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This is just a general design question. If you are developing a business component or service, (e.g. an object/service that exposes a relatively simple interface for handling billing transactions), what are some good ways to handle business-related errors?
Say the component/service must integrate with a few different external web services that all report back different errors, and must also do some database work on your side. There are many different things that can go wrong, both network/database-wise and business rule-wise. Would it be better to try to catch all types of errors within the component and report them back to the caller using an error-code scheme, or try to wrap all errors into various types of exceptions and throw them on to the caller.
This seems to be a struggle, because it gets awkward to deal with the business rule checks using exceptions, and I've read in several places to avoid using exceptions to control "non-exceptional" or business-logic flows. I feel that "exceptional" is often a debatable term, and it gets sticky trying to keep different cases defined as "exceptional" vs. "non-exceptional." (e.g. if your business logic checks for spending limits, age limitations, etc.). At the same time, using an error-code scheme is also awkward, because the caller might choose to ignore the error codes.
Any tips or references would be appreciated!
I'd go for the error code scheme, because you said "many different things that can go wrong". You might want to have a scheme that returns a List of Error objects, where Error would encapsulate the code, explanation, original data, etc.
An exception can only tell one thing that's wrong, and it should be of an "exceptional" nature, not a run-of-the-mill problem.
Or perhaps a hybrid where you throw an exception that has a List of Error objects as a private data member and an access method that lets you iterate over the errors.
Business related errors should not be exeptions. Exceptions are there if there is an event in the program that the program is not designed to handle or completly unforseen.
To this end, if you anticipate a large number of different exceptions, by all means implement some useful numbering scheme. Here I would recommend a guid for the reason of component reuse. Every component should expose some interface to query the list of possible exception numbers (this can then be added to a data store for documentation). Also as you reuse components, error codes will not conflict.
User or business data errors on the other hand almost warrant some numbering scheme. There is almost no way to anticipate all the silly things a user is capable of. However, I would strongly discourage formal language exceptions in favour of some custom error object. Language exceptions may incur significant overhead, or require specialized handling to prevent them from propogating up the call stack. A business error object may very well be a nice way to go.
Services are borderline, especially if they have to be hit from various languages that may not handle SOAP exceptions properly. There are also security concerns if it's on the WWW.
If it's not a service, is internal, and/or will be accessed by clients in known languages, use exceptions to handle failure conditions. Clients are not guaranteed to check error codes.
See
http://msdn.microsoft.com/en-us/library/ms229030.aspx
for a great discussion of this. (It's also in the book Framework Design Guidelines.)
There's absolutely nothing that could prevent you from making both business rules and technical rules violations exceptions. I'd use exceptions in two cases:
Precondition violation. If your function's specification says "the amount spent should not exceed the limit for this type of account", and user, knowing this, specifies greater limit anyway, throw the exception right into him!
Postcondition violation. If all preconditions are met, but due to unexpected denial (database connection, for example; or completion of the transaction violates anti-monopoly law) you can't supply the correct result, do not supply it. Throw an exception.
Since all business logic probably is within one component and doesn't rely on anything else, case 2 for business exceptions is quite rare. And what falls into case 1 should be checked for in the front-end application. This makes me think that the approach of not throwing business-logic exceptions is not that you "should not do", but likely "you mostly don't need to".
Anyway, from the perspective of the language you're programming, it hardly notices the difference between "technical" and "business" exceptions, so it'll handle them all correctly. Second, exceptions are caught somewhere. I guess, that, from architector's point of view, business and technical exceptions will be caught in the different components. Of course, unless they're effectively uncaught, merely displaying error messages without programmatical handling falls into this category. So the use of the same mechanism won't make you mix these errors.
So, my point is, that if you chose not to repulse exceptions as programming concept, you may use them for business logic as well.
I see a lot of code written where an exception is thrown if a parameter is not in the right form, or whatever. Basically "throw new ...".
What is the benefit of this? The exception can be avoided by checking the parameters (Eg if null, write message back to webpage/winform). Why is this approach not used when an exception is expensive?
Thanks
A few points are worth making here:
First, your supposition that exceptions are expensive is generally untrue - exceptions are, well ... exceptional. They shouldn't be occurring often enough to have any meaningful effect on program performance. And if you are seeing enough exceptions that performance is a problem then you have bigger fish to fry.
Second, a well written class, function or module program should be able to detect and handle invalid input somewhat gracefully. It helps the maintainers and debuggers of the code locate the problems as close to their introduction as possible. If arguments are not checked, they can often result in a failure much later in the code - far removed from the actual error. Debugging such problems can be very painful.
Third, you assume that all code is aware of the context in which it is executed. A method may be deep in a framework or library and have no knowledge of whether it is running in a web application, console app, NT service, etc. Besides, it'a terrible practice to pepper logic to display information about invalid arguments throughout the body of your code - that responsibility should be centralized and controlled - otherwise you UI could easily become a mess of errors interspersed with actual presentation content.
Finally, exceptions allow a program to sometimes handle and recover from a problem rather than exposing it to the user. Don't diminish this capability by directly displaying errors immediately when they occur. Now, granted, most often invalid arguments are a symptom of a programming defect (rather than an environmental or configuration issue) - and so in most cases they can't be handled. But, then again, sometimes they can be handled.
For example, if you're writing a library to be used by code you don't know about or doesn't exist yet, how that error is handled is down to the code that is making the call.
So throwing an exception is a natural thing to do. Allows you to leave the decision on how to handle that error scenario to the caller/consumer.
Throwing an exception:
makes it clear to other programmers that the situation is exceptional
allows software calling the method involved to clearly handle the problem
shows tools and the compiler that the situation is exceptional so that they can assist the programmer
allows information to be passed to handling routines in the exception object itself
Printing strings - well - doesn't, really.
