java compiler says this exception is never thrown in body of corresponding try statement - but it _is_ thrown

Question

I have the following code:

try {
    //jaw-ws service port operation
    port.login();
} catch (Exception e) {
    logger.error("Caught Exception in login(): " + e.getMessage());
}

When the above is run with an incorrect hostname, I get:

Caught Exception in login(): HTTP transport error: java.net.UnknownHostException: abc

That is correct and expected. I re-wrote the code to specifically catch UnknownHostException, as follows:

import java.net.UnknownHostException;
try {
    //jaw-ws service port operation
    port.login();
} catch (UnknownHostException uhe) {
    //do something specific to unknown host exception
} catch (Exception e) {
    logger.error(Caught Exception in login(): " + e.getMessage());
}

However, when I try to compile this I get:

[javac] foo.java: exception java.net.UnknownHostException is never thrown in body of corresponding try statement
[javac]         } catch (UnknownHostException uhe) {
[javac]                  ^

This is clearly false, since the exception is thrown, as I have caught it before. What am I missing here?

tia, rouble

Answer 1

It isn't throwing an UnknownHostException. It's just appearing in the message of the exception you actually caught. It's likely the underlying root cause of the exception you caught.

To determine the actual exception, you should print a bit more detail. E.g.

} catch (Exception e) {
    logger.error("Caught Exception in login(): " + e.getClass().getName() + ": " + e.getMessage());
}

or just using Throwable#toString() which already includes both exception type and message:

} catch (Exception e) {
    logger.error("Caught Exception in login(): " + e);
}

or just pass the Exception in as 2nd logger argument, if well configured its stacktrace will be printed:

} catch (Exception e) {
    logger.error("Caught Exception in login(): " + e.getMessage(), e);
}

---

Update as per your comments: your best bet is to update the catch as follows:

} catch (ClientTransportException e) {
    if (e.getCause() instanceof UnknownHostException) {
        // UHE.
    } else {
        // Other.
    }
}

You should absolutely not differentiate based on the message. It's receipe for portability trouble. The message is namely a sensitive subject for changes which may even be locale dependent!

Answer 2

There are various subtle (or in some cases unsubtle) ways to throw a checked exceptions in situations where they are not explicitly declared. Some are listed in this page.