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

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. i.e. it is only valid in the try block so you shouldn't be able to access it anywhere else.