Do You Need to Close Scanner in Java Practical Guide
Understand when to close a Java Scanner, how it affects input streams, and best practices using try with resources for safe and leak free code.
Closing a Scanner in Java refers to releasing the underlying input source used by a java.util.Scanner by calling close(), which helps free resources and avoid leaks.
What closing a Scanner does in Java
In Java, a Scanner is a convenience wrapper around an input source. The close() method signals that the Scanner’s underlying resource should be released. For file-based sources, this typically closes the file handle; for console input, it may close System.in. The important implication is that once you close the underlying stream, other readers or subsequent Scanner instances may be unable to reuse it. In practice, you can tie a Scanner to a resource with a try-with-resources statement to ensure it is closed automatically when you are done.
import java.util.Scanner;
import java.io.File;
public class FileReadExample {
public static void main(String[] args) throws Exception {
try (Scanner sc = new Scanner(new File("data.txt"))) {
while (sc.hasNextLine()) {
System.out.println(sc.nextLine());
}
}
}
}This pattern ensures the file handle is released even if an exception occurs, which is critical for servers and batch jobs that open many files over time.
Common Questions
Do you need to close a Scanner in Java?
Yes for files and other resources to prevent leaks; however, closing a Scanner that wraps System.in can terminate further console input. Plan the lifecycle of your scanners and prefer try-with-resources when possible.
Yes, close scanners that read from files or streams to free resources. If you read from the console, be careful because closing System.in can block additional input.
Should I always use try-with-resources with Scanner?
In most cases, yes. Try-with-resources guarantees the Scanner is closed even if an exception occurs, reducing the risk of leaks. For console input using System.in you may opt for a single Scanner kept open for the program.
Yes. Try-with-resources is the safest pattern for scanners reading from files or streams.
What happens if I close System.in?
Closing System.in closes the standard input stream, which prevents further reads unless the stream is reopened in some environments. This can break subsequent user prompts in the same program.
Closing System.in will stop you from reading more input from the console in the same run.
Can I reuse a Scanner after closing it?
No. After a Scanner is closed, it is not reusable. If you need more input, create a new Scanner for the appropriate source.
Once you close it, you should create a new Scanner if you need more input.
Is not closing a Scanner a memory leak?
Not closing a Scanner that handles a file or network resource can lead to resource leaks. Closing it promptly helps the OS reclaim handles and improves stability.
If you leave scanners open, you risk leaking resources like file handles.
How do I manage multiple scanners on the same input source?
Avoid sharing the same input stream across scanners. Close one only after all are done, or better, use a single Scanner per input source to prevent conflicts.
Don’t stack scanners on one stream. Use one per source and close it when done.
Key Takeaways
- Use try-with-resources to manage scanners automatically
- Be careful with System.in as closing it blocks further input
- Reuse a single scanner per input source when possible
- Remember that close affects the underlying stream
- Create a new Scanner for new input sources rather than reusing after close