Essay On Unsafe Injections

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Essay On Unsafe Injections

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It's rare that you need a singleton. The reason they're bad is that they feel like a global and they're a fully paid up member of the GoF Design Patterns book. Some coding snobs look down on them as just a glorified global. In the same way that many people hate the goto statement there are others that hate the idea of ever using a global. I have seen several developers go to extraordinary lengths to avoid a global because they considered using one as an admission of failure. Strange but true. In practice the Singleton pattern is just a programming technique that is a useful part of your toolkit of concepts. From time to time you might find it is the ideal solution and so use it. But using it just so you can boast about using a design pattern is just as stupid as refusing to ever use it because it is just a global.

Singletons are Pathological Liars has a unit testing example that illustrates how singletons can make it difficult to figure out dependency chains and start or test an application. It is a fairly extreme example of abuse, but the point that he makes is still valid:. Singletons are nothing more than global state. Global state makes it so your objects can secretly get hold of things which are not declared in their APIs, and, as a result, Singletons make your APIs into pathological liars. Where have all the Singletons Gone makes the point that dependency injection has made it easy to get instances to constructors that require them, which alleviates the underlying need behind the bad, global Singletons decried in the first article.

I think the confusion is caused by the fact that people don't know the real application of the Singleton pattern. I can't stress this enough. Singleton is not a pattern to wrap globals. Singleton pattern should only be used to guarantee that one and only one instance of a given class exists during run time. People think Singleton is evil because they are using it for globals. It is because of this confusion that Singleton is looked down upon. Please, don't confuse Singletons and globals.

If used for the purpose it was intended for, you will gain extreme benefits from the Singleton pattern. One rather bad thing about singletons is that you can't extend them very easily. You basically have to build in some kind of decorator pattern or some such thing if you want to change their behavior. Also, if one day you want to have multiple ways of doing that one thing, it can be rather painful to change, depending on how you lay out your code.

One thing to note, if you DO use singletons, try to pass them in to whoever needs them rather than have them access it directly Otherwise if you ever choose to have multiple ways of doing the thing that singleton does, it will be rather difficult to change as each class embeds a dependency if it accesses the singleton directly. I believe this sort of pattern is called dependency injection and is generally considered a good thing.

Like any pattern though Think about it and consider if its use in the given situation is inappropriate or not Rules are made to be broken usually, and patterns should not be applied willy nilly without thought. The singleton pattern is not a problem in itself. The problem is that the pattern is often used by people developing software with object-oriented tools without having a solid grasp of OO concepts. When singletons are introduced in this context they tend to grow into unmanageable classes that contain helper methods for every little use.

Singletons are also a problem from a testing perspective. They tend to make isolated unit-tests difficult to write. Inversion of control IoC and dependency injection are patterns meant to overcome this problem in an object-oriented manner that lends itself to unit testing. In a garbage collected environment singletons can quickly become an issue with regard to memory management. There is also the multi-threaded scenario where singletons can become a bottleneck as well as a synchronization issue. A singleton gets implemented using a static method.

Static methods are avoided by people who do unit testing because they cannot be mocked or stubbed. Most people on this site are big proponents of unit testing. The generally most accepted convention to avoid them is using the inversion of control pattern. Singletons are also bad when it comes to clustering. Because then, you do not have "exactly one singleton" in your application anymore. Consider the following situation: As a developer, you have to create a web application which accesses a database.

To ensure that concurrent database calls do not conflict each other, you create a thread-save SingletonDao :. So you are sure that only one singleton in your application exists and all database go through this one and only SingletonDao. Your production environment now looks like this:. Now, consider you want to set up multiple instances of your web application in a cluster. Now, you suddenly have something like this:. That sounds weird, but now you have many singletons in your application. And that is exactly what a singleton is not supposed to be: Having many objects of it.

This is especially bad if you, as shown in this example, want to make synchronized calls to a database. Of course this is an example of a bad usage of a singleton. But the message of this example is: You can not rely that there is exactly one instance of a singleton in your application - especially when it comes to clustering. After reading Singletons are Pathological Liars as suggested in jason's answer I came across this little tidbit that provides the best presented example of how singletons are often misused. Now, for anybody who hasn't really played monopoly, these standards are ideal at best. A defeat in monopoly is hard to swallow because, monopoly is about money, if you lose you have to painstakingly watch the rest of the players finish the game, and losses are usually swift and crushing.

