使用条件表达式尝试资源

Question

Intellij IDEA 14 gives me the warning "PrintStream used without try-with-resources" when it sees this code:

public static void main(String[] args) throws IOException {
    try (PrintStream out = args.length > 0 ? new PrintStream(args[0]) : null) {
        if (out != null)
            out.println("Hello, world!");
    }
}

Using javap -c I can see that the resource is closed as expected at the end of the try block.

The warning is issued only when creating the resource in a conditional expression, as above; it's not issued when done in the typical way.

Is this an IDEA bug or does it have a valid point?

Answer 1

I think IDEA is just confused by it. That looks like a valid try-with-resources to me. JLS§14.20.3 shows the Resource part of the statement as being:

Resource:
{VariableModifier} UnannType VariableDeclaratorId = Expression

...and doesn't seem to place restrictions on the Expression . So I don't see why an expression potentially yielding null would make it somehow invalid, and the translated "simple" example from §14.20.3.1 :

{
    final {VariableModifierNoFinal} R Identifier = Expression;
    Throwable #primaryExc = null;

    try ResourceSpecification_tail
        Block
    catch (Throwable #t) {
        #primaryExc = #t;
        throw #t;
    } finally {
        if (Identifier != null) {
            if (#primaryExc != null) {
                try {
                    Identifier.close();
                } catch (Throwable #suppressedExc) {
                    #primaryExc.addSuppressed(#suppressedExc);
                }
            } else {
                Identifier.close();
            }
        }
    }
}

...would be just fine with it.

Answer 2

Your code is in principle no problem so the warning that IntelliJ gives can be ignored.

However, your code would be much clearer if you wrote it like this:

public static void main(String[] args) throws IOException {
    if (args.length > 0) {
        try (PrintStream out = new PrintStream(args[0])) {
            out.println("Hello, world!");
        }
    }
}