I recently had a discussion with a friend, who is a relatively junior but very smart software developer. She asked me about exception handling. The questions were pointing to a tips and tricks kind of path and there is definitely a list of them. But I am a believer on context and motivation behind the way we write software so I decided to write my thoughts on exceptions from such a perspective.
Exceptions in programming (using Java as a stage for our story) are used to notify us that a problem occurred during the execution of our code. Exceptions are a special category of classes. What makes them special is that they extend the Exception class which in turn extends the Throwable class. Being implementations of Throwable allow us to "throw" them when necessary. So, how can an exception happen? Instances of exception classes are thrown either from the JVM or in a section of code using the throw statement. That is the how, but why?
I am sure that most of us cringe when we see exceptions occur, but they are a tool to our benefit. Before the inception of exceptions, special values or error codes were returned to let us know that an operation did not succeed. Forgetting (or being unaware) to check for such error codes, could lead to unpredictable behavior in our applications. So yay for exceptions!
There are 2 things that come to mind as I write the above. Exceptions are a bad event because when they are created we know a problem occurred. Exceptions are a helpful construct because they give us valuable information about what went wrong and allow us to behave in proper way for each situation.
Trying to distil the essence of this design issue: a method/request is triggered to do something but it might fail - how do we best notify the caller that it failed? How do we communicate information about what happened? How we help the client decide what to do next? The problem with using exceptions is that we “give up” and not just that; we do it in an “explosive” way and the clients/callers of our services have to handle the mess.
So my first advice when it comes to exceptions, since they are a bad event - try to avoid them. In the sections of software under your control, implement design that makes difficult for errors to happen. You can use features of your language that support this behavior. I believe the most common exception in java is the NullPointerException and Optionals can help us avoid them. Let’s consider we want to retrieve an employee with a specified id:
public Optional<Employee> tryGetEmployee(String employeeId) {
return Optional.ofNullable(employeeService.getEmployee(employeeId));
}
So much better now. But besides the features of our language, we can design our code in a way that makes it difficult for errors to occur. If we consider a method, which can only receive positive integers as an input, we can set our code up, so that it is extremely unlikely for clients to mistakenly pass invalid input. First we create a PositiveInteger class:
public class PositiveInteger {
private Integer integerValue;
public PositiveInteger(Integer inputValue) {
if(inputValue <= 0) {
throw new IllegalArgumentException("PositiveInteger instances can only be created out of positive integers");
}
this.integerValue = inputValue;
}
public Integer getIntegerValue() {
return integerValue;
}
}
Then for a method that can only use positive integer as an input:
public void setNumberOfWinners(PositiveInteger numberOfWinners) { … }
These are of course simple examples and I did argue that the heart of the issue is that occasionally problems occur and then we have to inform clients about what happened. So let’s say we retrieve a list of employees from an external back end system and things can go wrong. How to handle this?
We can set our response object to GetEmployeesResponse, which would look something like this:
public class GetEmployeesResponse {
private Ok ok;
private Error error;
…
class Ok {
private List<Employee> employeeList;
...
}
class Error {
private String errorMessage;
...
}
}
But let’s be realists, you do not have control on every part of your codebase and you are not going to change everything either. Exceptions do and will happen, so let’s start with brief background information on them.
As mentioned before, the Exception class extends the Throwable class. All exceptions are subclasses of the exception class. Exceptions can be categorized in checked and unchecked exceptions. That simply means that some exceptions, the checked ones, require from us to specify on compile time how the application will behave in case the exception occurs. The unchecked exceptions do not mandate compile time handling from us. To create such exceptions you extend the RuntimeException class which is a direct subclass of Exception. An old and common guideline when it comes to checked vs unchecked is that runtime exceptions are used to signal situations which the application usually cannot anticipate or recover from, while checked exceptions are situations that a well-written application should anticipate and recover from.
