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From: Daniel Pitts <newsgroup.nospam@virtualinfinity.net>
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Newsgroups: comp.lang.java.programmer
Subject: Re: Thumbs up for suppressable exceptions in JDK 1.7
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NNTP-Posting-Date: Thu, 29 Sep 2011 18:25:59 UTC
Date: Thu, 29 Sep 2011 11:25:57 -0700
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On 9/29/11 9:05 AM, Daniele Futtorovic wrote:
> On 27/09/2011 21:43, Tom Anderson allegedly wrote:
>> On Tue, 27 Sep 2011, Roedy Green wrote:
>>
>>> On Tue, 27 Sep 2011 09:31:03 +0200, Jan Burse<janburse@fastmail.fm>
>>> wrote, quoted or indirectly quoted someone who said :
>>>
>>>> Was just playing around with suppressable exceptions in JDK 1.7. This
>>>> looks like a great improvement for bug hunting!
>>>>          ... 5 more
>>>
>>> I think you need some exposition on why this is a good thing.
>>
>> It avoids this common mistake:
>>
>> try {
>>      doSomethingWhichMightThrowAnException();
>> }
>> finally {
>>      doSomeCleanupWhichMightThrowAnException();
>> }
>>
>> In that code, if both methods throw an exception, you will only see the
>> second. The first exception - the one which actually caused the problem
>> - will be lost. It's as if the VM has a very short attention span, and
>> can only focus on whatever exception was most recently thrown.
>>
>> In Java 7, you can put the cleanup into the close() method of an
>> (Auto)Closeable, and use the try-with-resources form:
>>
>> class Thing implements AutoCloseable {
>>      public void close() throws AnException {
>>          doSomeCleanupWhichMightThrowAnException();
>>      }
>> }
>>
>> try (Thing t = new Thing()) {
>>      doSomethingWhichMightThrowAnException();
>> }
>>
>> There, the compiler will arrange things so that if close() does throw an
>> exception, it will be 'suppressed', and tagged on to the exception
>> coming from doSomethingWhichMightThrowAnException() as a suppressed
>> exception.
>>
>> tom
>>
>
> Indeed, but even more generally, we can from now on register
> "suppressed" exceptions ourselves, as Jan's code and the JSE7 Javadoc
> for java.lang.Throwable show. Great Thing IMHO. Closes a hole that's
> been lurking there for a very long time.
>
> Thanks Jan for bringing this to my attention.
>

This will definitely be a big help debugging issues. It may also help 
fill up log files faster ;-).  I can see it being extremely useful in a 
try/catch/retry scenario (such as a service which tries a few times to 
connect to a remote machine, and gets a different error each time).

I want to read up on it, but I wonder if it will help with this 
(unfortunate) situation:

try {
    buggyCodeThrowsNullPointerException();
} catch (Exception oopsThisWasIgnoredByLazyProgrammer) {
    throw new BusinessLayerSpecificException(
         "Unable to process request for no good reason.");
}

This suppresses a bug, but the original programmer wasn't thinking about 
potential bugs, only recoverable exceptions.  The exception is 
effectively suppressed.