In terms of the 'expense' of an exception, exceptions should only be thrown in exceptional circumstances, i.e. rarely and as part of processing errors - I personally have not come across a situation where the 'expense' of an exception is a problem. More discussion on that point in this question.
This is known as Design by Contract.
The basic idea of Design by Contract is that objects have contracts between them, and if a caller does not fulfill the contract the receiver should fail with an exception rather than trying to guess the callers intention. At the end of the day, this leads to more stable software (in particular when more than one person is writing on project, since then the contract also become contracts between programmers).
PS: An important issue of Design by Contract that is often forgotten is the following. It must be possible for the client to know whether it fulfills the contract or not. So eg, if the contract of a stack is that client may only pop when the stack is not empty there must be an isEmpty method to check that and clients should use that method before calling pop. So this is why code that uses Design by Contract is cluttered with exceptions that are nevertheless never thrown.
It is better to throw an exception if the code will be compiled into a library and reused in multiple applications. In that case the client that calls into the library should handle the exceptions appropriately and report a user friendly message.
There are two main reasons I throw exceptions instead of writing an error message to standard out.
Debugging is easier - I know if the program has exited because of an error. Also, since exceptions in Java can be subclassed, I know exactly what type of error has occurred.
If you write an API, and then decide you want a GUI front-end, perhaps you want to take those exceptions and display them in a message dialog instead of writing them to standard out.
Generally and language-agnostically speaking, it is not correct assumption that exceptions are expensive. It depends on many factors.
Generally, exception is a generic way to signal an error condition and it is independent of any form of presentation. Sending out a page with error message would make the error reporting too tightly coupled with presentation, with UI. It is usually not a good idea in terms of flexible and scalable design.
The question is general and language-agnostic, thus the answer does not go deeply into details.
By the way, depending on a programming language, design of error handling, and number of other factors, approaches can be different. However, it's a good idea to learn about various options:
in C++, in Boost project, error handling guidelines say:
Don't worry too much about the what()
message. It's nice to have a message
that a programmer stands a chance of
figuring out, but you're very unlikely
to be able to compose a relevant and
user-comprehensible error message at
the point an exception is thrown (...)
Krzysztof Cwalina recommends a set of very useful Design Guidelines for .NET but they are in fact language-agnostic like Should Exceptions Carry Error Code Information
Given the guidelines above, after a while of consideration, it is not that clear what such error web page should display, what level of information, very technical or more user-friendly. Using exceptions, it gives more flexibility on various levels of the system as one of rules it catch when you need to handle (i.e. display error) ignore otherwise
Exceptions are, in most environments, easier to write tests for than is stuff written to the console:
it "should reject a negative initial balance" do
Account.new(-1).should raise_error(ArgumentError, "Invalid balance: -1")
end
Suppose you have a method with some pre and post-conditions.
Is it ok to create an exception class for each pre-condition that is not accomplished?
For example:
Not accomplishing pre1 means throwing a notPre1Exception instance.
Why wouldn't you want to define PreconditionFailedException(string precondition)? Throwing a different exception type for each failed precondition is overkill.
Yes and no.
Yes - Violating a precondition is certainly an appropriate time to throw an exception. Throwing a more specific exception will make catching that specific exception simpler.
No - Declaring a new exception class for every precondition in your program/api seems way overkill. This could result in hundreds or thousands of exceptions eventually. This seems a waste both mentally and computationally.
I would recommend throwing exceptions for precondition violations. I would not, however, recommend defining a new exception for each precondition. Instead, I would recommend creating broader classes of exceptions that cover a specific type of precondition violation, rather than a specific precondition violation. (I would also recommend using existing exceptions where they fit well.)
A failed precondition should throw an AssertException, or something similar. Before invoking a method, it's precondition must hold. If the caller doesn't do this check, it's a bug in the program, or an incorrect use of the method (API).
Only if not accomplishing the pre-conditions would be a rare exceptional occurrence.
Sounds like a useful use of exceptions to me. It certainly allows more fine-grained logging and debugging than just a general 'precondition failed' although you could also have a single 'preconditions failed' exception and put what preconditions have failed into the exception message.
I think it's ok to create a different exception for all your exceptions as long as you plan on using and handling them.
I have found that the better the error/exception handling the easier it is to debug software in the later stages.
For example: If you have a generic excpeiton to handle all bad input then you must look at everything that was passed into the method if there is an error. If you have an excpetion of all the types of bad conditions you will know exaclty where to look.
It looks possible to me, but if you want to continue this way of dealing with preconditions, you'll end up with N exception classes per class method. Looks like an explosive growth of 'non functional' classes.
I always liked code where the 'core' functionality didn't handle violation of preconditions (apart from asserting them - a great help!). This code can then be wrapped in a 'precondition-checker', which does throw exceptions or notifies non-satisfaction otherwise.
As a very very general way to determine if you should ever create a class or not (and an exception is a class), you should determine if there is some code that makes this class unique from all other classes (in this case exceptions).
If not, I'd just set the string in the exception and call it a day. If you are executing code in your exception (perhaps a generic recovery mechanism that can handle multiple situations by calling exception.resolve() or something), then it might be useful.
I realize that exceptions don't always follow this rule, but I think that's more or less because exceptions provided by the language can't have any business logic in them (because they don't know the business--libraries always tend to be full of exceptions to the OO rules)
I think Ben is on target here. What's the point in throwing different exceptions if you're not going to catch them? If you really want to throw a different one I would at least have a common "PreconditionFailedException" base class that they all derive from, and try to organize them into some sort of heirachy so you can catch groups of them. Personlly, I'd not have different ones and just have a common exception you throw with the details of the failure in each one.
No, you should not create a specific exception for each precondition because it will go against the principles of the Design-by-Contract.
