We Keep Coding

Java exception handling: using Exception (example: catch (Exception e)) vs specific exception (example: catch (IOException e))

Is there a reason to catch specific exceptions vs just using the Exception class if I don't have special cases/logging for the specific rule?
I current have
try {
//do IO stuff here and other things
} catch (IOException) {
system.out.println("Im here");
}
Since I don't have special logging required for the error, I don't see a reason why I should use IOException vs just using Exception which can capture al exceptions.
Thank you!

You do this because you might want to have a different behavior for an IOException (for example, asking the user to try again) versus an OutOfMemoryException, for which you might free up resources, versus some other kind of exception that means the program has to quit.
If you're handling all those cases the same way, then you can use a broader exception class, like Exception or Throwable.

Java know in finally that exception thrown without any variable [duplicate]

In Java, is there an elegant way to detect if an exception occurred prior to running the finally block? When dealing with "close()" statements, it's common to need exception handling within the finally block. Ideally, we'd want to maintain both exceptions and propagate them up (as both of them may contain useful information). The only way I can think of to do this is to have a variable outside the try-catch-finally scope to save a reference to a thrown exception. Then propagate the "saved" exception up with any that occur in the finally block.
Is there a more elegant way of doing this? Perhaps an API call that will reveal this?
Here's some rough code of what I'm talking about:
Throwable t = null;
try {
stream.write(buffer);
} catch(IOException e) {
t = e; //Need to save this exception for finally
throw e;
} finally {
try {
stream.close(); //may throw exception
} catch(IOException e) {
//Is there something better than saving the exception from the exception block?
if(t!=null) {
//propagate the read exception as the "cause"--not great, but you see what I mean.
throw new IOException("Could not close in finally block: " + e.getMessage(),t);
} else {
throw e; //just pass it up
}
}//end close
}
Obviously, there are a number of other similar kludges that might involve saving the exception as an member variable, returning it from a method, etc... but I'm looking for something a bit more elegant.
Maybe something like Thread.getPendingException() or something similar? For that matter, is there an elegant solution in other languages?
This question actually spawned from comments in another question that raised an interesting question.

Your idea about setting a variable outside the scope of the try/catch/finally is correct.
There cannot be more than one exception propagating at once.

Instead of using a Boolean flag, I would store a reference to the Exception object.
That way, you not only have a way to check whether an exception occurred (the object will be null if no exception occurred), but you'll also have access to the exception object itself in your finally block if an exception did occur. You just have to remember to set the error object in all your catch blocks (iff rethrowing the error).
I think this is a missing C# language feature that should be added. The finally block should support a reference to the base Exception class similar to how the catch block supports it, so that a reference to the propagating exception is available to the finally block. This would be an easy task for the compiler, saving us the work of manually creating a local Exception variable and remembering to manually set its value before re-throwing an error, as well as preventing us from making the mistake of setting the Exception variable when not re-throwing an error (remember, it's only the uncaught exceptions we want to make visible to the finally block).
finally (Exception main_exception)
{
try
{
//cleanup that may throw an error (absolutely unpredictably)
}
catch (Exception err)
{
//Instead of throwing another error,
//just add data to main exception mentioning that an error occurred in the finally block!
main_exception.Data.Add( "finally_error", err );
//main exception propagates from finally block normally, with additional data
}
}
As demonstrated above... the reason that I'd like the exception available in the finally block, is that if my finally block did catch an exception of its own, then instead of overwriting the main exception by throwing a new error (bad) or just ignoring the error (also bad), it could add the error as additional data to the original error.

You could always set a boolean flag in your catch(es). I don't know of any "slick" way to do it, but then I'm more of a .Net guy.

Use logging...
try {
stream.write(buffer);
} catch(IOException ex) {
if (LOG.isErrorEnabled()) { // You can use log level whatever you want
LOG.error("Something wrong: " + ex.getMessage(), ex);
}
throw ex;
} finally {
if (stream != null) {
try {
stream.close();
} catch (IOException ex) {
if (LOG.isWarnEnabled()) {
LOG.warn("Could not close in finally block", ex);
}
}
}
}

In vb.net, it's possible to use a "Catch...When" statement to grab an exception to a local variable without having to actually catch it. This has a number of advantages. Among them:
If nothing is going to 'ultimately' catch the exception, an unhandled exception trap will be fired from the spot of the original exception. Much nicer than having the debugger trap at the last rethrow, especially since information that might be needed for debugging won't yet have gone out of scope or been swept up by 'finally' statements.
Although a rethrow won't clear the stack trace the way "Throw Ex" would, it will still often jinx the stack trace. If the exception isn't caught, the stack trace will be clean.
Because this feature is unsupported in vb, it may be helpful to write a vb wrapper to implement the code in C (e.g. given a MethodInvoker and an Action(Of Exception), perform the MethodInvoker within a "Try" and the Action in a "Finally".
One interesting quirk: it's possible for the Catch-When to see an exception which will end up getting overwritten by a Finally-clause exception. In some cases, this may be a good thing; in other cases it may be confusing. In any event, it's something to be aware of.

