Android JavaMail API - Want to catch UnknownHostException

Question

I use the Android JavaMail-API to send E-Mails. Everything works as intended, but right now I'm testing some things when I have a bad internet connection. I do this by disabling WIFI and then click on the Send E-mail Button.

In my SendMail-method I have the following:

try{
    if(javaMailAPI.send()){
        // Do something on success
    }
    else{
        // Do something on fail
    }
}
catch(UnknownHostException ex){
    // Do something when I have no Internet Connection
}
catch(Exception ex){
    // Do something else when I have an other Exception
}

So, when I test this above when I have my WIFI disconnected, this is what I expect:

UnknownHostException is catched because we have no Internet Connection, code inside that catch is used.

What is the result instead:

Exception is catched with the following error Stacktrace:

07-02 15:00:47.699: W/System.err(6680): javax.mail.MessagingException: Unknown SMTP host: smtp.gmail.com; 07-02 15:00:47.699: W/System.err(6680): nested exception is: 07-02 15:00:47.699: W/System.err(6680): java.net.UnknownHostException: Host is unresolved: smtp.gmail.com 07-02 15:00:47.699: W/System.err(6680): at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1389) 07-02 15:00:47.709: W/System.err(6680): at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:412) 07-02 15:00:47.709: W/System.err(6680): at javax.mail.Service.connect(Service.java:310) 07-02 15:00:47.709: W/System.err(6680): at javax.mail.Service.connect(Service.java:169) 07-02 15:00:47.709: W/System.err(6680): at javax.mail.Service.connect(Service.java:118) 07-02 15:00:47.709: W/System.err(6680): at javax.mail.Transport.send0(Transport.java:188) 07-02 15:00:47.709: W/System.err(6680): at javax.mail.Transport.send(Transport.java:118) 07-02 15:00:47.719: W/System.err(6680): at external_software.JavaMailAPI.send(JavaMailAPI.java:118) 07-02 15:00:47.719: W/System.err(6680): at controllers.EMailSender.SendEmails(EMailSender.java:58) 07-02 15:00:47.719: W/System.err(6680): at activities.ChecklistResultActivity$3.run(ChecklistResultActivity.java:300) 07-02 15:00:47.719: W/System.err(6680): at java.lang.Thread.run(Thread.java:841) 07-02 15:00:47.719: W/System.err(6680): Caused by: java.net.UnknownHostException: Host is unresolved: smtp.gmail.com 07-02 15:00:47.729: W/System.err(6680): at java.net.Socket.connect(Socket.java:826) 07-02 15:00:47.729: W/System.err(6680): at java.net.Socket.connect(Socket.java:786) 07-02 15:00:47.729: W/System.err(6680): at com.sun.mail.util.SocketFetcher.createSocket(SocketFetcher.java:233) 07-02 15:00:47.729: W/System.err(6680): at com.sun.mail.util.SocketFetcher.getSocket(SocketFetcher.java:163) 07-02 15:00:47.729: W/System.err(6680): at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1359) 07-02 15:00:47.729: W/System.err(6680): ... 10 more

And the code inside the other catch is used.

As you can see it catches the JavaMail-API's MessagingException, which is nesting the UnknownHostException.

So, how can I catch UnknownHostException / MessagingException that is nesting UnknownHostException? Is it possible to somehow retrieve the nested Exception?

Should I use this (source):

...
catch(Exception ex){
    if(ex.getCause() != null && ex.getCause().getCause() instanceof UnknownHostException){
        throw new UnknownHostException(ex.getMessage());
    }
    else{
        // Do something else when I have an other Exception
    }
}

PS: When I throw the UnknownHostException like above, will this go to the UnknownHostException above this catched Exception?

The code you've suggested will do what you're asking (not sure why you have `getCause()` twice though), and no the exception you throw won't be caught by the catch above. However, there may be other exceptions that could be caused by the internet connection being unavailable. — Dave Morrissey, Jul 02 '14 at 14:31
@DaveMorrissey About the "no the exception you throw won't be caught by the catch above." part, how do I go to the UnknownHostException catch then? Or should I use duplicated code (then I would make a private method for it).. So instead of `throw new UnknownHostException(ex.getMessage());` I use `// Do something when I have no Internet Connection` again. And about the double `getCause()`, I just found that in the link. I just looked through the JavaMail-API, and they have a method `ex.getNextException()`, so I might as well use that instead of `getCause()`. — Kevin Cruijssen, Jul 03 '14 at 07:02

score 1 · Accepted Answer · answered Jul 03 '14 at 07:58

I've changed this:

try{
    if(javaMailAPI.send()){
        // Do something on success
    }
    else{
        // Do something on fail
    }
}
catch(UnknownHostException ex){
    // Do something when I have no Internet Connection
}
catch(Exception ex){
    // Do something else when I have an other Exception
}

to this:

try{
    if(javaMailAPI.send()){
        // Do something on success
    }
    else{
        // Do something on fail
    }
}
catch(UnknownHostException ex){
    onNoConnection();
}
catch(Exception ex){
    // Check if the thrown Exception is a MessagingException with a nested UnknownHostException
    if(ex instanceof MessagingException){
        if(((MessagingException)ex).getNextException() instanceof UnknownHostException){
            onNoConnection();
        }
    }
    else{
        // Do something else when I have an other Exception
    }
}

private void onNoConnection(){
    // Do something when I have no Internet Connection
}

And it works as intended.

Android JavaMail API - Want to catch UnknownHostException

1 Answers1