Piping cat output into a Java jar through the terminal

I have a JAR file that will require use of the standard input. I need to use this command and cannot change it:

cat filename | java -jar jarname.jar

I want this to be accessible to the JAR somehow, easiest way being through String[] args. The problem is that this the result of cat filename doesn't show up in String[] args, or in any way for me to make use of it. I've tried searching this up for some time, but all I get is questions pertaining to writing java output to files using the pipe.

Solution

easiest way being through String[] args

impossible.

The file will be available as System.in.

Here's a trivial program that prints sysin right back out, adding line numbers visually. cat with extra steps. Now you know how to use System.in. Note, if System.in contains byte-based data, obviously don't wrap it in readers (which are for char based data). Just read bytes straight from it.

> cat Test.java
import java.io.*;

class Test {
  public static void main(String[] args) throws Exception {
    var in = new BufferedReader(new InputStreamReader(System.in));
    int lineNum = 1;
    while (true) {
      String line = in.readLine();
      if (line == null) break;
      System.out.printf("%6d. %s\n", lineNum++, line);
    }
  }
}

> cat hello.txt
Hello!
World!
> cat hello.txt | java Test.java
     1. Hello!
     2. World!