Catch separate exceptions or use instanceof - Java 6

Assume that this piece of code is in 20 places and always the same
try {
// do something
} catch (FirstException e) {
// log it
} catch (SecondException e) {
// log it
}
Wouldn't be better to use something like this or instanceof is not good solution?
try {
// do something
} catch(Exception e) {
logException(e);
}
void logException(Exception e) {
if (e instanceof FirstException) {
// log it
} else if (e instanceof SecondException) {
// log it differently
} else {
// do something with other exception
}
}
The only thing that I really hate about the solution is catching Exception which is definitelly not the best way... Is there any better way?

In Java 7, use catch (FirstException1 | SecondException | ...)
There may be nothing wrong with catch (Exception e)—you do want to log all exceptions, don't you? I would in fact advise catch (Throwable t) because OutOfMemoryError s and StackOverflowErrors also want to be logged.
An advice from many years of experience with logging exceptions is to log them all the same way. The exception message is enough as human-readable text, and what the developer really needs for debugging is the stack trace.
Just be careful about one thing: never catch exceptions too early: catch them at a single place for the whole application, the so-called exception barrier—it is at the level where you enter and exit a unit of work.
If checked exceptions are giving you trouble at the lower level, wrap them into RuntimeException:
try {
...
}
catch (RuntimeException e) {throw e;}
catch (Exception e) {throw new RuntimeException(e);}
Only if you know precisely and in advance that there is an exception which has business-level meaning to your application, and will not abort the current unit of work, but redirect its flow, is it appropriate to catch that exception at a lower level. In practice such exceptions are rare compared to the totality of all possible exceptions thrown by the application code.

The first approach is definitely better. Generally it is a bad practice to catch Exception because in this case you catch RuntimeExceptions too.

Former is clean and great solution if you only have to log the exceptions.
Else first approach is better.

In the book "Refactoring to Patterns" one of the common refactorings is "replace instanceof with polymorphism" - in other words whenever you use instanceof, consider if polymporphism would in fact work better. . .
Having said that, for this particular question the Spring philosophy of replacing checked exceptions with runtime exceptions springs to mind (excuse the pun).
The idea is that checked exceptions can be overused - is the exception something that could be recovered from? If yes, ok. . . if not, just let it propagate up the chain. You can do this by:
Re-throwing it. . . (but better still)
Wrap it in a RuntimeException
Create a logging Aspect:
Another thing to consider is that if you do need to log these exceptions at precisely the point they occur rather than having them propagate up the chain, and that they occur in 20 different places, then they're a cross-cutting concern. . . You could have the regular method just rethrow the exception, and then write an aspect to catch and log them. . . . again using Spring makes this easy.

Is it a bad practice to catch Throwable?

Is it a bad practice to catch Throwable?
For example something like this:
try {
// Some code
} catch(Throwable e) {
// handle the exception
}
Is this a bad practice or we should be as specific as possible?

You need to be as specific as possible. Otherwise unforeseen bugs might creep away this way.
Besides, Throwable covers Error as well and that's usually no point of return. You don't want to catch/handle that, you want your program to die immediately so that you can fix it properly.

This is a bad idea. In fact, even catching Exception is usually a bad idea. Let's consider an example:
try {
inputNumber = NumberFormat.getInstance().formatNumber( getUserInput() );
} catch(Throwable e) {
inputNumber = 10; //Default, user did not enter valid number
}
Now, let's say that getUserInput() blocks for a while, and another thread stops your thread in the worst possible way ( it calls thread.stop() ). Your catch block will catch a ThreadDeath Error. This is super bad. The behavior of your code after catching that Exception is largely undefined.
A similar problem occurs with catching Exception. Maybe getUserInput() failed because of an InterruptException, or a permission denied exception while trying to log the results, or all sorts of other failures. You have no idea what went wrong, as because of that, you also have no idea how to fix the problem.
You have three better options:
1 -- Catch exactly the Exception(s) you know how to handle:
try {
inputNumber = NumberFormat.getInstance().formatNumber( getUserInput() );
} catch(ParseException e) {
inputNumber = 10; //Default, user did not enter valid number
}
2 -- Rethrow any exception you run into and don't know how to handle:
try {
doSomethingMysterious();
} catch(Exception e) {
log.error("Oh man, something bad and mysterious happened",e);
throw e;
}
3 -- Use a finally block so you don't have to remember to rethrow:
Resources r = null;
try {
r = allocateSomeResources();
doSomething(r);
} finally {
if(r!=null) cleanUpResources(r);
}

