Scanner object has a class attribute instead of in code initialisation

Today, I have encounter a problem with Scanner object while programming a server/client application. I have solved the problem but I don't understand why my solution solved the problem.

At first, in my application I had some memory leak warning when compiling the project (JAVA 8, eclipse)
because I was using Scanner to ask input from the user but didn't close it. When I try to close the scanner my other class where throwing exception like java.util.NoSuchElementException. I learned that I had a similar problem to : https://stackoverflow.com/questions/29053580/cannot-use-multiple-scanner-objects-in-java but between classes. When I was closing a scanner in a class the other class (thread) scanner couldn't execute. (java.util.NoSuchElementException)

The solution I found to solve this problem was to put a Scanner function as a class function member:

public class ClientThread extends Thread {

    private Scanner scanner;

   /*some code*/

}

When doing so I lost my memory leak warning.

Why having the scanner as a class member solved the warning ? Is it because of the class destructor ?

Usually, when you create a Scanner, you need to close it once you no longer need it.

The Scanner class has several, overloaded constructors where each one takes a different type of argument. There is a constructor that takes a File argument. There is a constructor that takes a Path argument. And there is a constructor that takes a InputStream argument. In order for your Java program to accept input from the user, via the computer keyboard, you use the constructor that takes an InputStream like so:

Scanner scanner = new Scanner(System.in);

This is because the static member in of class java.lang.System has type InputStream. You should think of System.in as encapsulating the keyboard on your computer. If you close System.in, it's like closing your keyboard and that is something you shouldn't do.

Now the Eclipse IDE is not smart enough to realize that your code is creating a Scanner that essentially encapsulates the computer keyboard (and therefore should not be closed). Eclipse only knows that any Scanner that you create should eventually be closed and since your code does not close the Scanner you created, Eclipse displays a warning that you haven't closed a Scanner that you created. In the displayed warning you are offered three suggestions on how to get rid of the warning. The first two refer to an annotation that you can add, namely

@SuppressWarnings("resource")

The third suggestion requires you to change the Eclipse configuration. You do this by selecting the Preferences item of the Window menu that appears at the top of the Eclipse IDE. In the Preferenes window, select <tt>Java > Compiler > Errors/Warnings</tt> and change the Resource leak setting to <tt>Ignore</tt> as shown in the following screen capture:

Note however, that this configuration setting applies to all resources, not just Scanner, so if you open a regular file in your java code and don't close it, Eclipse will not warn you.

Personally, I simply ignore the warning since I know that I should not close a Scanner created with System.in or I add an annotation.

Although Java usually auto-release the memory when you leave the scope some classes require special treatment.

Generally these are classes who has something to do with files and connections (to db or just sockets).
The latter is required because the underlying OS requires those resource to be explicitly released (in general).

In the Java 8 there was introduced Autoclosable and it solves some of the problems but ultimately you need to be careful.

Here is an example for Scanner with autoclosable:

File file = new File("filename here");
try (Scanner in = new Scanner(file)) {
    // do something here, note you dont need close as it is autoclosable
}