UPDATE:

This feature is now available for some Run/Debug configurations. At the moment supported types for Java-based run configurations are: Application, Java Scratch, JUnit, JarApplication.

Use the Redirect input from option:

Original answer from 2017 with the workaround:

IntelliJ IDEA doesn't support it at the moment:

• IDEA-88717 No way to configure STDIN reading from a file at Run/Debug configurations

You can adjust the code so that it accepts the file name as a parameter and opens it for reading. Or you can create a wrapper class that will redefine System.in, then start your original Main class:

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class MainWrapper {
  public static void main(String[] args) throws IOException {
    FileInputStream is = new FileInputStream(new File("1.txt"));
    System.setIn(is);
    Main.main(args);
  }
}

Make sure to either specify the full path to the file or to change the working directory in IntelliJ IDEA Run/Debug configuration to the location of 1.txt.

Now you can run MainWrapper class instead of the Main class and it will work the same as running

java Main < 1.txt

If you need to test with different file names, replace new File("1.txt") with args[0] and pass the file name in the MainWrapper Run/Debug configuration Program arguments field.

You can actually do that now, starting from version 2018.3:
- Select "Run/Debug configurations"
- Set path to input file at "Redirect input from" field

The < 1.txt syntax mean that the command processor ensures that standard input (System.in in Java) provides the contents of the 1.txt file. The typical Java launcher do not support this.

You can emulate this by manually rewire System.in to read from the file using your own Java snippet to do this.