Why does my Java program hang when I call it from Perl?

Question

Inside a Perl script, I want to run a Java program that takes in 2 inputs, which will be passed by the command line.

So I do:

if (scalar @ARGV == 2)
{ print `java myProg $ARGV[0] $ARGV[1]`; } 
elsif (scalar @ARGV == 1) 
{ print `java myProg $ARGV[0]`; }

I works if I enter 2 args, but still hangs if I enter only 1 arg

How should I correct it?

BTW, the Java program works.

---

I changed my Perl script to:

print scalar @ARGV;
print `$ARGV[0]`;
print `$ARGV[1]`;

And if I run 'perl myPerl.pl abc def' in command line, it only prints 2. And not my two inputs. WHY!?

Answer 1

The java program is hanging. Try it straight from the command line.

To check if there are enough args you can do:

 die "needs 2 args" unless (scalar @ARGV == 2);

Try this code to prove it is the java:

if (scalar @ARGV == 2)
{ 
    print `cat myProg $ARGV[0] $ARGV[1]`; 
} 
elsif (scalar @ARGV==1) 
{ 
    print `cat myProg $ARGV[0]`; 
}

If all you want to do is print the arguments:

if (scalar @ARGV == 2)
{ 
    print "arguments = [$ARGV[0]] [$ARGV[1]]\n"; 
} 
elsif (scalar @ARGV==1) 
{ 
    print "arguments = [$ARGV[0]]\n"; 
}

Answer 2

Use $#ARGV to find the index of the last array element of @ARGV. Add 1 to that and you can get the arg count.

my $argCount = $#ARGV + 1;
if ($argCount == 1)
{
    die "Please enter 2 arguments!\n";
}

Answer 3

To answer your latest question:

print scalar @ARGV;
print `$ARGV[0]`;
print `$ARGV[1]`;

The above code doesn't print the data you expect because you are using backticks with some random input data. Perl is trying to execute the shell commands "abc" and "def" (as per your example) and captures the output of running those commands, of which there isn't any. Try printing the values directly:

print scalar @ARGV;
print $ARGV[0], "\n";
print $ARGV[1], "\n";

Answer 4

The second code sample is heading off the rails, but the first code sample looks correct, more-or-less. Let's have a look at the Java code, your environment, or anything else that can help us figure out why

{ print `java myProg $ARGV[0] $ARGV[1]`; }

works and

{ print `java myProg $ARGV[0]`; }

doesn't.

Answer 5

Try this:

use strict;
use warnings;

my $BASE_COMMAND = 'java MyProgram';

die "Illegal argument count - must have one or two arguments."
    unless @ARGV == 1 or @ARGV == 2;

# Arrays interpolate into space separated strings (unless you change $" - see perlvar)
my $command = "$BASE_COMMAND @ARGV";

print "Running '$command'\n";
print `$command`;