do a lot of work in java), but there are aspects of the design I described above that I find beneficial. The benefits of having a robust logging system that someone else has already spent significant time debugging has outweighed the need for what could be considered a cleaner design (that's obviously subjective, especially given the lack of detail in this post).
I have had issues with static loggers causing permgen memory issues (at least, I think that's what the problem is), so I'll probably be revisiting loggers soon.
To avoid global variables, I propose to create a global REGISTRY and register your globals there.
For logging, I prefer to provide a singleton class or a class which provides some static methods for logging.
Actually, I'd use one of the existing logging frameworks.
One other possible solution is to have a Log class which encapsulates the logging/stored procedure. That way you can just instantiate a new Log(); whenever you need it without having to use a singleton.
This is my preferred solution, because the only dependency you need to inject is the database if you're logging via database. If you're using files potentially you don't need to inject any dependencies. You can also entirely avoid a global or static logging class/function.
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Singletons are a hotly debated design pattern, so I am interested in what the Stack Overflow community thought about them.
Please provide reasons for your opinions, not just "Singletons are for lazy programmers!"
Here is a fairly good article on the issue, although it is against the use of Singletons:
scientificninja.com: performant-singletons.
Does anyone have any other good articles on them? Maybe in support of Singletons?
In defense of singletons:
They are not as bad as globals because globals have no standard-enforced initialization order, and you could easily see nondeterministic bugs due to naive or unexpected dependency orders. Singletons (assuming they're allocated on the heap) are created after all globals, and in a very predictable place in the code.
They're very useful for resource-lazy / -caching systems such as an interface to a slow I/O device. If you intelligently build a singleton interface to a slow device, and no one ever calls it, you won't waste any time. If another piece of code calls it from multiple places, your singleton can optimize caching for both simultaneously, and avoid any double look-ups. You can also easily avoid any deadlock condition on the singleton-controlled resource.
Against singletons:
In C++, there's no nice way to auto-clean-up after singletons. There are work-arounds, and slightly hacky ways to do it, but there's just no simple, universal way to make sure your singleton's destructor is always called. This isn't so terrible memory-wise -- just think of it as more global variables, for this purpose. But it can be bad if your singleton allocates other resources (e.g. locks some files) and doesn't release them.
My own opinion:
I use singletons, but avoid them if there's a reasonable alternative. This has worked well for me so far, and I have found them to be testable, although slightly more work to test.
Google has a Singleton Detector for Java that I believe started out as a tool that must be run on all code produced at Google. The nutshell reason to remove Singletons:
because they can make testing
difficult and hide problems with your
design
For a more explicit explanation see 'Why Singletons Are Controversial' from Google.
A singleton is just a bunch of global variables in a fancy dress.
Global variables have their uses, as do singletons, but if you think you're doing something cool and useful with a singleton instead of using a yucky global variable (everyone knows globals are bad mmkay), you're unfortunately misled.
The purpose of a Singleton is to ensure a class has only one instance, and provide a global point of access to it. Most of the time the focus is on the single instance point. Imagine if it were called a Globalton. It would sound less appealing as this emphasizes the (usually) negative connotations of a global variable.
Most of the good arguments against singletons have to do with the difficulty they present in testing as creating test doubles for them is not easy.
There's three pretty good blog posts about Singletons by Miško Hevery in the Google Testing blog.
Singletons are Pathological Liars
Where Have All the Singletons Gone?
Root Cause of Singletons
Singleton is not a horrible pattern, although it is misused a lot. I think this misuse is because it is one of the easier patterns and most new to the singleton are attracted to the global side effect.
Erich Gamma had said the singleton is a pattern he wishes wasn't included in the GOF book and it's a bad design. I tend to disagree.
If the pattern is used in order to create a single instance of an object at any given time then the pattern is being used correctly. If the singleton is used in order to give a global effect, it is being used incorrectly.
Disadvantages:
You are coupling to one class throughout the code that calls the singleton
Creates a hassle with unit testing because it is difficult to replace the instance with a mock object
If the code needs to be refactored later on because of the need for more than one instance, it is more painful than if the singleton class were passed into the object (using an interface) that uses it
Advantages:
One instance of a class is represented at any given point in time.
By design you are enforcing this
Instance is created when it is needed
Global access is a side effect
Chicks dig me because I rarely use singleton and when I do it's typically something unusual. No, seriously, I love the singleton pattern. You know why? Because:
I'm lazy.
Nothing can go wrong.
