Is there a way to mark (e.g. annotate) a Java method as never returning?

Question

I just started using the new Eclipse 4.2 (Juno) Null Analysis.

On code like this:

x = foo();
if (x == null)
    fail("x is null");
return x.bar();

I'm getting warnings that x may be null. But it can't be, because fail always throws and therefore never returns. (With better inter-procedural analysis it could presumably determine this automatically, but it currently doesn't seem to.)

Obviously, there are ways to rewrite the code to get around the warning, but what I'd like is a way to indicate (e.g. an annotation) that fail never returns.

I also tried to suppress the warning with @SuppressWarnings("null") but that didn't work.

One way to get rid of the warning is to add: assert x != null; (assuming you have turned on the setting to include asserts in null analysis)

In GCC C++ I can do: void fn __attribute__ ((noreturn))

I'm not aware of this situation. Do you have a need for this, or is it more of an academic inquiry? — BlackVegetable, Jun 28 '12 at 17:10
It's not critical since I can easily work around it, but it is from actual production code. — Andrew McKinlay, Jun 28 '12 at 17:17

score 15 · Accepted Answer · answered Jun 28 '12 at 17:12

15

One semi-traditional way to do this is as follows:

public RuntimeException fail(String message) {
  throw new RuntimeException(message);
}

so you can write throw fail("x is null"). Of course, fail will always end up doing the throwing, not the throw, but it's enough to reassure the compiler that that line will always throw.

answered Jun 28 '12 at 17:12

Louis Wasserman

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Interesting workaround. Does anyone recommend this pattern to be used? – BlackVegetable Jun 28 '12 at 17:13
6

For reference, it's the pattern used by Guava in the `Throwables` utility class. – Louis Wasserman Jun 28 '12 at 17:15
Why is this better than `public RuntimeException fail(String message) { return new RuntimeException(message); }`? I am just asking a question, not expressing a preference. – emory Jun 28 '12 at 17:42
2

Mostly that you can use it in either way; if you don't need to prove anything to the compiler, then you can just call it without the `throw`. – Louis Wasserman Jun 28 '12 at 17:52
1

I wondered that at first, but then I realized it was "safer" in case you called fail without throwing. Especially since that's how it used to work in my code. – Andrew McKinlay Jun 28 '12 at 18:03
Which Java version supports this? – Vladimir Feb 24 '23 at 16:33
Every single one? – Louis Wasserman Feb 24 '23 at 16:38

score 0 · Answer 2 · answered Jun 28 '12 at 17:12

0

have you tried org.eclipse.jdt.annotation.Nullable annotation?

http://git.eclipse.org/c/jdt/eclipse.jdt.core.git/tree/org.eclipse.jdt.annotation/src/org/eclipse/jdt/annotation/Nullable.java

answered Jun 28 '12 at 17:12

Gergely Szilagyi

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How does this address the user's problem? He's already doing an explicit check for null. – Louis Wasserman Jun 28 '12 at 17:12

Is there a way to mark (e.g. annotate) a Java method as never returning?

2 Answers2

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