Try-with-resources and return statements in java

Question

I\'m wondering if putting a return statement inside a try-with-resources block prevents the resource to be automatically closed.

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Answer 1

The AutoCloseable interface can make the execution order of code confusing at first glance. Lets run through this with an example:

public class Main {

    // An expensive resource which requires opening / closing
    private static class Resource implements AutoCloseable {

        public Resource() {
            System.out.println("open");
        }
        
        @Override public void close() throws Exception {
            System.out.println("close");
        }
    }
    
    // find me a number!
    private static int findNumber() {
        // open the resource
        try(Resource resource = new Resource()) {
            // do some business logic (usually involving the resource) and return answer
            return 2 + 2;
        } catch(Exception e) {
            // resource encountered a problem
            throw new IllegalStateException(e);
        }
    }
    
    public static void main(String[] args) {
        System.out.println(findNumber());
    }
}

The above code attempts to open some Resource and conduct some business logic using the resource (just some arithmetic in this case). Running the code will print:

open
close
4

Therefore the Resource is closed before exiting the try-with-resource block. To make it clear what exactly is going on, lets reorganise the findNumber() method.

    private static int findNumber() {
        // open the resource
        int number;
        try(Resource resource = new Resource()) {
            // do some business logic and return answer
            number = 2 + 2;
        } catch(Exception e) {
            // resource encountered a problem
            throw new IllegalStateException(e);
        }
        return number;
    }

Conceptually, this is what happens under the hood when return is placed inside a try-with-resource block. The return operation is moved to after the try-with-resource block to allow the AutoCloseable object to close before returning.

Therefore we can conclude that a return operation inside a try-with-resource block is just syntactic sugar and you need not worry about returning before an AutoCloseable has closed.