Sure, the "experts" will throw around a bunch of talk about "unit testing" and "dependency injection" but that's all a load of dingo's kidneys. You say the singleton is hard to unit test? No problem! Just declare everything public and turn your class into a fun house of global goodness. You remember the show Highlander from the 1990's? The singleton is kind of like that because: A. It can never die; and B. There can be only one. So stop listening to all those DI weenies and implement your singleton with abandon. Here are some more good reasons...
Everybody is doing it.
The singleton pattern makes you invincible.
Singleton rhymes with "win" (or "fun" depending on your accent).
I think there is a great misunderstanding about the use of the Singleton pattern. Most of the comments here refer to it as a place to access global data. We need to be careful here - Singleton as a pattern is not for accessing globals.
Singleton should be used to have only one instance of the given class. Pattern Repository has great information on Singleton.
One of the colleagues I have worked with was very Singleton-minded. Whenever there was something that was kind of a manager or boss like object he would make that into a singleton, because he figured that there should be only one boss. And each time the system took up some new requirements, it turned out there were perfectly valid reasons to allow multiple instances.
I would say that singleton should be used if the domain model dictates (not 'suggests') that there is one. All other cases are just accendentally single instances of a class.
I've been trying to think of a way to come to the poor singelton's rescue here, but I must admit it's hard. I've seen very few legitimate uses of them and with the current drive to do dependency injection andd unit testing they are just hard to use. They definetly are the "cargo cult" manifestation of programming with design patterns I have worked with many programmers that have never cracked the "GoF" book but they know 'Singelton' and thus they know 'Patterns'.
I do have to disagree with Orion though, most of the time I've seen singeltons oversused it's not global variables in a dress, but more like global services(methods) in a dress. It's interesting to note that if you try to use Singeltons in the SQL Server 2005 in safe mode through the CLR interface the system will flag the code. The problem is that you have persistent data beyond any given transaction that may run, of course if you make the instance variable read only you can get around the issue.
That issue lead to a lot of rework for me one year.
Holy wars! Ok let me see.. Last time I checked the design police said..
Singletons are bad because they hinder auto testing - instances cannot be created afresh for each test case.
Instead the logic should be in a class (A) that can be easily instantiated and tested. Another class (B) should be responsible for constraining creation. Single Responsibility Principle to the fore! It should be team-knowledge that you're supposed to go via B to access A - sort of a team convention.
I concur mostly..
Many applications require that there is only one instance of some class, so the pattern of having only one instance of a class is useful. But there are variations to how the pattern is implemented.
There is the static singleton, in which the class forces that there can only be one instance of the class per process (in Java actually one per ClassLoader). Another option is to create only one instance.
Static singletons are evil - one sort of global variables. They make testing harder, because it's not possible to execute the tests in full isolation. You need complicated setup and tear down code for cleaning the system between every test, and it's very easy to forget to clean some global state properly, which in turn may result in unspecified behaviour in tests.
Creating only one instance is good. You just create one instance when the programs starts, and then pass the pointer to that instance to all other objects which need it. Dependency injection frameworks make this easy - you just configure the scope of the object, and the DI framework will take care of creating the instance and passing it to all who need it. For example in Guice you would annotate the class with #Singleton, and the DI framework will create only one instance of the class (per application - you can have multiple applications running in the same JVM). This makes testing easy, because you can create a new instance of the class for each test, and let the garbage collector destroy the instance when it is no more used. No global state will leak from one test to another.
For more information:
The Clean Code Talks - "Global State and Singletons"
Singleton as an implementation detail is fine. Singleton as an interface or as an access mechanism is a giant PITA.
A static method that takes no parameters returning an instance of an object is only slightly different from just using a global variable. If instead an object has a reference to the singleton object passed in, either via constructor or other method, then it doesn't matter how the singleton is actually created and the whole pattern turns out not to matter.
It was not just a bunch of variables in a fancy dress because this was had dozens of responsibilities, like communicating with persistence layer to save/retrieve data about the company, deal with employees and prices collections, etc.
I must say you're not really describing somthing that should be a single object and it's debatable that any of them, other than Data Serialization should have been a singelton.
I can see at least 3 sets of classes that I would normally design in, but I tend to favor smaller simpler objects that do a narrow set of tasks very well. I know that this is not the nature of most programmers. (Yes I work on 5000 line class monstrosities every day, and I have a special love for the 1200 line methods some people write.)
I think the point is that in most cases you don't need a singelton and often your just making your life harder.
The biggest problem with singletons is that they make unit testing hard, particularly when you want to run your tests in parallel but independently.
