Java 7 - Precise rethrow with a final Exception

Question

In previous versions of java, rethrowing an exception was treated as throwing the type of the catch parameter.

For example:

public static void test() throws Exception{     DateFormat df = new SimpleDateFormat("yyyyMMdd");     try {         df.parse("x20110731");         new FileReader("file.txt").read();     } catch (Exception e) {         System.out.println("Caught exception: " + e.getMessage());         throw e;     } } 

In Java 7, you can be more precise about the exception being thrown, if you declare the exception final:

//(doesn't compile in Java<7) public static void test2() throws ParseException, IOException{     DateFormat df = new SimpleDateFormat("yyyyMMdd");     try {         df.parse("x20110731");         new FileReader("file.txt").read();     } catch (final Exception e) {         System.out.println("Caught exception: " + e.getMessage());         throw e;     } } 

My question: The docs say that I need to declare the Exception final. But if I don't, the code above still compiles and works. Am I missing something?

References:

Project Coin: multi-catch and final rethrow
Add more flexible checking for rethrown exceptions

Answer 1

I believe I saw a tweet from Josh Bloch saying that the "final" restriction had been lifted late on. I'll see if I can find a post about it, but I suspect it's just that any "early" documentation you read is now inaccurate.

EDIT: I can't find the exact "it's changed" post, but the Java 7 documentation states shows an example with it not being final. It talks about exception variables being implicitly final when a catch block declares more than one type, but that's slightly separate.

EDIT: I've now found the source of my confusion, but it's an internal mailing list post :( Anyway, it doesn't have to be declared as final, but I believe the compiler treats it as implicitly final - just like in the multi-catch scenario.

Answer 2

The reason why both compile is that an exception in a uni catch clause that is not subsequently modified is implicitly final (JLS 14.20).

So for your example not to compile, you need to modify e in some way, for example:

public static void test2() throws ParseException, IOException {     DateFormat df = new SimpleDateFormat("yyyyMMdd");     try {         df.parse("x20110731");         new FileReader("file.txt").read();     } catch (Exception e) {         if (e instanceof ParseException) {             e = new ParseException("Better message", 0);         } else {             e = new IOException("Better message");         }         System.out.println("Caught exception: " + e.getMessage());         throw e; //does not compile any more     } }