try-with-resources on existing variables

Definition

Before Java 9 a resource had to be declared inside the try (...) parentheses. Now you can pass a variable that already exists, as long as it is final or effectively final, and Java still closes it for you.

Example code

Using an existing variable as the resource

class Connection implements AutoCloseable {
    Connection() { System.out.println("opened"); }
    void query() { System.out.println("query"); }
    @Override public void close() { System.out.println("closed"); }
}

class Demo {
    public static void main(String[] args) {
        Connection connection = new Connection();      // declared earlier

        try (connection) {                              // Java 9: no need to declare it again
            connection.query();
        }
    }
}

// Output:
// opened
// query
// closed