Well, I am an advocate of only using runtime exceptions. And if I use a library that has a method with checked exception, I create a wrapper method that turns it into a runtime. Why not checked exceptions then? Uncle Bob in his “Clean Code” book argues, they break the Open/Closed principle, since a change in the signature with a new throws declaration could have effects in many levels of our program calling the method.
Now, checked or unchecked, since exceptions are a construct to give us insights on what went wrong, they should be as specific and as informative as possible on what happened. So try to use standard exceptions, others will understand what happened easier. When seeing a NullPointerException, the reason is clear to anyone. If you make your own exceptions, make it sensible and specific. For example, a ValidationException lets me know a certain validation failed, an AgeValidationException points me to the specific validation failure. Being specific, allows both to diagnose easier what happened but also to specify a different behavior based on what happened (type of exception). That is the reason why you should always catch the most specific exception first! So here comes another common advice that instructs to not catch on “Exception”. It is a valid advice which I occasionally do not follow. In the boundaries of my api (let’s say the endpoints of my REST service) I always have generic catch Exception clauses. I do not want any surprises and something that I did not manage to predict or guard against in my code, to potentially reveal things to the outside world.
Be descriptive but also provide exceptions according to the level of abstraction. Consider creating a hierarchy of exceptions that provide semantic information in different abstraction levels. If an exception is thrown from the lower levels of our program, such as a database related exception, it does not have to provide the details to the caller of our API. Catch the exception and throw a more abstract one, that simply informs callers that their attempted operation failed. This might seem like it comes against the common approach of “catch only when you can handle”, but it is not. Simply in this case our “handling” is the triggering of a new exception. In these cases make the whole history of the exception available from throw to throw, by passing the original exception to the constructor of the new exception.
The word “handle” was used many times. What does it mean? An exceptions is considered to be handled when it gets “caught” in our familiar catch clause. When an exception is thrown, first it will search for exception handling in the code from where it happens, if none is found it will go to the calling context of the method it is enclosed and so on until an exception handler is found or the program will terminate.
One nice piece that I like from uncle Bob again, is that the try-catch-finally blocks define a scope within the program. And besides the lexical scope we should think of its conceptual scope, treat the try block as a transaction. What should we do if something goes wrong? How do we make sure to leave our program in a valid state? Do not ignore exceptions! I am guessing many hours of unhappiness for programmers were caused by silent exceptions. The catch and finally block are the place where you will do your cleaning up. Make sure you wait until you have all the information to handle the exception properly. This can be tied to the throw early-catch late principle. We throw early so we don’t make operations that we have to revert later because of the exception and we catch late in order to have all the information to correctly handle the exception. And by the way, when you catch exceptions, only log when you resolve them, else a single exception event would cause clutter in your logs. Finally, for exception handling, I personally prefer to create an error handling service that I can use in different parts of my code and take appropriate actions in regards to logging, rethrowing, cleaning resources, etc. It centralizes my error handling behavior, avoids code repetition and help me keep more high level perspective of how errors are handled in the application.
So now that we have enough context, paradoxes, rules and their exceptions, we could summarise:
- Try to avoid exceptions. Use the language features and proper design in order to achieve it
- Use runtime exceptions, wrap methods with checked exceptions and turn them into runtime
- Try to use standard exceptions
- Make your exceptions specific and descriptive
- Catch the most specific exception first
- Do not catch on Exception
- But catch on Exception on the boundaries of your api. Have complete control on what comes out to the world
- Create a hierarchy of exceptions that matches the layers and functionalities of your application
- Throw exceptions at the proper abstraction level. Catch an exception and throw a higher level one as you move from layer to layer
- Pass the complete history of exceptions when rethrowing by providing the exception in the constructor of the new one
- Think of the try-catch-finally block as a transaction. Make sure you leave your program in a valid state when something goes wrong
- Catch exception when you can handle it
- Never have empty catch clauses
- Log an exception when you handle it
- Have a global exception handling service and have a strategy on how you handle errors
That was it! Go on and be exceptional!