The corollaries of implementing pre-conditions is that they should be part of the documentation and that you should provide the necessary methods for the caller to check that all the preconditions are valid. (i.e. if a method execution is relying on the status of an object, the method to check the status should be available to the caller).
So, the caller should be able to check if all prerequisites are met before calling your method.
Implementing specific exceptions for each violated pre-condition would/could encourage the caller to use the try/catch pattern around the method call, what is in contradiction with the philosophy of the Design-by-Contract.
I would say it's okay as long as you make them unchecked exceptions (subclass of RuntimeException in Java). However, in Java it is better to just use assertions.
If you do this, make sure they all inherit from another common custom exception.
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It's my understanding that common wisdom says to only use exceptions for truly exceptional conditions (In fact, I've seen that statement here at SO several times).
However, Krzysztof Cwalina says:
One of the biggest misconceptions about exceptions is that they are for “exceptional conditions.” The reality is that they are for communicating error conditions. From a framework design perspective, there is no such thing as an “exceptional condition”. Whether a condition is exceptional or not depends on the context of usage, --- but reusable libraries rarely know how they will be used. For example, OutOfMemoryException might be exceptional for a simple data entry application; it’s not so exceptional for applications doing their own memory management (e.g. SQL server). In other words, one man’s exceptional condition is another man’s chronic condition.
He then also goes on to say that exceptions should be used for:
Usage errors
Program errors
System failures
Considering Krzysztof Cwalina is the PM for the CLR team at MS I ask: What do you think of his statement?
This sounds over-simplistic, but I think it makes sense to simply use exceptions where they are appropriate. In languages like Java and Python, exceptions are very common, especially in certain situations. Exceptions are appropriate for the type of error you want to bubble up through a code path and force the developer to explicitly catch. In my own coding, I consider the right time to add an exception when the error either can't be ignored, or it's simply more elegant to throw an exception instead of returning an error value to a function call etc.
Some of the most appropriate places for exceptions that I can think of offhand:
NotImplementedException - very appropriate way of designating that a particular
method or function isn't available, rather than simply returning without doing
anything.
OutOfMemory exceptions - it's difficult to imagine a better way of handling this
type of error, since it represents a process-wide or OS-wide memory allocation
failure. This is essential to deal with, of course!
NullPointerException - Accessing a null variable is a programmer mistake, and IMO
this is another good place to force an error to bubble to the surface
ArrayIndexException - In an unforgiving language like C, buffer overflows
are disastrous. Nicer languages might return a null value of some type, or in
some implementations, even wrap around the array. In my opinion, throwing an
exception is a much more elegant response.
This is by no means a comprehensive list, but hopefully it illustrates the point. Use exceptions where they are elegant and logical. As always with programming, the right tool for the right job is good advice. There's no point going exception-crazy for nothing, but it's equally unwise to completely ignore a powerful and elegant tool at your disposal.
For people who write frameworks, perhaps it's interesting.
For the rest of us, it's confusing (and possibly useless.) For ordinary applications, exceptions have to be set aside as "exceptional" situations. Exceptions interrupt the ordinary sequential presentation of your program.
You should be circumspect about breaking the ordinary top-to-bottom sequential processing of your program. The exception handling is -- intentionally -- hard to read. Therefore, reserve exceptions for things that are outside the standard scenarios.
Example: Don't use exceptions to validate user input. People make input mistakes all the time. That's not exceptional, that's why we write software. That's what if-statements are for.
When your application gets an OutOfMemory exception, there's no point in catching it. That's exceptional. The "sequential execution" assumption is out the window. Your application is doomed, just crash and hope that your RDBMS transaction finishes before you crash.
It is indeed difficult to know what exactly construes an "exceptional condition" which warrants the use of an exception in a program.
One instance that is very helpful for using communicating the cause of errors. As the quote from Krzysztof Cwalina mentions:
One of the biggest misconceptions
about exceptions is that they are for
“exceptional conditions.” The reality
is that they are for communicating
error conditions.
To give a concrete example, say we have a getHeader(File f) method that is reading some header from a file and returns a FileHeader object.
There can be several problems which can arise from trying to read data from a disk. Perhaps the file specified doesn't exist, file contains data that can't be read, unexpected disk access errors, running out of memory, etc. Having multiple means of failure means that there should be multiple ways to report what went wrong.
If exceptions weren't used, but there was a need to communicate the kind of error that occurred, with the current method signature, the best we can do is to return a null. Since getting a null isn't very informative, the best communication we get from that result is that "some kind of error happened, so we couldn't continue, sorry." -- It doesn't communicate the cause of the error.
(Or alternatively, we may have class constants for FileHeader objects which indicate FileNotFound conditions and such, emulating error codes, but that really reeks of having a boolean type with TRUE, FALSE, FILE_NOT_FOUND.)
If we had gotten a FileNotFound or DeviceNotReady exception (hypothetical), at least we know what the source of the error was, and if this was an end user application, we could handle the error in ways to solve the problem.
Using the exception mechanism gives a means of communication that doesn't require a fallback to using error codes for notification of conditions that aren't within the normal flow of execution.
However, that doesn't mean that everything should be handled by exceptions. As pointed out by S.Lott:
Don't use exceptions to validate user
input, for example. People make
mistakes all the time. That's what
if-statements are for.
That's one thing that can't be stressed enough. One of the dangers of not knowing when exactly to use exceptions is the tendency to go exception-happy; using exceptions where input validation would suffice.
There's really no point in defining and throwing a InvalidUserInput exception when all that is required to deal in such a situation is to notify the user of what is expected as input.
Also, it should be noted that user input is expected to have faulty input at some point. It's a defensive measure to validate input before handing off input from the outside world to the internals of the program.
It's a little bit difficult to decide what is exceptional and what is not.
