Would you ever NOT catch an exception, or throw an

问题:

I've dealt with instances where I would throw/rethrow an exception knowing that the code surrounding it would catch the specific exception. But is there any time you would want to throw an exception, knowing that it wouldn't be caught?

Or at least, NOT catch an exception?

Exceptions immediately halt the application unless their handled right? So I guess I'm asking if you would ever want to purposely let your application die?

回答1:

If your application is primarily going to be used by other clients and is not standalone, it generally makes sense to throw exceptions if a condition arises that you don't know how to (or don't want to) handle, and there's no sensible way for you to recover from it. Clients should be able to decide how they want to handle any exceptions that you might throw.

On the other hand, if your application is the endpoint, throwing an exception essentially becomes a notification mechanism to alert people that something has gone terribly wrong. In such cases, you need to consider a few things:

• How important is the continued running of the application? Is this error really unrecoverable? Throwing an exception and terminating your program is not something you want to be doing on the space shuttle.

• Are you using exceptions as a proxy for real logging? There's almost never a reason to do this; consider a real logging mechanism instead. Catch the exception and have the logger work out what happened.

• What are you trying to convey by throwing the exception yourself? Ask yourself what the value in throwing a new exception is, and consider carefully whether there isn't a better way to do what you want.

• Not catching an exception may leave resources in a bad state. If you don't gracefully exit, things are generally not cleaned up for you. Make sure you understand what you're doing if you need to do this -- and if you're not going to catch it, at least consider a try-finally block so you can do some tidying up.

回答2:

There's a very good rule that I came across a while ago:

Throw an exception when a method can't do what its name says it does.

The idea is that an exception indicates that something has gone wrong. When you are implementing a method, it is not your responsibility to be aware of whether it will be used correctly or not. Whether the code using your method catches the exception or not is not your responsibility, but the responsibility of the person using your method.

Another rule to follow is:

Don't catch an exception unless you know what you want to do with it.

Obviously, you should include cleanup code in a try...finally block, but you should never just catch an exception just for the sake of catching it. And you should never swallow exceptions silently. While there are occasions when you may want to catch all exceptions (e.g. by doing catch (Exception ex) in C#), these are fairly uncommon and generally have a very specific technical reason. For example, when you are using threads in .NET 2.0 or later, if an exception escapes from your thread, it will cause the entire application domain to unload. In these cases, however, at the very minimum you should log the exception details as an error and provide an explanation in the comments.

回答3:

Generally, and certainly in early iterations of your application, don't catch the exception. More often than not, the recovery from an exception will require a business rule of some sort, and, more often than not, those business rules are not defined for you. If you "handle" the exception instead of letting the application die then you will most likely be inventing business rules for your customer. Not good.

The general pattern of catching every exception just for the sake of catching it has caused me more headaches than I can count. It usually happens that someone puts some sort of generic exception handling code throughout the application, which inevitably ends up hiding a bug or creating some behavior that is unwanted. (incidentally, catching and then not rethrowing is even worse.)

So, I'd suggest that you ask instead: "When should I catch an exception?"

回答4:

Sure. For example, if you're trying to load some bytes into a string in Java:

try {
  String myString = new String(byteArray, "UTF-8");
} catch (UnsupportedEncodingException e) {
  // Platform doesn't support UTF-8?  What is this, 1991?
  throw new RuntimeExceptione(e);
}

In this case, there is no graceful degradation, the platform simply can't support the operation desired. You can check for this condition at initialization all you want, but the constructor for String still throws this exception, and you have to deal with it. Either that, or use Charset.forName() :)

回答5:

Here's the thing ... it is about "layers", or "encapsulation", or "low coupling". At some place in your codebase, you're writing a method to do something. Say it's a public method. Therefore, it should not assume much or anything about the caller ... rather, it should merely do the job it is supposed to do, regardless of who is calling it and what context the caller is in.

And if, for some reason, it cannot complete its job, then it needs to tell the caller "Sorry, I couldn't do that, and here's why". Exceptions are an excellent mechanism to let it tell the caller that (not the only mechanism, but the best mechanism I've ever seen for most cases).

So, when you throw the exception, you have no idea whether it will be caught or not ... because you're exposing a public method and you have no idea who might choose to call it and why.

