contents ... Exception Care ... Understand Exceptions

As written in the introduction, deep knowledge of how exceptions work is critical to developing in Java.  At the simplest of levels, here is how exceptions work:

1) An exceptional condition is detected in a block of code.  The code throws an exception:


try
{
    :
    File file = new File(filename);
    if (!file.exists())
    {
        throw new IOException(“File does not exist” + filename);
    }
    parse(file);
    :
}
catch (IOException e)
{
    logger.error(e);
 }

2) Normal program flow is modified – execution does not continue at the same location.

3) If a try {..} catch(..) block is around the code (or around the call to the function) execution will continue where the exception is caught.


try
{
    :
    File file = new File(filename);
    if (!file.exists())
    {
        throw new IOException(“File does not exist” + filename);
    }
    parse(file);
    :
}
catch (IOException e)
{
    logger.error(e);
 }

4) Normally the catching block of code handles the exception elegantly.

try
{
    :
}
catch (IOException e)
{
    logger.error(e);
    // There is a chance we parsed the XML OK last time so restore it
    xmlRoot = lastXmlRoot;
 }

Having caught the exception, the block of code can also decide not to deal with the exception at all, and rethrow it to be caught elsewhere.

    :
}
catch (IOException e)
{
    logger.error(e);
    // There is a chance we previously parsed the XML so restore it
    xmlRoot = lastXmlRoot;
 }
catch (XmlParseException e)
{
    // let the framework deal with it
    throw e;
}

 
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