IntelliJ IDEA: Run java with args from external file

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:

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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.