为什么 try-with-resources 不适用于字段变量？

Question

This is my very first question on SO and I'm confused there isn't a similar question yet!

So the question is:

Why doesn't try-with-resources work with field variables?

Or in other words: Why do I always need a local variable for that?

Here goes some example code:

public class FileWriteTest {

    public FileWriter file;

    public void workingDemo() {

        try(FileWriter file = new FileWriter(new File("someFilePath")) {
            // do something
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    public void notWorkingDemo() {

        file = null;

        try(file = new FileWriter(new File("someFilePath")) {
            // do something
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

May anyone explain me why there is this convention?

Answer 1

An instance variable may be changed at any point during the execution of the try-with-resources block. This would break its invariant and prevent the cleanup. Note that the local variable is implictly final, for the same reason.

BTW a better question is, why does Java force us to declare a local variable, even if we don't refer to it within the block. C#, for example, doesn't require this.

Update: with version 9, Java has stopped forcing us:

private final Some obj = new Some();

try (obj) { 
  // obj captured in a hidden local variable, resource closed in the end
}

Answer 2

I suspect the designers considered using a field a bad idea as this allow the object to escape the region of usage. ie it is only valid in the try block so you shouldn't be able to access it anywhere else.

Answer 3

Section 14.20.3 of the Java Language Specification states it will only work with local variables.

Why is this? My guess is checking for definite assignment and escapage (the local variable doesn't escape into the scope of another method). A field may be initialized anywhere in the class. My guess is that by validating it's a local variable, it's much simpler to analyse.

Answer 4

With Java 9, They added support for try with resources with variables.

    // Original try-with-resources statement from JDK 7 or 8
    try (Resource r1 = resource1;
         Resource r2 = resource2) {
        // Use of resource1 and resource 2 through r1 and r2.
    }

    // New and improved try-with-resources statement in JDK 9
    try (resource1;
         resource2) {
        // Use of resource1 and resource 2.
    }

https://blogs.oracle.com/darcy/more-concise-try-with-resources-statements-in-jdk-9

Answer 5

First off, I think it would be bad practice to have a variable/resource which is used at multiple places. If it is not opened in the try , then you cannot close it afterwards, if it is opened there, then you won't need a non-local variable. This leads to "second": If you have a resource open already, then you need to close it somewhere else explicitly, otherwise the autoclose wouldn't know if it is open or not.

So, IMHO it makes only sense to handle it the way it is specified in the specification.

Answer 6

It may have to do with consistency with the language specifications. Whenever a variable is declared between two brackets, it is encapsulated inside and cannot be accessed from the outside:

anything
{
int var;
}

// cannot access var from here!

Why shoul try { } be an exception ?