Since I usually program in Python, and in that language exceptions are everywhere, to me an exception may represent anything from a system error to a completely legitimate condition.
For example, the "pythonic" way to check if a string contains an integer is to try int(theString) and see if it raises an exception. Is that an "exceptional error"?
Again, in Python the for loop is always thought of as acting on an iterator, and an iterator must raise a 'StopIteration' exception when it finishes its job (the for loop catches that exception). Is that "exceptional" by any means?
I think the closer to the ground are you are the less appropriate exceptions as a means of error communication become. At a higher abstraction such as in Java or .net, an exception may make for an elegant way to pass error messages to your callers. This however is not the case in C. This is also a framework vs api design decision.
If you practice "tell, don't ask" then an exception is just the way a program says "I can't do that". It is "exceptional" in that you say "do X" and it cannot do X. A simple error-handling situation. In some languages it is quite common to work this way, in Java and C++ people have other opinions because exceptions become quite costly.
General: exception just means "I can't"
Pragmatic: ... if you can afford to work that way in your language.
Citizenship: ... and your team allows it.
Here is the definition for exception: An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
Therefore, to answer your question, no. Exceptions are for disruptive events, which may or may not be exceptional. I love this definition, it's simple and works every time - if you buy into exceptions like I do. E.g., a user submits an incorrect un/pw, or you have an illegal argument/bad user input. Throwing an exception here is the most straightforward way of solving these problems, which are disruptive, but not exceptional, nor even unanticipated.
They probably should have been called disruptions, but that boat has sailed.
I think there are a couple of good reasons why exceptions should be used to catch unexpected problems.
Firstly, they create an object to encapsulate the exception, which by definition must make it a lot more expensive than processing a simple if-statement. As a Java example, you should call File.exists() rather than routinely expecting and handling a FileNotFoundException.
Secondly, exceptions that are caught outside the current method (or maybe even class) make the code much harder to read than if the handling is all there in in the one method.
Having said that, I personally love exceptions. They relieve you of the need of explicitly handling all of those may-happen-but-probably-never-will type errors, which cause you to repetitively write print-an-error-and-abort-on-non-zero-return-code handling of every method call.
My bottom line is... if you can reasonably expect it to happen then it's part of your application and you should code for it. Anything else is an exception.
I've been wondering about this myself. What do we mean by "exceptional"? Maybe there's no strict definition, but are there any rules of thumb that we can use to decide what's exceptional, in a given context?
For example, would it be fair to say that an "exceptional" condition is one that violates the contract of a function?
KCwalina has a point.
It will be good to identify cases where the code will fail (upto a limit)
I agree with S.Lott that sometimes validating is better than to throw Exception.
Having said that, OutOfMemory is not what you might expect in your application (unless it is allocating a large memory & needs memory to go ahead).
I think, it depends on the domain of the application.
The statement from Krzysztof Cwalina is a little misleading. The original statement refers 'exceptional conditions', for me it is natural that I am the one who defines what's exceptional or not. Nevertheless, I think the message passed through OK, since I think we are all talking about 'developer' exceptions.
Exceptions are great for communication, but with a little hierarchy design they are also great for some separation of concerns, specially between layers (DAO, Business, etc). Of course, this is only useful if you treat these exceptions differently.
A nice example of hierarchy is spring's data access exception hierarchy.
I think he is right. Take a look at number parsing in java. You cant even check input string before parsing. You are forced to parse and retrieve NFE if something went wrong. Is parse failure something exceptional? I think no.
I certainly believe exceptions should be used only if you have an exceptional condition.
The trouble is in the definition of "exceptional". Here is mine:
A condition is exceptional if it is outside the assumed normal
behaviour of the part of the system that raises the exception.
This has some implications:
Exceptional depends on your assumptions. If a function assumes that it is passed valid parameters, then throwing an IllegalArgumentException is OK. However if a function's contract says that it will correct input errors in input in some way, then this usage is "normal" and it shouldn't throw an exception on an input error.
Exceptional depends on sub-system layering. A network IO function could certainly raise an exception if the network is discommented, as it assumes a valid connection. A ESB-based message broker however would be expected to handle dropped connections, so if it used such a network IO function internally then it would need to catch and handle the error appropriately. In case it isn't obvious, try/catch is effectively equivalent to a subsystem saying "a condition which is exceptional for one of my components is actually considered normal by me, so I need to handle it".
The saying that exceptions should be used for exceptional circumstances is used in "Effective Java Second Edition": one of the best java books.
The trouble is that this is taken out of context. When the author states that exceptions should be exceptional, he had just shown an example of using exceptions to terminate a while loop - a bad exception use. To quote:
exceptions are, as their name implies, to
be used only for exceptional conditions; they should never be used for ordinary
control flow.
So it all depends on your definition of "exception condition". Taken out of context you can imply that it should very rarely be used.
Using exceptions in place of returning error codes is good, while using them in order to implement a "clever" or "faster" technique is not good. That's usually what is meant by "exceptional condition".
Checked exception - minor errors that aren't bugs and shouldn't halt execution. ex. IO or file parsing
Unchecked exception - programming "bug" that disobeys a method contract - ex. OutOfBoundsException. OR a error that makes continuing of execution a very bad idea - ex IO or file parsing of a very important file. Perhaps a config file.
What it comes down to is what tool is needed to do the job.
Exceptions are a very powerful tool. Before using them ask if you need this power and the complexity that comes with it.
Exceptions may appear simple, because you know that when the line with the exception is hit everything comes to a halt. What happens from here though?
Will an uncaught exception occur?
Will the exception be caught by global error handling?
Will the exception be handled by more nested and detailed error handling?
You have to know everything up the stack to know what that exception will do. This violates the concept of independence. That method now is dependent on error handling to do what you expect it to.
