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

The exception parameter in the catch is implicitly final. You don't need to explicitly state it as final. You can, obviously, but you don't have to. — tapavko, May 21 '22 at 18:45

Jon Skeet · Accepted Answer · 2011-07-31T12:01:58.300

25

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.

edited Jul 31 '11 at 12:01

answered Jul 31 '11 at 11:51

Jon Skeet

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6

Hey @jon-skeet , I know it's a +1y old post, but one thing on that last edit: in this scenario, if `e` is not declared `final`, the compiler **won't** _implicitly_ treat it as `final` (as in multi catch). However, if the `e` reference is changed, then `Exception` has to be included in the `throws` clause. – betomontejo Sep 02 '12 at 20:10
@betomontejo BullsEye. – Kumar Abhinav Sep 22 '14 at 15:16

score 4 · Answer 2 · answered Aug 21 '12 at 10:04

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
    }
}

Related: http://stackoverflow.com/questions/12051791/is-there-ever-a-reason-to-not-use-the-final-keyword-when-catching-an-exception/12051907#12051907 — assylias, Aug 21 '12 at 10:22

score 0 · Answer 3 · answered Jul 31 '11 at 11:50

0

Without the final it is still valid java. You just lose the benefit of it being 'precise'.

answered Jul 31 '11 at 11:50

Brett Walker

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1
18
36

1

I think you are missing the thrown exceptions bit of test2() - it *IS* more precise, so a "throw Exception" would normally fail. – Maarten Bodewes Jul 31 '11 at 22:38

Java 7 - Precise rethrow with a final Exception

3 Answers3

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