The file 2.jar contains a file 2.class that was generated manually (without javac). You can download it here.
Running java -jar 2.jar works perfectly and should output
Hello World!
Here is the output of running javap -c -v 2.class:
Classfile 2.class
Last modified 14 Dec, 2016; size 314 bytes
MD5 checksum 58880f3fe46c502feb88e671fe54498c
public class 2
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Class #2 // "2"
#2 = Utf8 2
#3 = Class #4 // java/lang/Object
#4 = Utf8 java/lang/Object
#5 = Utf8 main
#6 = Utf8 ([Ljava/lang/String;)V
#7 = Fieldref #8.#10 // java/lang/System.out:Ljava/io/PrintStream;
#8 = Class #9 // java/lang/System
#9 = Utf8 java/lang/System
#10 = NameAndType #11:#12 // out:Ljava/io/PrintStream;
#11 = Utf8 out
#12 = Utf8 Ljava/io/PrintStream;
#13 = Class #14 // java/io/PrintStream
#14 = Utf8 java/io/PrintStream
#15 = String #16 // Hello World!
#16 = Utf8 Hello World!
#17 = Methodref #13.#18 // java/io/PrintStream.println:(Ljava/lang/String;)V
#18 = NameAndType #19:#20 // println:(Ljava/lang/String;)V
#19 = Utf8 println
#20 = Utf8 (Ljava/lang/String;)V
#21 = Class #22 // java/lang/String
#22 = Utf8 java/lang/String
#23 = Utf8 Code
#24 = Utf8 StackMapTable
{
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #15 // String Hello World!
5: invokevirtual #17 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
}