If I have a method I shouldn't care what is outside of that method. I should only care what the input is, how to process it, and how to return the response.
When you use an exception you are essentially saying, I don't care what happens from here, something went wrong and I don't want it getting any worse, do whatever needs to be done to mitigate the issue.
Now if you care about how to handle the error, you will do some more thinking and build that into the interface of the method e.g. if you are attempting to find some object possibly return the default of that object if one can't be found rather than throwing some exception like "Object not found".
When you build error handling into your methods interface, not only is that method's signature more descriptive of what it can do, but it places the responsibility of how to handle the error on the caller of the method. The caller method may be able to work through it or not, and it would report again up the chain if not. Eventually you will reach the application's entry point. Now it would be appropriate to throw an exception, since you better have a good understanding of how exceptions will be handled if you're working with the applications public interface.
Let me give you an example of my error handling for a web service.
Level 1. Global error handling in global.asax - That's the safety net to prevent uncaught exceptions. This should never intentionally be reached.
Level 2. Web service method - Wrapped in a try/catch to guarantee it will always comply with its json interface.
Level 3. Worker methods - These get data, process it, and return it raw to the web service method.
In the worker methods it's not right to throw an exception. Yes I have nested web service method error handling, but that method can be used in other places where this may not exist.
Instead if a worker method is used to get a record and the record can't be found, it just returns null. The web service method checks the response and when it finds null it knows it can't continue. The web service method knows it has error handling to return json so throwing an exception will just return the details in json of what happened. From a client's perspective it's great that it got packaged into json that can be easily parsed.
You see each piece just knows what it needs to do and does it. When you throw an exception in the mix you hijack the applications flow. Not only does this lead to hard to follow code, but the response to abusing exceptions is the try/catch. Now you are more likely to abuse another very powerful tool.
All too often I see a try/catch catching everything in the middle of an a application, because the developer got scared a method they use is more complex than it appears.
I have exceptions created for every condition that my application does not expect. UserNameNotValidException, PasswordNotCorrectException etc.
However I was told I should not create exceptions for those conditions. In my UML those ARE exceptions to the main flow, so why should it not be an exception?
Any guidance or best practices for creating exceptions?
My personal guideline is: an exception is thrown when a fundamental assumption of the current code block is found to be false.
Example 1: say I have a function which is supposed to examine an arbitrary class and return true if that class inherits from List<>. This function asks the question, "Is this object a descendant of List?" This function should never throw an exception, because there are no gray areas in its operation - every single class either does or does not inherit from List<>, so the answer is always "yes" or "no".
Example 2: say I have another function which examines a List<> and returns true if its length is more than 50, and false if the length is less. This function asks the question, "Does this list have more than 50 items?" But this question makes an assumption - it assumes that the object it is given is a list. If I hand it a NULL, then that assumption is false. In that case, if the function returns either true or false, then it is breaking its own rules. The function cannot return anything and claim that it answered the question correctly. So it doesn't return - it throws an exception.
This is comparable to the "loaded question" logical fallacy. Every function asks a question. If the input it is given makes that question a fallacy, then throw an exception. This line is harder to draw with functions that return void, but the bottom line is: if the function's assumptions about its inputs are violated, it should throw an exception instead of returning normally.
The other side of this equation is: if you find your functions throwing exceptions frequently, then you probably need to refine their assumptions.
Because they're things that will happen normally. Exceptions are not control flow mechanisms. Users often get passwords wrong, it's not an exceptional case. Exceptions should be a truly rare thing, UserHasDiedAtKeyboard type situations.
My little guidelines are heavily influenced by the great book "Code complete":
Use exceptions to notify about things that should not be ignored.
Don't use exceptions if the error can be handled locally
Make sure the exceptions are at the same level of abstraction as the rest of your routine.
Exceptions should be reserved for what's truly exceptional.
It is NOT an exception if the username is not valid or the password is not correct. Those are things you should expect in the normal flow of operation. Exceptions are things that are not part of the normal program operation and are rather rare.
I do not like using exceptions because you can not tell if a method throws an exception just by looking at the call. Thats why exceptions should only be used if you can't handle the situation in a decent manner (think "out of memory" or "computer is on fire").
One rule of thumb is to use exceptions in the case of something you couldn't normally predict. Examples are database connectivity, missing file on disk, etc. For scenarios that you can predict, ie users attempting to log in with a bad password you should be using functions that return booleans and know how to handle the situation gracefully. You don't want to abruptly end execution by throwing an exception just because someone mistyped their password.
Others propose that exceptions should not be used because the bad login is to be expected in a normal flow if the user mistypes. I disagree and I don't get the reasoning. Compare it with opening a file.. if the file doesn't exist or is not available for some reason then an exception will be thrown by the framework. Using the logic above this was a mistake by Microsoft. They should have returned an error code. Same for parsing, webrequests, etc., etc..
I don't consider a bad login part of a normal flow, it's exceptional. Normally the user types the correct password, and the file does exist. The exceptional cases are exceptional and it's perfectly fine to use exceptions for those. Complicating your code by propagating return values through n levels up the stack is a waste of energy and will result in messy code. Do the simplest thing that could possibly work. Don't prematurely optimize by using error codes, exceptional stuff by definition rarely happens, and exceptions don't cost anything unless you throw them.
I think you should only throw an exception when there's nothing you can do to get out of your current state. For example if you are allocating memory and there isn't any to allocate. In the cases you mention you can clearly recover from those states and can return an error code back to your caller accordingly.
