does instanceof work for subclassed exceptions?

问题

java.net.ConnectException extends java.net.SocketException

If I do the following, will it cater for both exceptions? ie if I catch a "parent" exception using instanceof, does that include any subclassed exceptions?

catch (Exception e)
{
   if (e instanceof java.net.SocketException)
   {
      System.out.println("You've caught a SocketException, OR a ConnectException");
   }
}

(and for the record, yes I know catching plain Exceptions is bad, just using it for this example ;) )

回答1:

Exceptions are regular classes, so instanceof works fine for them.

But you don't need such a thing. The following achieves the same result:

try {
    throw new ConnectException();
} catch (SocketException e) {
    System.out.println("You've caught a SocketException, OR a ConnectException");
}

回答2:

Yes, it will cater for both. Because ConnectionException IS A SocketException, it also is an instance of it.

回答3:

Bozho already has given the right answer. I don't know your particular usecase, but you'd rather catch different exceptions:

try {
  ...
}
catch (SocketException ex) {
  System.out.println("You've caught a SocketException, OR a ConnectException");
}
catch (Exception ex) {
  ...
}

回答4:

I know that it's now a good way but if you want to do custom action in a many places in code you can do something like this: public class ImageIOExecption extends Exception {

Exception ex;

public ImageIOExecption(Exception ex) {
    this.ex = ex;
    doCatch();
 }

private void doCatch() {
    System.err.println(ex.getClass());

    if (ex instanceof java.net.SocketTimeoutException) {
        System.out.println("You've caught a SocketTimeoutException, OR a ConnectException");
    }

    if (ex instanceof java.net.SocketException) {
        System.out.println("You've caught a SocketException, OR a ConnectException");
    }


}
}

 public BufferedImage getBufferedImage() {
        try {
            BufferedImage srcImage = ImageIO.read(is);
            is.close();
            return  srcImage;
        } catch (Exception ex) {
            new ImageIOExecption(ex);
        }
        return null;
    }

回答5:

Yes, that is how instanceof works. For exceptions it is more common to use something like this if you care about different exceptions. It works because the JVM will work down the list of catch statements in order and execute the first one that matches.

catch(ConnectException e)
{
    //got ConnectException  
}
catch(SocketException e)
{
    /got a SocketException
}
catch(Exception e)
{
   //got some other exception
}

Or below if you dont care about the difference between Connection and Socket Exception

catch(SocketException e)
{
   //got a SocketException or a subclass e.g. ConnectionException
}
catch(Exception e)
{
   //got another type of exception
}

来源：https://stackoverflow.com/questions/4154080/does-instanceof-work-for-subclassed-exceptions

标签

java

exception-handling