I am currently working on the maintenance of a piece of code that is a little bit "Exception Happy." Basically, ever method or anything throws Exception. I'm going to work to take care of that, but, in the meantime, I am wondering what is the best way to handle individual exceptions within a smaller block of code, such as a method. Is it better to do something like this:
public void aMethod()
try {
//Lots of code in here. Many lines.
} catch(Exception e) {
// TODO - Handle All Exceptions, but fairly generically
}
}
Or something like this:
public void bMethod() {
try {
// One line of code.
} catch(Exception e) {
// TODO - Handle a specific Exception (may even involve rethrowing it with more information)
}
// More code.
try {
// Another line of code.
} catch(Exception e) {
// TODO - Handle another specific exception.
}
}
I realize this is a pretty basic question, but after looking at hundreds of methods with Exceptions coming out of every one, I'm starting to wonder how best to handle all of them and what a best practice may be here.
First off, you should only put code in try/catch blocks that is exception worthy. For instance, having an unexpected value is not necessarily an exception, but trying to read from a file that doesn't exist is.
To answer your main question, you should put the exceptionable code in the same try {} block and catch specific questions in order of granularity in multiple catch blocks after the main try.
//Regular code that doesn't need to be covered by a try/catch block
try {
//critical code only
} catch (NullPointerException npe) {
//Code
} catch (RandomException re) {
//code
} catch (Exception e) {
//code
}
The answer to your question is: it depends.
if the code in the try block is coherent where it makes no sense to proceed in the event of an error, the first approach is best
if the code is taking seperate steps that are relatively unrelated (parsing numbers for instance) and can be recovered without aborting the rest of the method the seconds appraoch makes sense
A general remark on the code you inherited; it sounds like exceptions are abused to pass state around, I would refactor so that exceptions are caught at the spot where they can be handled and introduce return values or attributes to handle the state.
your bMethod isn't very useful illustration. Try to reprase it. Anyway, you have two options:
catch exceptions and log them.
catch exceptions and throw new RuntimeException(ex) (rethrow a runtime exception, setting the original as a cause)
Also, you will need to differentiate between exceptions. If your system has many custom exceptions, they are probably defined for a reason, and specific behaviour is expected when one of them is thrown.
If the try/catch block isn't adding useful information, just allow the method to throw the exception and have it handled where the caller can sensibly do something with the exception. This could cut down the number of try/catch significantly (by an order of magnitude).
BTW a trick for rethrowing any exception is.
try {
// do something
} catch (Throwable t) {
Thread.currentThread().stop(t); // rethrow any exception.
}
In addition to the suggestions already made you may also want to consider extracting try/catch blocks from the "meat" of the function.
public void delete(Page page) {
try {
deletePageAndAllReferences(page);
}
catch (Exception e) {
logError(e);
}
}
private void deletePageAndAllReferences(Page page) throws Exception {
deletePage(page);
registry.deleteReference(page.name);
configKeys.deleteKey(page.name.makeKey());
}
private void logError(Exception e) {
logger.log(e.getMessage());
}
This lets you focus your attention on the function you are really interested in without the exception handling getting in your way.
Related
How do I make it so the code runs only if there was no exception thrown?
With finally code runs whether there was an exception or not.
try {
//do something
} catch (Exception e) {}
//do something only if nothing was thrown
Here are two ways:
try {
somethingThatMayThrowAnException();
somethingElseAfterwards();
} catch (...) {
...
}
Or if you want your second block of code to be outside the try block:
boolean success = false;
try {
somethingThatMayThrowAnException();
success = true;
} catch (...) {
...
}
if (success) {
somethingElseAfterwards();
}
You could also put the if statement in a finally block, but there is not enough information in your question to tell if that would be preferable or not.
try {
doSomething();
doSomething2();
} catch (Exception e) {
doSomething3();
}
In this example, doSomething2() will only be executed if no exception is thrown by doSomething().
If an exception is thrown by doSomething(), doSomething2(); will be skipped and execution will jump to doSomething3();
Also note, doSomething3() will be executed if there is an exception thrown by doSomething2();
If no exception is thrown, doSomething3(); will not be executed.