So, the rules usually get twisted at some point to serve the self-interest of some of the players at the expense of the others. So you're playing monopoly with friends Bob, Joe, and Ed. You're swiftly building your empire and consuming market share at an exponential rate. Your opponents are weakening and you start to smell blood figuratively. Your buddy Bob put all of his money into gridlocking as many low-value properties as possible but his isn't receiving a high return on investment the way he expected. Bob, as a stroke of bad luck, lands on your Boardwalk and is excised from the game.

Now the game goes from friendly dice-rolling to serious business. Bob has been made the example of failure and Joe and Ed don't want to end up like 'that guy'. So, being the leading player you, all of a sudden, become the enemy. Joe and Ed start practicing under-the-table trades, behind-the-back money injections, undervalued house-swapping and generally anything to weaken you as a player until one of them rises to the top. Then, instead of one of them winning, the process starts all over. All of a sudden, a finite set of rules becomes a moving target and the game degenerates into the type of social interactions that would make up the foundation of every high-rated reality TV show since Survivor.

So, if a rulebook for a game accurately represented a singleton, the monopoly rulebook would be an example of abuse. Aside from all of the obvious thread-safety and synchronization issues that mutable singletons present Personally, I have seen a programmer abuse a singleton by using it as some sort of twisted cross-thread database store within an application. A Singleton is only an option if you need what a singleton provides. A write-one read-only instance of an object. Unlike other answers I don't want to talk about what is wrong with Singletons but to show you how powerful and awesome they are when used right!

Solution : Well, dispose them! Implement a callback in your app to properly dispose a singletons, you should remove any data linked to them and finally: remove them from the Factory. It is also considered an anti-pattern by some people, who feel that it is overly used, introducing unnecessary limitations in situations where a sole instance of a class is not actually required. My answer on how Singletons are bad is always, "they are hard to do right". Many of the foundational components of languages are singletons classes, functions, namespaces and even operators , as are components in other aspects of computing localhost, default route, virtual filesystem, etc. While they cause trouble and frustration from time to time, they also can make a lot of things work a LOT better.

Everyone talks about Singleton's as globals, because they basically are. However, much sadly, not all of the badness in a global comes not intrinsically from being global, but how you use it. Same goes for Singletons. Actually more so as "single instance" really doesn't need to mean "globally accessible". It is more a natural byproduct, and given all the bad that we know comes from it, we shouldn't be in such a hurry to exploit global accessibility.

Once programmers see a Singleton they seem to always access it directly through its instance method. Instead, you should navigate to it just like you would any other object. Most code shouldn't even be aware it is dealing with a Singleton loose coupling, right? If only a small bit of code accesses the object like it is a global, a lot of harm is undone. I recommend enforcing it by restricting access to the instance function. The Singleton context is also really important. The other, less common, mistake is to ignore the Singleton lifestyle. Just because there is only one doesn't mean a Singleton is some omnipotent "always was and always will be", nor is it generally desirable objects without a begin and end violate all kinds of useful assumptions in code, and should be employed only in the most desperate of circumstances.

If you avoid those mistakes, Singletons can still be a PITA, bit it is ready to see a lot of the worst problems are significantly mitigated. Imagine a Java Singleton, that is explicitly defined as once per classloader which means it needs a thread safety policy , with defined creation and destruction methods and a life cycle that dictates when and how they get invoked, and whose "instance" method has package protection so it is generally accessed through other, non-global objects. Still a potential source of trouble, but certainly much less trouble. Sadly, rather than teaching good examples of how to do Singletons. We teach bad examples, let programmers run off using them for a while, and then tell them they are a bad design pattern. It's not that singletons themselves are bad but the GoF design pattern is.

The only really argument that is valid is that the GoF design pattern doesn't lend itself in regards to testing, especially if tests are run in parallel. Using a single instance of an class is a valid construct as long as you apply the following means in code:. Make sure the class that will be used as a singleton implements an interface. This allows stubs or mocks to be implemented using the same interface. During the runtime of you application, where singletons need to be passed to a given object, use a class factory that builds that object and have the class factory pass the singleton instance to the class that needs it. Don't use the class factor that creates that object under test that needs the singleton during test as it will pass the single global instance of it, which defeats the purpose.

