On Fri, Oct 25, 2013 at 9:01 AM, Jörg Schaible
<[email protected]> wrote:
> Hi Damjan,
>
> Damjan Jovanovic wrote:
>
>> On Thu, Oct 24, 2013 at 11:29 PM, Gary Gregory <[email protected]>
>> wrote:
>>> On Thu, Oct 24, 2013 at 4:31 PM, Jörg Schaible
>>> <[email protected]>wrote:
>>>
>>>> Hi Damjan,
>>>>
>>>> Damjan Jovanovic wrote:
>>>>
>>>> > As one of the perpetrators of the problem, I have now fixed it. The
>>>> > reasons I swallowed exceptions were simple:
>>>>
>>>> [snip]
>>>>
>>>> > * when an exception is thrown and close() then throws another
>>>> > exception, the close() exception is propagated and the original
>>>> > exception - which could reveal the cause of the problem - swallowed
>>>>
>>>> [snip]
>>>>
>>>> And this *is* a real problem. And it is exactly why the IOException of
>>>> close() are normally ignored. While I don't like
>>>> IOUtils.closeQietly(closeable), I use normally a method
>>>> "IO.closeLogged(closeable)" resp. "IO.closeLogged(closeable, logger)".
>>>>
>>>> If e.g. an image is corrupted and later on an additional exception
>>>> occurs closing any resource, you will simply *never* learn about the
>>>> corrupted image that caused the problem in first case. The original
>>>> exception must never be swallowed!
>>>>
>>>
>>> Nest'em!
>>>
>>> G
>>
>> What's the way forward though?
>>
>> 1. Catching both exceptions and nesting with setCause():
>>
>> InputStream is = null;
>> Exception ex = null;
>> try {
>> is = factoryMethodThatCanThrow();
>> is.methodThatCanThrow();
>> } catch (Exception exx) {
>> ex = exx;
>> } finally {
>> if (is != null) {
>> try {
>> is.close();
>> } catch (IOException closeEx) {
>> if (ex != null) {
>> closeEx.setCause(ex);
>> }
>> throw closeEx;
>> }
>> }
>> }
>>
>> which only gets worse, as each type of exception has to be separately
>> caught and rethrown...
>
>
> Right, you could have got even an OOME reading an image ... :-/
And setCause() isn't really appropriate since the exception thrown by
close() can be causally unrelated to the original exception.
>> 2. Swallowing close() exceptions when a succeeded flag at the end of
>> the try wasn't set:
>>
>> InputStream is = null;
>> boolean succeeded = false;
>> try {
>> is = factoryMethodThatCanThrow();
>> is.methodThatCanThrow();
>> succeeded = true;
>> } finally {
>> if (is != null) {
>> try {
>> is.close();
>> } catch (IOException closeEx) {
>> if (succeeded) {
>> throw closeEx;
>> }
>> }
>> }
>> }
>>
>> which now also requires making sure you don't "return;" before the end
>> of the try...
>
>
> IMHO this is the best approach so far. Maybe we can introduce a utility
> method somewhere to avoid the boilerplate copies:
>
> =============== %< ==================
> public static void closeSafely(boolean mayThrow, Closeable... closeables)
> throws IOException {
> IOException ioex = null;
> for(Closeable closeable : closeables) {
> if (closeable != null) {
> try {
> closeable.close();
> } catch (IOException ex) {
> ioex = ex; // keep first or last ?!?
> }
> }
> }
> if (mayThrow && ioex != null) {
> throw ioex;
> }
> }
> =============== %< ==================
I like that method, but I'd rather we always set succeeded correctly. Read on.
>> 3. Java 7 + try-with-resources, which will cripple portability to
>> Android...
>
> I would first write a unit test to see what Java 7 actually does with
> multiple open resources in the different error cases.
>
> Cheers,
> Jörg
Java 7 compiles:
try (InputStream is = factoryMethodThatCanThrow()) {
is.methodThatCanThrow();
}
into:
final InputStream is = factoryMethodThatCanThrow();
Throwable primaryException = null;
try {
is.methodThatCanThrow();
} catch (Throwable t) {
primaryException = t;
throw t;
} finally {
if (is != null) {
if (primaryException != null) {
try {
is.close();
} catch (Throwable t) {
primaryException.addSuppressed(t);
}
} else {
is.close();
}
}
}
When try-with-resources has a catch and/or finally, it compiles into 2
nested try blocks, this being the inner, and the one with
catch/finally being the outer. When multiple resources are being
managed by a try-with-resources block, each resource compiles into its
own try-finally block as above, nested in the previous resource's try
body. The innermost try body contains the body of the original
try-with-resources block. For examples see
http://stackoverflow.com/questions/7860137/what-is-the-java-7-try-with-resources-bytecode-equivalent-using-try-catch-finall
We would be able to adapt that for Java < 1.7 by swallowing the close
exception instead of calling addSuppressed() on the primary exception,
but the show stopper is catching and rethrowing the primary exception
(Throwable), which javac < 1.7 refuses to compile because it doesn't
do "Rethrowing Exceptions with More Inclusive Type Checking"
(http://docs.oracle.com/javase/7/docs/technotes/guides/language/catch-multiple.html).
But this would work and always sets succeeded correctly without catch/re-throw:
final InputStream is = factoryMethodThatCanThrow();
boolean succeeded = false;
try {
try {
is.methodThatCanThrow();
} finally {
}
succeeded = true;
} finally {
closeSafely(!succeeded, is);
}
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