The second is that people often believe that lazy initialisation with double-checked locking is a good way to implement them.
Finally, unless your singletons are immutable, then they can easily become a performance problem when you try and scale your application up to run in multiple threads on multiple processors. Contended synchronization is expensive in most environments.
Singletons have their uses, but one must be careful in using and exposing them, because they are way too easy to abuse, difficult to truly unit test, and it is easy to create circular dependencies based on two singletons that accesses each other.
It is helpful however, for when you want to be sure that all your data is synchronized across multiple instances, e.g., configurations for a distributed application, for instance, may rely on singletons to make sure that all connections use the same up-to-date set of data.
I find you have to be very careful about why you're deciding to use a singleton. As others have mentioned, it's essentially the same issue as using global variables. You must be very cautious and consider what you could be doing by using one.
It's very rare to use them and usually there is a better way to do things. I've run into situations where I've done something with a singleton and then had to sift through my code to take it out after I discovered how much worse it made things (or after I came up with a much better, more sane solution)
I've used singletons a bunch of times in conjunction with Spring and didn't consider it a crutch or lazy.
What this pattern allowed me to do was create a single class for a bunch of configuration-type values and then share the single (non-mutable) instance of that specific configuration instance between several users of my web application.
In my case, the singleton contained client configuration criteria - css file location, db connection criteria, feature sets, etc. - specific for that client. These classes were instantiated and accessed through Spring and shared by users with the same configuration (i.e. 2 users from the same company). * **I know there's a name for this type of application but it's escaping me*
I feel it would've been wasteful to create (then garbage collect) new instances of these "constant" objects for each user of the app.
I'm reading a lot about "Singleton", its problems, when to use it, etc., and these are my conclusions until now:
Confusion between the classic implementation of Singleton and the real requirement: TO HAVE JUST ONE INSTANCE OF a CLASS!
It's generally bad implemented. If you want a unique instance, don't use the (anti)pattern of using a static GetInstance() method returning a static object. This makes a class to be responsible for instantiating a single instance of itself and also perform logic. This breaks the Single Responsibility Principle. Instead, this should be implemented by a factory class with the responsibility of ensuring that only one instance exists.
It's used in constructors, because it's easy to use and must not be passed as a parameter. This should be resolved using dependency injection, that is a great pattern to achieve a good and testable object model.
Not TDD. If you do TDD, dependencies are extracted from the implementation because you want your tests to be easy to write. This makes your object model to be better. If you use TDD, you won't write a static GetInstance =). BTW, if you think in objects with clear responsibilities instead classes, you'll get the same effect =).
I really disagree on the bunch of global variables in a fancy dress idea. Singletons are really useful when used to solve the right problem. Let me give you a real example.
I once developed a small piece of software to a place I worked, and some forms had to use some info about the company, its employees, services and prices. At its first version, the system kept loading that data from the database every time a form was opened. Of course, I soon realized this approach was not the best one.
Then I created a singleton class, named company, which encapsulated everything about the place, and it was completely filled with data by the time the system was opened.
It was not just a bunch of variables in a fancy dress because this was had dozens of responsibilities, like communicating with persistence layer to save/retrieve data about the company, deal with employees and prices collections, etc.
Plus, it was a fixed, system-wide, easily accessible point to have the company data.
Singletons are very useful, and using them is not in and of itself an anti-pattern. However, they've gotten a bad reputation largely because they force any consuming code to acknowledge that they are a singleton in order to interact with them. That means if you ever need to "un-Singletonize" them, the impact on your codebase can be very significant.
Instead, I'd suggest either hiding the Singleton behind a factory. That way, if you need to alter the service's instantiation behavior in the future, you can just change the factory rather than all types that consume the Singleton.
Even better, use an inversion of control container! Most of them allow you to separate instantiation behavior from the implementation of your classes.
One scary thing on singletons in for instance Java is that you can end up with multiple instances of the same singleton in some cases. The JVM uniquely identifies based on two elements: A class' fully qualified name, and the classloader responsible for loading it.
That means the same class can be loaded by two classloaders unaware of each other, and different parts of your application would have different instances of this singleton that they interact with.
Write normal, testable, injectable objects and let Guice/Spring/whatever handle the instantiation. Seriously.
This applies even in the case of caches or whatever the natural use cases for singletons are. There's no need to repeat the horror of writing code to try to enforce one instance. Let your dependency injection framework handle it. (I recommend Guice for a lightweight DI container if you're not already using one).