We've used Singletons in our solutions with a great deal of success that are testable ensuring deterministic behavior in parallel test run streams. Vince Huston has these criteria, which seem reasonable to me:. If ownership of the single instance, when and how initialization occurs, and global access are not issues, Singleton is not sufficiently interesting. I'd like to address the 4 points in the accepted answer, hopefully someone can explain why I'm wrong. Why is hiding dependencies in your code bad?

There are already dozens of hidden dependencies C runtime calls, OS API calls, global function calls , and singleton dependencies are easy to find search for instance. If you're passing an object through 10 functions in a call stack just to avoid a singleton, is that so great? Single Responsibility Principle: I think this is a bit vague and depends on your definition of responsibility. A relevant question would be, why does adding this specific "responsibility" to a class matter? Why does passing an object to a class make it more tightly coupled than using that object as a singleton from within the class? Why does it change how long the state lasts? Singletons can be created or destroyed manually, so the control is still there, and you can make the lifetime the same as a non-singleton object's lifetime would be.

From a pratical point of view, a singleton is a trade-off developing time vs complexity. If you know your application won't change that much they are pretty OK to go with. Just know that you may need to refactor things up if your requirements change in an unexpected way which is pretty OK in most cases. Singletons sometimes also complicate unit testing. There is nothing inherently wrong with the pattern, assuming it is being used for some aspect of your model which is truly single. I believe the backlash is due to its overuse which, in turn, is due to the fact that it's the easiest pattern to understand and implement.

Using dependency injection has pretty much removed my requirements for singleton, servicelocators and factories. I find this a much more productive and clean environment, at least for the type of work I do Java-based web applications. The bad part of a singleton is generally the user or should I say the inappropriate use of a singleton for things it is not designed to do.

The biggest offender is using a singleton as a fake global variable. Rather than using singletons, you should pass all the needed utility objects from function to function. That can be simplified if you wrap all them into a helper object, like this:. The problems with singletons is the issue of increased scope and therefore coupling. There is no denying that there are some of situations where you do need access to a single instance, and it can be accomplished other ways.

I now prefer to design around an inversion of control IoC container and allow the the lifetimes to be controlled by the container. This gives you the benefit of the classes that depend on the instance to be unaware of the fact that there is a single instance. The lifetime of the singleton can be changed in the future. Once such example I encountered recently was an easy adjustment from single threaded to multi-threaded. Note: Coding Without Comments is no longer valid. However, The article being linked to has been cloned by another user. Singletons are NOT bad. It's only bad when you make something globally unique that isn't globally unique. However, there are "application scope services" think about a messaging system that makes components interact - this CALLS for a singleton, a "MessageQueue" - class that has a method "SendMessage Too many people put objects which are not thread safe in a singleton pattern.

In most cases "singletonity" is a detail of implementation for some class rather than characteristic of its interface. Inversion of Control Container may hide this characteristic from class users; you just need to mark your class as a singleton with Singleton annotation in Java for example and that's it; IoCC will do the rest. You don't need to provide global access to your singleton instance because the access is already managed by IoCC. Thus there is nothing wrong with IoC Singletons. GoF Singletons in opposite to IoC Singletons are supposed to expose "singletonity" in the interface through getInstance method, and so that they suffer from everything said above. Because they are basically object oriented global variables, you can usually design your classes in such a way so that you don't need them.

Firstly a class and its collaborators should firstly perform their intended purpose rather than focusing on dependents. Lifecycle management when instances are created and when they go out of scope should not be part of the classes responsibility. The accepted best practice for this is to craft or configure a new component to manage dependencies using dependency injection. Often software gets more complicated it makes sense to have multiple independent instances of the Singleton class with different state. Committing code to simply grab the singleton is wrong in such cases. Using Singleton. No class should be thought of as a singleton but rather that should be an application of it's usage or how it is used to configure dependents.

For a quick and nasty this does not matter - just luke hard coding say file paths does not matter but for bigger applications such dependencies need to be factored out and managed in more appropriate way using DI. The test suite and the many tests are each individual and separate something that is not compatible with hard coding a singleton. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. What are drawbacks or disadvantages of singleton pattern?

