Android Don’t Catch Generic Exception

Bjarne

Sometimes it is tempting to be lazy when catching exceptions and do something like this:

try {
    someComplicatedIOFunction();        // may throw IOException 
    someComplicatedParsingFunction();   // may throw ParsingException 
    someComplicatedSecurityFunction();  // may throw SecurityException 
    // phew, made it all the way 
} catch (Exception e) {                 // I'll just catch all exceptions 
    handleError();                      // with one generic handler!
}

You should not do this. In almost all cases it is inappropriate to catch generic Exception or Throwable, preferably not Throwable, because it includes Error exceptions as well. It is very dangerous. It means that Exceptions you never expected (including RuntimeExceptions like ClassCastException) end up getting caught in application-level error handling. It obscures the failure handling properties of your code. It means if someone adds a new type of Exception in the code you’re calling, the compiler won’t help you realize you need to handle that error differently. And in most cases you shouldn’t be handling different types of exception the same way, anyway.

There are rare exceptions to this rule: certain test code and top-level code where you want to catch all kinds of errors (to prevent them from showing up in a UI, or to keep a batch job running). In that case you may catch generic Exception (or Throwable) and handle the error appropriately. You should think very carefully before doing this, though, and put in comments explaining why it is safe in this place.

Alternatives to catching generic Exception:

    • Catch each exception separately as separate catch blocks after a single try. This can be awkward but is still preferable to catching all Exceptions. Beware repeating too much code in the catch blocks.

 

  • Refactor your code to have more fine-grained error handling, with multiple try blocks. Split up the IO from the parsing, handle errors separately in each case.
  • Rethrow the exception. Many times you don’t need to catch the exception at this level anyway, just let the method throw it.

Remember: exceptions are your friend! When the compiler complains you’re not catching an exception, don’t scowl. Smile: the compiler just made it easier for you to catch runtime problems in your code.

Android source: http://source.android.com/source/code-style.html