You will see plenty of advice, including in answers to this question, that you should throw exceptions only in "exceptional" circumstances. That seems superficially reasonable, but is flawed advice, because it replaces one question ("when should I throw an exception") with another subjective question ("what is exceptional"). Instead, follow the advice of Herb Sutter (for C++, available in the Dr Dobbs article When and How to Use Exceptions, and also in his book with Andrei Alexandrescu, C++ Coding Standards): throw an exception if, and only if
a precondition is not met (which typically makes one of the following
impossible) or
the alternative would fail to meet a post-condition or
the alternative would fail to maintain an invariant.
Why is this better? Doesn't it replace the question with several questions about preconditions, postconditions and invariants? This is better for several connected reasons.
Preconditions, postconditions and invariants are design characteristics of our program (its internal API), whereas the decision to throw is an implementation detail. It forces us to bear in mind that we must consider the design and its implementation separately, and our job while implementing a method is to produce something that satisfies the design constraints.
It forces us to think in terms of preconditions, postconditions and invariants, which are the only assumptions that callers of our method should make, and are expressed precisely, enabling loose coupling between the components of our program.
That loose coupling then allows us to refactor the implementation, if necessary.
The post-conditions and invariants are testable; it results in code that can be easily unit tested, because the post-conditions are predicates our unit-test code can check (assert).
Thinking in terms of post-conditions naturally produces a design that has success as a post-condition, which is the natural style for using exceptions. The normal ("happy") execution path of your program is laid out linearly, with all the error handling code moved to the catch clauses.
Exceptions are a somewhat costly effect, if for example you have a user that provides an invalid password, it is typically a better idea to pass back a failure flag, or some other indicator that it is invalid.
This is due to the way that exceptions are handled, true bad input, and unique critical stop items should be exceptions, but not failed login info.
I would say there are no hard and fast rules on when to use exceptions. However there are good reasons for using or not using them:
Reasons to use exceptions:
The code flow for the common case is clearer
Can return complex error information as an object (although this can also be achieved using error "out" parameter passed by reference)
Languages generally provide some facility for managing tidy cleanup in the event of the exception (try/finally in Java, using in C#, RAII in C++)
In the event no exception is thrown, execution can sometimes be faster than checking return codes
In Java, checked exceptions must be declared or caught (although this can be a reason against)
Reasons not to use exceptions:
Sometimes it's overkill if the error handling is simple
If exceptions are not documented or declared, they may be uncaught by calling code, which may be worse than if the the calling code just ignored a return code (application exit vs silent failure - which is worse may depend on the scenario)
In C++, code that uses exceptions must be exception safe (even if you don't throw or catch them, but call a throwing function indirectly)
In C++, it is hard to tell when a function might throw, therefore you must be paranoid about exception safety if you use them
Throwing and catching exceptions is generally significantly more expensive compared to checking a return flag
In general, I would be more inclined to use exceptions in Java than in C++ or C#, because I am of the opinion that an exception, declared or not, is fundamentally part of the formal interface of a function, since changing your exception guarantee may break calling code. The biggest advantage of using them in Java IMO, is that you know that your caller MUST handle the exception, and this improves the chance of correct behaviour.
Because of this, in any language, I would always derive all exceptions in a layer of code or API from a common class, so that calling code can always guarantee to catch all exceptions. Also I would consider it bad to throw exception classes that are implementation-specific, when writing an API or library (i.e. wrap exceptions from lower layers so that the exception that your caller receives is understandable in the context of your interface).
Note that Java makes the distinction between general and Runtime exceptions in that the latter need not be declared. I would only use Runtime exception classes when you know that the error is a result of a bug in the program.
If it's code running inside a loop that will likely cause an exception over and over again, then throwing exceptions is not a good thing, because they are pretty slow for large N. But there is nothing wrong with throwing custom exceptions if the performance is not an issue. Just make sure that you have a base exception that they all inherite, called BaseException or something like that. BaseException inherits System.Exception, but all of your exceptions inherit BaseException. You can even have a tree of Exception types to group similar types, but this may or may not be overkill.
So, the short answer is that if it doesn't cause a significant performance penalty (which it should not unless you are throwing a lot of exceptions), then go ahead.
Exception classes are like "normal" classes. You create a new class when it "is" a different type of object, with different fields and different operations.
As a rule of thumb, you should try balance between the number of exceptions and the granularity of the exceptions. If your method throws more than 4-5 different exceptions, you can probably merge some of them into more "general" exceptions, (e.g. in your case "AuthenticationFailedException"), and using the exception message to detail what went wrong. Unless your code handles each of them differently, you needn't creates many exception classes. And if it does, may you should just return an enum with the error that occured. It's a bit cleaner this way.
the rule of thumb for throwing exceptions is pretty simple. you do so when your code has entered into an UNRECOVERABLE INVALID state. if data is compromised or you cannot wind back the processing that occurred up to the point then you must terminate it. indeed what else can you do? your processing logic will eventually fail elsewhere. if you can recover somehow then do that and do not throw exception.
in your particular case if you were forced to do something silly like accept money withdrawal and only then check user/pasword you should terminate the process by throwing an exception to notify that something bad has happened and prevent further damage.
I agree with japollock way up there--throw an acception when you are uncertain about the outcome of an operation. Calls to APIs, accessing filesystems, database calls, etc. Anytime you are moving past the "boundaries" of your programming languages.
I'd like to add, feel free to throw a standard exception. Unless you are going to do something "different" (ignore, email, log, show that twitter whale picture thingy, etc), then don't bother with custom exceptions.
I'd say that generally every fundamentalism leads to hell.
You certainly wouldn't want to end up with exception driven flow, but avoiding exceptions altogether is also a bad idea. You have to find a balance between both approaches. What I would not do is to create an exception type for every exceptional situation. That is not productive.
What I generally prefer is to create two basic types of exceptions which are used throughout the system: LogicalException and TechnicalException. These can be further distinguished by subtypes if needed, but it is not generally not necessary.