Asked 13 years ago. Active 3 months ago. Viewed k times. Improve this question. I have to say that using a singleton design has burned me recently as I have tried to adapt the code. As I do it in my free time, I'm almost too lazy to refactor this. Bad news for productivity. There's a lot of 'cons' in the answers, but I'd also like to see some good examples of when the pattern is good, to contrast with the bad I wrote a blog post on the subject a few months ago: jalf. I can't personally think of a single situation where a singleton is the right solution. That doesn't mean such a situation does not exist, but AdamSmith it doesn't mean you have to, but it means you can access it like that.

And if you don't intend to access it like that, then there's little reason to make it a singleton in the first place. So your argument is effectively "there's no harm in making a singleton if we don't treat it as a singleton. Yeah, great. My car also doesn't pollute if I don't drive in it. But then it's easier to just not acquire a car in the first place. The worst part of this whole topic is that the people who hate singletons rarely give concrete suggestions for what to use instead. The links to journal articles and self-published blogs all through this SO article, for example, go on and on about why not to use singletons and they're all excellent reasons , but they're extremely slim on replacements.

Lots of handwaving, though. Those of us trying to teach new programmers why not to use singletons don't have many good third-party counterexamples to point to, only contrived examples. It's wearying. Show 19 more comments. Active Oldest Votes. Paraphrased from Brian Button: They are generally used as a global instance, why is that so bad? Improve this answer. I disagree with you. Since comments are allowed only chars, I have written a Blog Post to comment on this, please see the link below. On point 1 and 4, I think singletons are useful, and in fact almost perfect for caching data especially from a DB. The increase in performance far outway the complexities involved in modeling unit tests.

Dai Bok: "caching data especially from a DB " use the proxy pattern to do this Wow, great responses. I probably used overly agressive wording here, so please keep in mind I was answering a negatively geared question. My answer was meant to be a short list of the 'anti patterns' that occur from bad Singleton usage. Full disclosure; I too use Singletons from time to time. There are more neutrally posed questions on SO that would make excellent forums for when Singletons can be considered a good idea. For instance, stackoverflow. They aren't so great for caching in an multithreaded environment. You can easily defeat a cache with multiple threads fighting over a limited resource. Show 20 more comments. Singletons solve one and only one problem. Resource Contention.

If you have some resource that 1 can only have a single instance, and 2 you need to manage that single instance, you need a singleton. When you think you need a global, you're probably making a terrible design mistake. Hardware is also an example right? Embedded systems have lots of hardware that might use singletons - or maybe one big one? Totally agree. There's a lot of determined "this practice is bad" talk floating around without any recognition that the practice may have its place. Often, the practice is only "bad" because it's frequently misused. There's nothing inherently wrong with the Singleton pattern as long as it's applied appropriately. Real, by-principle singletons are very rare and a printer queue certainly isn't one, and neither is a log file - see log4j.

More often than not, hardware is a singleton by coincidence rather than by principle. All hardware connected to a PC is at best a coincidental singleton think multiple monitors, mice, printers, sound cards. In telecommunications, neither phone number nor the physical phone are singletons think ISDN, call center. Hardware may have only one instance of a variety of resources, but hardware isn't software. There's no reason that a single serial port on a development board should be modeled as a Singleton. Indeed, modeling it so will only make it harder to port to the new board that has two ports!

So you'd write two identical classes, duplicating a lot of code, just so you can have two singletons representing two ports? How about just using two global SerialPort objects? How about not modelling the hardware as classes at all? And why would you use singletons, which also imply global access? Do you want every function in your code to be able to access the serial port? Show 18 more comments.

The failure I see in Singleton is that folks use them instead of globals because they're somehow "better. Construct-on-first-use is trivial to implement in a non-singleton, for example, even if it's not actually using the constructor to do it. The reason "we" look down upon them is that "we" very frequently see them used wrong, and way too often. Phil, You stated "from time to time you might find it is the ideal solution and so use it". Ok, so exactly in which case would we find a singleton useful? Pacerier, whenever all of following conditions hold: 1 you only need one of something, 2 you need to pass that one thing as an argument in a large number of method calls, 3 you are willing to accept a chance of having to refactor someday in exchange for an immediate reduction in the size and complexity of your code by not passing the darn thing around everywhere.

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