Also be aware that when you catch Throwable, you can also catch InterruptedException which requires a special treatment. See Dealing with InterruptedException for more details.
If you only want to catch unchecked exceptions, you might also consider this pattern
try {
...
} catch (RuntimeException exception) {
//do something
} catch (Error error) {
//do something
}
This way, when you modify your code and add a method call that can throw a checked exception, the compiler will remind you of that and then you can decide what to do for this case.

straight from the javadoc of the Error class (which recommends not to catch these):
* An <code>Error</code> is a subclass of <code>Throwable</code>
* that indicates serious problems that a reasonable application
* should not try to catch. Most such errors are abnormal conditions.
* The <code>ThreadDeath</code> error, though a "normal" condition,
* is also a subclass of <code>Error</code> because most applications
* should not try to catch it.
* A method is not required to declare in its <code>throws</code>
* clause any subclasses of <code>Error</code> that might be thrown
* during the execution of the method but not caught, since these
* errors are abnormal conditions that should never occur.
*
* #author Frank Yellin
* #version %I%, %G%
* #see java.lang.ThreadDeath
* #since JDK1.0

It's not a bad practice if you absolutely cannot have an exception bubble out of a method.
It's a bad practice if you really can't handle the exception. Better to add "throws" to the method signature than just catch and re-throw or, worse, wrap it in a RuntimeException and re-throw.

Catching Throwable is sometimes necessary if you are using libraries that throw Errors over-enthusiastically, otherwise your library may kill your application.
However, it would be best under these circumstances to specify only the specific errors thrown by the library, rather than all Throwables.

The question is a bit vague; are you asking "is it OK to catch Throwable", or "is it OK to catch a Throwable and not do anything"? Many people here answered the latter, but that's a side issue; 99% of the time you should not "consume" or discard the exception, whether you are catching Throwable or IOException or whatever.
If you propagate the exception, the answer (like the answer to so many questions) is "it depends". It depends on what you're doing with the exception—why you're catching it.
A good example of why you would want to catch Throwable is to provide some sort of cleanup if there is any error. For example in JDBC, if an error occurs during a transaction, you would want to roll back the transaction:
try {
…
} catch(final Throwable throwable) {
connection.rollback();
throw throwable;
}
Note that the exception is not discarded, but propagated.
But as a general policy, catching Throwable because you don't have a reason and are too lazy to see which specific exceptions are being thrown is poor form and a bad idea.

Throwable is the base class for all classes than can be thrown (not only exceptions). There is little you can do if you catch an OutOfMemoryError or KernelError (see When to catch java.lang.Error?)
catching Exceptions should be enough.

it depends on your logic or to be more specific to your options / possibilities. If there is any specific exception that you can possibly react on in a meaningful way, you could catch it first and do so.
If there isn't and you're sure you will do the same thing for all exceptions and errors (for example exit with an error-message), than it is not problem to catch the throwable.
Usually the first case holds and you wouldn't catch the throwable. But there still are plenty of cases where catching it works fine.

Although it is described as a very bad practice, you may sometimes find rare cases that it not only useful but also mandatory. Here are two examples.
In a web application where you must show a meaning full error page to user.
This code make sure this happens as it is a big try/catch around all your request handelers ( servlets, struts actions, or any controller ....)
try{
//run the code which handles user request.
}catch(Throwable ex){
LOG.error("Exception was thrown: {}", ex);
//redirect request to a error page.
}
}
As another example, consider you have a service class which serves fund transfer business. This method returns a TransferReceipt if transfer is done or NULL if it couldn't.
String FoundtransferService.doTransfer( fundtransferVO);
Now imaging you get a List of fund transfers from user and you must use above service to do them all.
for(FundTransferVO fundTransferVO : fundTransferVOList){
FoundtransferService.doTransfer( foundtransferVO);
}
But what will happen if any exception happens? You should not stop, as one transfer may have been success and one may not, you should keep go on through all user List, and show the result to each transfer. So you end up with this code.
for(FundTransferVO fundTransferVO : fundTransferVOList){
FoundtransferService.doTransfer( foundtransferVO);
}catch(Throwable ex){
LOG.error("The transfer for {} failed due the error {}", foundtransferVO, ex);
}
}
You can browse lots of open source projects to see that the throwable is really cached and handled. For example here is a search of tomcat,struts2 and primefaces:
https://github.com/apache/tomcat/search?utf8=%E2%9C%93&q=catch%28Throwable
https://github.com/apache/struts/search?utf8=%E2%9C%93&q=catch%28Throwable
https://github.com/primefaces/primefaces/search?utf8=%E2%9C%93&q=catch%28Throwable

Generally speaking you want to avoid catching Errors but I can think of (at least) two specific cases where it's appropriate to do so:
You want to shut down the application in response to errors, especially AssertionError which is otherwise harmless.
Are you implementing a thread-pooling mechanism similar to ExecutorService.submit() that requires you to forward exceptions back to the user so they can handle it.