Just put the code in the try block. If an exception is thrown, it will skip to the catch block. If no exception is thrown, the code will just run.
try {
someMethodThatMayThrowException();
codeThatShouldBeRunIfNoExceptionThrown();
} catch (Exception e) {...}
An enhancement to the proposed
try {
somethingThatMayThrowAnException();
somethingElseAfterwards();
} catch (...) {
...
}
from the accepted answer. What you should do is:
void foo() {
try {
doStuff();
} catch (...) {
handleException();
}
}
The above feels like overkill to someone who hasn't been exposed to "clean code thinking".
But the point here: you do not want to mix different abstractions within one method. In other words: you don't have one try block, and more code following behind that within the same method.
You make sure that each and any method contains a straight forward path - you avoid anything that complicates your reading flow. As soon as you get used to writing and reading such kind of code you will find that it takes you dramatically less time to understand your code.
Exceptions for flow control is kind of a bad practice. If you insist, use a boolean variable.
boolean thrown = false;
try {
//do something
} catch (Exception e) {
thrown = true;
}
//do something only if nothing was thrown
if (!thrown) {
// do stuff
}
I was trying to solve this exact problem when I came across this question, and the answers here helped me think it through and realize, at least in my particular case, this was the wrong question I should have been asking.
I wanted to create this method because I was already doing it in main() and wanted to do it elsewhere for portability. So I copied a block of code that contained a try/catch block. However, it turns out, I don't want to copy the catch block at all, because if the creation of the Connection failed, I just wanted to fail completely.
It seems obvious now, but I never wanted to actually catch the exception in the first place. That code was only there because I copy and pasted it. So if you find yourself asking this question because you're in a try block and you might not generate a value, then consider if you just wanted to fail completely and not return anything that way this extra code is unnecessary.
We all write from time to time code like this:
try {
// ... some code.
} catch (SomeException e) {
// ... No action is required, just ignore.
}
Is there any standard code fragment like annotation to show we really intend to ignore exception? Something that shows to other team members and static analyzers we really need to skip this situation like InterruptedException after Thread.sleep()? Something like:
Exception.ignore(e);
Googled around but have not found something standard for such case.
This is especially relevant to tests that assure exceptions:
try {
action();
fail("We expected this to fail.");
} catch (ExpectedException e) {
ignore(e, "OK, so far so good.");
}
The only way to ignore an exception is to catch & swallow it, being very specific on the exception of course, you wouldn't want to catch Exception e, that would be a very bad idea.
try{
... //code
}
catch( VerySpecificException ignore){
Log(ignore);
}
Logging is obviously optional but a good practice.
in order to keep the code up with your exception handling or ignoring, it's nice to name the exception var as ignored:
try {
action();
} catch (ExpectedException ignored ) {}
Concerning your update referring to testing, if you use a testing framework, you can annotate the test in such a way that an exception is expected, e.g. using TestNG
#Test(expectedExceptions = ClassNotFoundException.class)
public void testSomething() {
// will succeed if a ClassNotFoundException is thrown
}
We need to have two functions. They do the same thing but one handles the exceptions and one throws the exception. Note: If to handle exception, the exception has to be handled in the commonFunc() below. It would not work to handle the exception in functionHandlesEx() because to handle the function it needs the variable only available in commonFunc(). In order to re-use code, we got the following. Do you see any problem with it (I can see one problem is: the function throws exception but actually no exception is thrown when isExceptionHandled is true----but Java allows this)? Any better way to do this? We are in Java 6. Thanks.
public void functionThrowsEx() throws Exception e
{
commonFunc(false); //false means does not handle exception
}
public void functionHandlesEx()
{
try
{
commonFunc(true); //true means handle exception
}
catch (Exception e)
{
// do nothing as exception handled
}
}
private void commonFunc(final boolean isExceptionHandled) throws Exception
{
try
{
....
}
catch (Exception e)
{
if (isExceptionHandled)
{
//handle the exception
}
else
{
throw e;
}
}
}
This seems like an overly complicated approach to handling the exception. I understand its toy code to show what you are doing, but I haven't encountered many reasons to create nested calls in order to handle exceptions.
