How to grant access to a file with security manager enabled?

Question:

How to grant access to a file with security manager enabled?

Code:

import java.security.*;
import java.io.*;
 
public class SecurityManagerTest {
 
  public static void main(String [] args) {
 
    SecurityManager sm = System.getSecurityManager();
    if(sm != null){
      try {
        sm.checkPermission(new FilePermission("/tmp/myfile.txt", "read,write"));
        System.out.println("Permission granted");
      } catch (Exception e){
        System.out.println(e.toString());
        System.out.println("Permission NOT granted");
      }
    }
 
  }
}

Output:

$ java -Djava.security.manager SecurityManagerTest
java.security.AccessControlException: 
access denied ("java.io.FilePermission" "/tmp/myfile.txt" "read,write")
Permission NOT granted

$ java -Djava.security.manager -Djava.security.policy=./sample.policy SecurityManagerTest
Permission granted

$ cat ./sample.policy
grant {
  permission java.io.FilePermission "/tmp/myfile.txt", "read,write";
};