Throwable is the superclass of all the errors and excetions.
If you use Throwable in a catch clause, it will not only catch all exceptions, it will also catch all errors. Errors are thrown by the JVM to indicate serious problems that are not intended to be handled by an application. Typical examples for that are the OutOfMemoryError or the StackOverflowError. Both are caused by situations that are outside of the control of the application and can’t be handled. So you shouldn't catch Throwables unless your are pretty confident that it will only be an exception reside inside Throwable.

If we use throwable, then it covers Error as well and that's it.
Example.
public class ExceptionTest {
/**
* #param args
*/
public static void m1() {
int i = 10;
int j = 0;
try {
int k = i / j;
System.out.println(k);
} catch (Throwable th) {
th.printStackTrace();
}
}
public static void main(String[] args) {
m1();
}
}
Output:
java.lang.ArithmeticException: / by zero
at com.infy.test.ExceptionTest.m1(ExceptionTest.java:12)
at com.infy.test.ExceptionTest.main(ExceptionTest.java:25)

A more differentiated answer would be: it depends.
The difference between an Exception and an Error is that an Exception is a state that has to be expected, while an Error is an unexpected state, which is usually fatal. Errors usually cannot be recovered from and require resetting major parts of the program or even the whole JVM.
Catching Exceptions is something you should always do to handle states that are likely to happen, which is why it is enforced by the JVM. I.E. opening a file can cause a FileNotFoundException, calling a web resource can result in a TimeoutException, and so on. Your code needs to be prepared to handle those situations as they can commonly occur. How you handle those is up to you, there is no need to recover from everything, but your application should not boot back to desktop just because a web-server took a little longer to answer.
Catching Errors is something you should do only if it is really necessary. Generally you cannot recover from Errors and should not try to, unless you have a good reason to. Reasons to catch Errors are to close critical resources that would otherwise be left open, or if you i.E. have a server that runs plugins, which can then stop or restart the plugin that caused the error. Other reasons are to log additional information that might help to debug that error later, in which case you of course should rethrow it to make sure the application terminates properly.
Rule of thumb: Unless you have an important reason to catch Errors, don't.
Therefore use catch (Throwable t) only in such really important situation, otherwise stick to catch (Exception e)

How do I add state information without writing repetitive code when catching multiple Java exceptions?

I've read a few other posts like this one about avoiding repetition in Java catch blocks. Apparently, what I really want is "multi-catch", but seeing as Java 7 isn't here yet, is there a good pattern to let me add state to my exceptions, then re-throw them, without catching the kitchen sink?
Specifically, I have some code that makes library calls, which can throw exceptions, but don't provide nearly enough context for successful debuging. I find myself having a problem, then going in, wrapping the library call in a try/catch, catching the specific exception, then adding extra state information in the catch block and re-throwing the caught exception. I then re-iterate through that loop over and over, each time finding a new error condition to log. I wind up with
try {
make_library_call();
}
catch (SomeException e){
throw new SomeException ("Needed local state information is " + importantInfo, e);
}
catch (SomeOtherException e){
throw new SomeOtherException ("Needed local state information is " + importantInfo, e);
}
catch (YetAnotherException e){
throw new YetAnotherException ("Needed local state information is " + importantInfo, e);
}
etc etc. I think what I'd really like to see is more along the lines of
try {
make_lib_call();
}
catch(Exception e){
e.AddSomeInformationSomehow("Needed Info" + importantInfo);
throw e;
}
assuming that in that case e would keep its actual runtime type when re-thrown -- in this case, even catching unchecked would probably be OK since I'd be re-throwing them, and they'd benefit from carrying the extra state info as well. I guess an alternate might be something like
try{
make_lib_call();
}
finally {
if (/*an exception happened*/)
logger.debug("Some state info: " + importantInfo);
}
but I don't know how to write the conditional. Is that crazy / wrong?

You're probably expecting this, but no, there isn't a good (and by good I mean clean) way to do this. You're probably expecting it because you subconsciously knew that if there was a good pattern for doing this, it probably wouldn't have been worth adding to Java 7.
It's actually the final rethrow feature that you'd find really handy in this case.

For the alternative:
boolean isOk = false;
try{
make_lib_call();
isOk = true;
}
finally {
if (!isOk)
logger.debug("Some state info: " + importantInfo);
}