The catching of the exception is the job of the "context". For example, say you're writing a library with public methods that might throw exceptions. Then, say you're using that library from a Windows Forms app. The Windows Forms app might catch exceptions and show a message box to the user.

But later, you might use the same library from a Windows Service. The Service would be more likely to catch the exception, log it, return an error to the original caller, but keep running so it can process further requests.

So the exception is like a contractual agreement between the caller and the provider. The provider says, "I'll either do the job or tell you why I can't. What you do from there is your own business." And the caller says, "OK, if you can't do the job, just tell me why, and I'll decide what to do in that case."

回答6:

But is there any time you would want to throw an exception, knowing that it wouldn't be caught?

I would say that if you're manually throwing an exception, most of the time you don't know if it will be caught. If you knew it would be caught you could just handle it yourself rather than throwing the exception in the first place.

To be fair, I suppose that depends in part on the kind of programming you're doing, and sometimes the same programmer ends up building both the library and the code that consumes said library.

Would you ever NOT catch an exception?

If you didn't expect/weren't aware an exception could be thrown. But putting that aside and assuming you are aware of the exception, sometimes you know about it at one tier but know the next tier up is the more appropriate place to handle it.

回答7:

It depends on the type of application. Web applications can continue running even after exceptions have bubbled up to the execution context.

It is common practice to 'throw/rethrow' an exception if you catch the exception at a level where it can't be dealt with. But, you would almost always add context to the issue, at the very least add some logging at the higher level to say that it was caught and rethrown.

for example

A calls B calls C (throws exception)

B catches/rethrows

A catches.

In this case, you would want B to add some logging so that you can differentiate between B generating and throwing an error, and C generating and throwing an error. That would allow you a greater ability to debug and fix problems later.

In general you will almost NEVER want an exception to kill your program. The best practice is to catch the except and exit gracefully. This allows you to save any currently open information and release resources that are being used so they don't become corrupted. If you intend to exit, you can create your own 'core-dump' information report that includes the things you were doing when you caught the fatal exception.

If you let the exception kill your process you are eliminating your chance to get custom tailored crash information, and you are also skipping the part where you provide the user with a friendly error message and then exit.

So, I would recommend ALWAYS catching exceptions, and never voluntarily letting them run amok in your program.

EDIT

If you are writing a library, you have to choose ahead of time whether your function will throw an exception, or be exception safe. In those cases, sometimes you will throw an exception and have no idea if the calling party will catch it. But in that case, catching it is not your responsibility, as long as the api declares that the function could throw exceptions. (I'm looking for a word that means 'could possibly throw exception'... anyone know what it is? It's going to bug me all day.)

回答8:

Firstly, there absolutely are situations where it is better to not catch an exception.

Sometimes, an exception can sometimes tell you that your program is in an unknown state. There are a number of exceptions where this is pretty much intrinsically true given the exception type. A NullReferenceException essentially tells you "there is a bug". And by catching such an exception, you may hide the bug, which sounds good in the short term, but in the long term you'd be happier to fix it. The product may not crash, but it certainly won't have the expected behaviour.

But this is also true for exception types we invent for ourselves. Sometimes, the fact that exception A has been thrown should be "impossible" - and yet it has happened, so there's a bug.

Also, something very important happens when you catch an exception: the finally blocks for the whole call stack inside the try block (and anything it calls) will be executed. What do those finally blocks do? Well, anything. And if the program is in an unknown state, I really do mean anything. They could erase valuable customer data from the disk. They could throw more exceptions. They could corrupt data in memory, making the bug impossible to diagnose.

So when an exception indicates an unknown state, you don't want to run any more code, so whatever you do, don't catch the exception. Let it fly past, and your program will terminate harmlessly, and Windows Error Reporting will be able to capture the state of the program as it was when the problem was originally detected. If you catch the exception, you will cause more code to execute, which will screw up the state of the program further.

Secondly, should you throw an exception knowing it won't be caught? I think that question misunderstands the nature of reusable methods. The whole idea of a method is that it has a "contract" that it follows: it accepts certain parameters and returns a certain value, plus also it throws certain exceptions under certain conditions. That's the contract - it's up to the caller what they do with it. For some callers, exception A might indicate a recoverable condition. For other callers, it might indicate a bug. And from what I said above, it should be clear that if an exception indicates a bug, it must not be caught.