Ideally, the code that can cause the exception should have the means to handle the exception in it, using try...catch, and maybe finally. This logical flow looks like it would be pretty complex in a non-trivial application, and would lead to increased complexity and difficulty in debugging.
You should get rid of the two functions, and avoid nesting try/catch blocks. If you must nest them, make it has straigtfoward as possible by keeping the logic in the same method.
From the code, I'm guessing you want to handle the exception differently based on the state of the program. This code is too complex, and will be difficult to understand for a non trivial app. A better way would be to keep functionHandlesEx and put the exception recovery logic in the handler function.
public void functionHandlesEx() {
try {
//logic here;
}
}
catch (Exception e) {
if(iWantToHandleException) {
//handle exception
}
//do nothing otherwise. Use the finally block for anything else they have in common.
}
}
This is much simpler to understand, and it shouldn't be too hard to refactor it into this form.
Hopefully I can explain this clearly. If I have a main method with lots of steps that can generate different exceptions, some fatal, some not, do I have to catch the "recoverable" ones separately? It seems like that would result in potentially a lot of try/catch blocks, like so:
public static void main (String[] args) {
try {
//...
for (int i=0;someArray.length;i++) {
try{
System.out.println("I = " + i);
doSometing(i);
} catch (RecoverableException e) {
//recover, continue, whatever
//log warning
//keep
}
}//end for loop
try {
doSomethingElse();
} catch (AnotherRecoverableException e) {
//not fatal, keep on chugging
}
//...
//do more stuff that can throw unrecoverable exceptions
} catch (UnrecoverableException e) {
System.out.println("I can't handle this, it's too much!");
e.printStackTrace();
}
}
Is there a better way to do this?
My patterns are to let exceptions propagate when they would represent programming bugs and handle them as closely to the "problem" when you can handle them.
The problem is that many potential Programming bugs throw checked exceptions which is really bad (Checked exceptions are kind of a failed experement, the newer languages have gotten rid of them).
So:
Handle checked and unchecked exceptions that you can deal with immediately.
If you don't know how to handle a checked exception rethrow it as an unchecked exception.
any "top level" loop like in main or a thread should be surrounded by a try/catch/log of "Exception" to ensure that any exception that bubbles up doesn't kill the thread (but log it loudly because it represents an unknown programming bug!)
Any critical loop that should continue regardless of exceptions should have a try/catch/log of "Exception" inside the loop construct so it will continue.
Catch exception, not throwable at this high level. Throwable includes unrecoverable exceptions that you probably never want to catch.
If you really must throw an exception you think you want the caller to catch (try to avoid this--It means you are using Exceptions as code flow!), throw an unchecked exception but document it and have the method "throw" the unchecked exception (it doesn't HAVE to be handled, but this acts as additional documentation/hint).
Just as a background for why I dislike checked exceptions so--it makes code like this happeen:
try {
Thread.sleep(1000);
} catch(InterruptedException e) {}
This can hide some INCREDABLY annoying to find program-flow related bugs. In this case it simply means you might have some thread-control issues, but in others it can mean your code-flow "Magically" vanishes mid-method with no indication whatsoever (because an exception was picked up by a higher level try/catch).
If you are using Java 7 then you can club exceptions in the catch block using pipe as separator. This will reduce your number of catch blocks. But you need to decide how you want to handle them by putting appropriate code in the catch block.
In Java 7, you can do this:
try
{
...
}
catch(Exception1 | Exception2 | Exception3 e)
{
//Handle
}
catch(Exception4 | Exception5 | Exception6 e)
{
//Handle differently
}
You can use. defaultUncaughtExceptionHandler, but it only triggers if the Thread doesn't have a uncaughtExceptionHandler set.
For java versions other than 7, a cleaner approach is to handle exceptions in called methods. This makes code readable as shown below:
doSomething(int i){
//code
try{
//code
}catch(Exception1 e1){
//handle
}
catch(Exception2 e2){
//handle
}
}
doSomethingElse(){
//code
try{
}catch(Exception1 e1){
//handle
}
catch(Exception2 e2){
//handle
}
}
public static void main (String[] args) {
for (int i=0;someArray.length;i++) {
doSometing(i);
}//end for loop
doSomethingElse();
}
I do not recommend using generic Exception to catch all errors in one block. This makes difficult to know specific exceptions and prevents specific handling of them.
