I maintain a online judge which uses CMS isolate sandbox. I was trying to support Java so I looked at CMS itself, and found that they are using gcj (GNU Compiler for Java) as you can see in this commit.

https://github.com/cms-dev/cms/commit/cf949952424677e0b0762eec5e461e33f85eecb3

Also you can see this blame:

In my server, I have several bugs:

(Of course these bugs are related to I/O and since IOI uses interface file so it may not be a problem.)

public class Task {
  public static void main(String[] args) {
    System.out.format("$%.3f\n", 0.9876);
  }
}

Expected output: $0.988
Program output: $

import java.util.*;
import java.io.*;
public class Task {
  public static void main(String[] args) throws IOException {
    Scanner in = new Scanner(System.in);
    System.out.print((char) System.in.read());
    System.out.println(in.nextInt());
  }
}

Input: A5
Expected Output: A5
Program: 
(strange character)Exception in thread "main" java.util.InputMismatchException: "A5" is not an integer
   at java.util.Scanner.myNextInt(libgcj.so.15)
   at java.util.Scanner.nextInt(libgcj.so.15)
   at java.util.Scanner.nextInt(libgcj.so.15)
   at Task.main(Task.java)