And if you're wondering what this means for the Microsoft Enterprise Library's Exception Handling Block: yes, it's pretty broken. They tell you to catch (Exception x) and then decide whether to rethrow based on your policy; too late - the finally blocks have already executed by that point. Don't do that.

回答9:

You probably wouldn't want an uncaught exception anywhere where the end-users can see it, but it is often acceptable to let clients of your API (other programmers) decide how to handle exceptions.

For example, suppose you are designing a Java class library. You expose a public method that takes in a String. In your application, a null input value would cause an error. Instead of handling the error yourself, it would be acceptable to check for a null value, then throw an IllegalArgumentException.

You must, of course, document that your method throws this exception in this circumstance. This behavior becomes part of your method's contract.

回答10:

It depends on what you mean by 'being caught'. Something, somewhere eventually catches the exception whether it be the underlying OS or something else.

We have a workflow system that executes job plans comprised of individual jobs. Each job runs a unit of code. For some of the exceptions, we don't want to handle them in the code but throw it up the stack so that the external workflow system catches it (which happens completely outside of the thrower's process).

回答11:

If you're writing the entire application, then your reasons are your own. I can think of a few situations where you might want to throw the exception and let the app die, most of them are not very good reasons though.

The best reason is usually when debugging. I frequently disable exceptions while debugging to allow me to know better where something is failing. You can also just turn on thrown exception breaks in the debugger if you're running it on a machine with the debugger.

Another possible reason is when continuing after an exception is thrown doesn't make sense or would result in possible irrecoverable data corruption or worse (think Robots with laser beams, but then you should be damn sure your applicaiton deals with these situations IMO, crashing the program is just the lazy way).

If you're writing API code, or Framework code that you won't use yourself, then you have no idea if someone will catch your exceptions.

回答12:

Yup, it's my ONLY opportunity to slap the developer consuming the service/object to tell them "Ur dO1n it WrOnG!!!!".

That and getting rid of possibilities that you don't want to permit or are seemingly "impossible". Apps that catch all exceptions and continue are just a walled garden surrounded by chaos.

回答13:

If I need a moderately large system that is somehow processing data in what I believe to be a consistent manner.

And

Somewhere along the line, I detect that the application's state has become inconsistent.

And

The system doesn't (yet) know how to fix the inconsistency and recover gracefully

Then, yes, I would throw an exception with as much detail as possible and cause the application to die as quickly as possible, to avoid doing any further harm to the data. If it can be recovered, it'd be important not to exacerbate the problem by trying feebly to cover up the mess.

Later along the line, once the chain of events that led to the inconsistency is better understood, I higher facility can catch that exception, repair the state, and continue with minimal interruption.

回答14:

A library will often throw exceptions based on defensive programming checks, should a condition arise that shouldn't have been allowed to arise by the application code. Applications code will often be written such that most of those invalid conditions will never arise, and therefore the exceptions will never be thrown, so there's no point catching them.

Depending on language (I'm mostly thinking in terms of C++ rather than C#, and not that clear what the differences are) the effect of an uncaught exception actually being thrown is probably the same as what used to be done in the days before exceptions were invented. A common policy for defensive programming in C libraries, for example, was to terminate the program immediately with an error message.

The difference is that if the exception throw does turn out to be possible (hopefully this will be discovered through unit testing), it is often relatively easy to add an exception handler that can recover from the problem in a more constructive way. You don't have to rewrite the library, or add complex checks in application code to ensure the condition cannot arise before the exception-throwing call is made.

I have quite a few exception throws that are never caught. They are all for defensive purposes, and while being uncaught is bad for an exception that does happen, this only ever happens during development and testing, for error conditions I failed to consider in the application code so far. And when it happens, it is unusual for the fix to be awkward - no need for a large-scale refactoring, no need for the applications code to be massively complicated with error condition checks, just a catch clause with a relatively simple recovery or "I'm sorry, Dave, I'm afraid I can't do that." without failing out the whole app.