I have a very basic question about best practice of using try/catch.
I have a simple function (DAO) like this:
public void addVehicle(Vehicle vehicle) {
em.getTransaction().begin();
em.persist(vehicle);
em.getTransaction().commit();
}
and using DAO function inside web service:
#WebMethod(operationName = "addVehicle")
public void addVehicle(Vehicle vehicle) {
try {
vehicleDAO.addVehicle(vehicle);
System.out.print("Vehicle added");
} catch (Exception e) {
e.printStackTrace();
}
}
OR is better using try/catch inside DAO function like this:
public void addVehicle(Vehicle vehicle) {
try {
em.getTransaction().begin();
em.persist(vehicle);
em.getTransaction().commit();
} catch (Exception e) {
e.printStackTrace();
}
}
There is no perfect rule for that.
Often code is clearer and less complex if exceptions are catched as early as needed, but as late as possible.
You should think who has to take an action when that Exception happens, this decides if you catch it inside the method (addVehicle) or if you throw it such that the caller has to catch it.
E.g:
public void addVehicle(Vehicle vehicle) throws SQLException{
em.getTransaction().begin();
em.persist(vehicle);
em.getTransaction().commit();
}
In this example the caller has to catch.
Further only in few situations you should catch Exception or RunTimeException, better
catch that specific Exception, like IOException instead of Exception.
Somewhere in your code you will need a "last line of defense" where it make sense to catch (Exception ex). This is needed to handle errors that should not happen.
When deciding where to handle a specific kind of exception, the best rule of thumb is to stop looking at the micro details of your code, take a step back to reason about your program's logic and consider these things:
Is the exception something that your program's current operation cannot recover from? If yes, it only makes sense to put the exception at the topmost level of that operation, to ensure that it doesn't continue.
If your program can work around that particular exception (perhaps by trying something else before giving up), take each layer of nested functions (starting from the highest) and each time ask yourself: If the exception occurs during the execution of some line of code in this function, would it make sense for this function to continue? As long as the answer is "yes", move to the deeper level. As soon the answer is "no", chances are this is the best place to put the handler for that exception.
Alternatively to the previous one, you could decide what would your program's alternate "plan of attack" be in case the exception is raised. Then, go to the line of code that would raise that exception and ask yourself: Does this function have enough context information to perform the workaround I have in mind? As long as the answer is "no", move to the caller function. As soon as the answer becomes "yes", consider putting your exception handler there.
That being said, you should only catch reasonably specialized exceptions and keep the catch(Exception ex) construct only as a last resort only at the top level and only after all the other possible catch blocks, reserving it only for kinds of exceptions you really couldn't predict at the time of writing. (I know you said this is not the point of the example, but since we're at it, I thought it should be mentioned to make this answer more complete.)
You should only catch those exceptions which you want to handle. You may include a topmost exception handler to turn any unhandled exceptions into something somewhat useful for the end user.
Instead of e.printStackTrace();, try return proper exception message.
find out more about exception handling here
Here is more discussion about exception handling.
AFAIK the best practice will be smth like that:
public void addVehicle(Vehicle vehicle) {
em.getTransaction().begin();
try {
em.persist(vehicle);
em.getTransaction().commit();
} catch (Exception e) {
if (em.getTransaction().isActive()) {
try {
em.getTransaction().rollback();
} catch (Exception e) {
// Log rollback failure or something
}
}
throw new RuntimeException(e);
}
}
Use both, the only reason is to use catch RuntimeException or even Throwable. Because this kind of exception is typically thrown by the underlying frameworks. An you should catch exactly this kind of exception if you want to make some actions, for example logging, print stack trace, etc., before you re-throw it again. If you don't do in such way you may loose the cause of exception.
#Transactional
public void addVehicle(Vehicle vehicle) {
try {
//do whatever with session
} catch (RuntimeException e) {
e.printStackTrace();
throw new Exception(e);
}
}