The technical exception denotes the really unexpected exception like database server being down, the connection to the web service threw the IOException and so on.
On the other hand the logical exceptions are used to propagate the less severe erroneous situation to the upper layers (generally some validation result).
Please note that even the logical exception is not intended to be used on regular basis to control the program flow, but rather to highlight the situation when the flow should really end. When used in Java, both exception types are RuntimeException subclasses and error handling is highly aspect oriented.
So in the login example it might be wise to create something like AuthenticationException and distinguish the concrete situations by enum values like UsernameNotExisting, PasswordMismatch etc. Then you won't end up in having a huge exception hierarchy and can keep the catch blocks on maintainable level. You can also easily employ some generic exception handling mechanism since you have the exceptions categorized and know pretty well what to propagate up to the user and how.
Our typical usage is to throw the LogicalException during the Web Service call when the user's input was invalid. The Exception gets marshalled to the SOAPFault detail and then gets unmarshalled to the exception again on the client which is resulting in showing the validation error on one certain web page input field since the exception has proper mapping to that field.
This is certainly not the only situation: you don't need to hit web service to throw up the exception. You are free to do so in any exceptional situation (like in the case you need to fail-fast) - it is all at your discretion.
In general you want to throw an exception for anything that can happen in your application that is "Exceptional"
In your example, both of those exceptions look like you are calling them via a password / username validation. In that case it can be argued that it isn't really exceptional that someone would mistype a username / password.
They are "exceptions" to the main flow of your UML but are more "branches" in the processing.
If you attempted to access your passwd file or database and couldn't, that would be an exceptional case and would warrant throwing an exception.
Firstly, if the users of your API aren't interested in specific, fine-grained failures, then having specific exceptions for them isn't of any value.
Since it's often not possible to know what may be useful to your users, a better approach is to have the specific exceptions, but ensure they inherit from a common class (e.g., std::exception or its derivatives in C++). That allows your client to catch specific exceptions if they choose, or the more general exception if they don't care.
Exceptions are intended for events that are abnormal behaviors, errors, failures, and such. Functional behavior, user error, etc., should be handled by program logic instead. Since a bad account or password is an expected part of the logic flow in a login routine, it should be able to handle those situations without exceptions.
The simple answer is, whenever an operation is impossible (because of either application OR because it would violate business logic). If a method is invoked and it impossible to do what the method was written to do, throw an Exception. A good example is that constructors always throw ArgumentExceptions if an instance cannot be created using the supplied parameters. Another example is InvalidOperationException, which is thrown when an operation cannot be performed because of the state of another member or members of the class.
In your case, if a method like Login(username, password) is invoked, if the username is not valid, it is indeed correct to throw a UserNameNotValidException, or PasswordNotCorrectException if password is incorrect. The user cannot be logged in using the supplied parameter(s) (i.e. it's impossible because it would violate authentication), so throw an Exception. Although I might have your two Exceptions inherit from ArgumentException.
Having said that, if you wish NOT to throw an Exception because a login failure may be very common, one strategy is to instead create a method that returns types that represent different failures. Here's an example:
{ // class
...
public LoginResult Login(string user, string password)
{
if (IsInvalidUser(user))
{
return new UserInvalidLoginResult(user);
}
else if (IsInvalidPassword(user, password))
{
return new PasswordInvalidLoginResult(user, password);
}
else
{
return new SuccessfulLoginResult();
}
}
...
}
public abstract class LoginResult
{
public readonly string Message;
protected LoginResult(string message)
{
this.Message = message;
}
}
public class SuccessfulLoginResult : LoginResult
{
public SucccessfulLogin(string user)
: base(string.Format("Login for user '{0}' was successful.", user))
{ }
}
public class UserInvalidLoginResult : LoginResult
{
public UserInvalidLoginResult(string user)
: base(string.Format("The username '{0}' is invalid.", user))
{ }
}
public class PasswordInvalidLoginResult : LoginResult
{
public PasswordInvalidLoginResult(string password, string user)
: base(string.Format("The password '{0}' for username '{0}' is invalid.", password, user))
{ }
}
Most developers are taught to avoid Exceptions because of the overhead caused by throwing them. It's great to be resource-conscious, but usually not at the expense of your application design. That is probably the reason you were told not to throw your two Exceptions. Whether to use Exceptions or not usually boils down to how frequently the Exception will occur. If it's a fairly common or an fairly expectable result, this is when most developers will avoid Exceptions and instead create another method to indicate failure, because of the supposed consumption of resources.
Here's an example of avoiding using Exceptions in a scenario like just described, using the Try() pattern:
public class ValidatedLogin
{
public readonly string User;
public readonly string Password;
public ValidatedLogin(string user, string password)
{
if (IsInvalidUser(user))
{
throw new UserInvalidException(user);
}
else if (IsInvalidPassword(user, password))
{
throw new PasswordInvalidException(password);
}
this.User = user;
this.Password = password;
}
public static bool TryCreate(string user, string password, out ValidatedLogin validatedLogin)
{
if (IsInvalidUser(user) ||
IsInvalidPassword(user, password))
{
return false;
}
validatedLogin = new ValidatedLogin(user, password);
return true;
}
}
for me Exception should be thrown when a required technical or business rule fails.
for instance if a car entity is associated with array of 4 tires ... if one tire or more are null ... an exception should be Fired "NotEnoughTiresException" , cuz it can be caught at different level of the system and have a significant meaning through logging.
besides if we just try to flow control the null and prevent the instanciation of the car .
we might never never find the source of the problem , cuz the tire isn't supposed to be null in the first place .
the main reason for avoiding throwing an exception is that there is a lot of overhead involved with throwing an exception.
One thing the article below states is that an exception is for an exceptional conditions and errors.
A wrong user name is not necessarily a program error but a user error...
Here is a decent starting point for exceptions within .NET:
http://msdn.microsoft.com/en-us/library/ms229030(VS.80).aspx
Throwing exceptions causes the stack to unwind, which has some performance impacts (admitted, modern managed environments have improved on that). Still repeatedly throwing and catching exceptions in a nested situation would be a bad idea.
Probably more important than that, exceptions are meant for exceptional conditions. They should not be used for ordinary control flow, because this will hurt your code's readability.
I have three type of conditions that I catch.
Bad or missing input should not be an exception. Use both client side js and server side regex to detect, set attributes and forward back to the same page with messages.
The AppException. This is usually an exception that you detect and throw with in your code. In other words these are ones you expect (the file does not exist). Log it, set the message, and forward back to the general error page. This page usually has a bit of info about what happened.
The unexpected Exception. These are the ones you don't know about. Log it with details and forward them to a general error page.
Hope this helps
Security is conflated with your example: You shouldn't tell an attacker that a username exists, but the password is wrong. That's extra information you don't need to share. Just say "the username or password is incorrect."
I have philosophical problems with the use of exceptions. Basically, you are expecting a specific scenario to occur, but rather than handling it explicitly you are pushing the problem off to be handled "elsewhere." And where that "elsewhere" is can be anyone's guess.
To my mind, the fundamental question should be whether one would expect that the caller would want to continue normal program flow if a condition occurs. If you don't know, either have separate doSomething and trySomething methods, where the former returns an error and the latter does not, or have a routine that accepts a parameter to indicate whether an exception should be thrown if it fails). Consider a class to send commands to a remote system and report responses. Certain commands (e.g. restart) will cause the remote system to send a response but then be non-responsive for a certain length of time. It is thus useful to be able to send a "ping" command and find out whether the remote system responds in a reasonable length of time without having to throw an exception if it doesn't (the caller would probably expect that the first few "ping" attempts would fail, but one would eventually work). On the other hand, if one has a sequence of commands like:
exchange_command("open tempfile");
exchange_command("write tempfile data {whatever}");
exchange_command("write tempfile data {whatever}");
exchange_command("write tempfile data {whatever}");
exchange_command("write tempfile data {whatever}");
exchange_command("close tempfile");
exchange_command("copy tempfile to realfile");
one would want failure of any operation to abort the whole sequence. While one could check each operation to ensure it succeeded, it's more helpful to have the exchange_command() routine throw an exception if a command fails.
Actually, in the above scenario it may be helpful to have a parameter to select a number of failure-handling modes: never throw exceptions, throw exceptions for communication errors only, or throw exceptions in any cases where a command does not return a "success" indication.
You may use a little bit generic exceptions for that conditions. For e.g. ArgumentException is meant to be used when anything goes wrong with the parameters to a method (with the exception of ArgumentNullException). Generally you would not need exceptions like LessThanZeroException, NotPrimeNumberException etc. Think of the user of your method. The number of the conditions that she will want to handle specifically is equal to the number of the type of the exceptions that your method needs to throw. This way, you can determine how detailed exceptions you will have.
By the way, always try to provide some ways for users of your libraries to avoid exceptions. TryParse is a good example, it exists so that you don't have to use int.Parse and catch an exception. In your case, you may want to provide some methods to check if user name is valid or password is correct so your users (or you) will not have to do lots of exception handling. This will hopefully result in more readble code and better performance.
Ultimately the decision comes down to whether it is more helpful to deal with application-level errors like this using exception handling, or via your own home-rolled mechanism like returning status codes. I don't think there's a hard-and-fast rule about which is better, but I would consider:
Who's calling your code? Is this a public API of some sort or an internal library?
What language are you using? If it's Java, for example, then throwing a (checked) exception puts an explicit burden on your caller to handle this error condition in some way, as opposed to a return status which could be ignored. That could be good or bad.
How are other error conditions in the same application handled? Callers won't want to deal with a module that handles errors in an idiosyncratic way unlike anything else in the system.
How many things can go wrong with the routine in question, and how would they be handled differently? Consider the difference between a series of catch blocks that handle different errors and a switch on an error code.
Do you have structured information about the error you need to return? Throwing an exception gives you a better place to put this information than just returning a status.
Some useful things to think about when deciding whether an exception is appropriate:
what level of code you want to have run after the exception candidate occurs - that is, how many layers of the call stack should unwind. You generally want to handle an exception as close as possible to where it occurs. For username/password validation, you would normally handle failures in the same block of code, rather than letting an exception bubble up. So an exception is probably not appropriate. (OTOH, after three failed login attempts, control flow may shift elsewhere, and an exception may be appropriate here.)
Is this event something you would want to see in an error log? Not every exception is written to an error log, but it's useful to ask whether this entry in an error log would be useful - i.e., you would try to do something about it, or would be the garbage you ignore.
"PasswordNotCorrectException" isn't a good example for using exceptions. Users getting their passwords wrong is to be expected, so it's hardly an exception IMHO. You probably even recover from it, showing a nice error message, so it's just a validity check.
Unhandled exceptions will stop the execution eventually - which is good. If you're returning false, null or error codes, you will have to deal with the program's state all by yourself. If you forget to check conditions somewhere, your program may keep running with wrong data, and you may have a hard time figuring out what happened and where.
Of course, you could cause the same problem with empty catch statements, but at least spotting those is easier and doesn't require you to understand the logic.
So as a rule of thumb:
Use them wherever you don't want or simply can't recover from an error.
I would say that exceptions should be thrown if an unexpected behaviour is occuring that wasnt meant to be.
Like trying to update or delete a non existing entity. And it should be catched where the Exception can be handled and has meaning. For working in an alternative way to continue, add logging or returning a specific result on Api level.
If you expect something to be the case, you should build code to check and ensure it.