JEP proposed to target JDK 11: 330: Launch Single-File Source-Code Programs (original) (raw)

Stuart Marks stuart.marks at oracle.com
Thu May 24 05:08:03 UTC 2018

On 5/22/18 7:43 AM, Peter Levart wrote:

If this JEP added a feature that specifying source file name as "-" would read STDIN as the Java source file, one could write executable scripts in the following "hybrid" way:

#!/bin/bash exec /path/to/java - $* <<EOF public class Hello { public static void main(String[] args) { System.out.println("Hello world"); } } EOF Notice that this does not need shebang support from java launcher. The benefit of this approach would also be that /path/to/java could be computed by the bash script prefixing the Java source. Java typically does not have a standard installation "place" like bash, so shebang scripts with hard-coded java path would be less portable as bash scripts.

Support for "-" STDIN source in java launcher does have one drawback. Scripts written this way can not process STDIN passed to the script as STDIN of java launcher is redirected to obtain Java source.

Hi Peter,

This is an excellent point: one can use a shell script "wrapper" to do all kinds of interesting things in addition to computing the path to java, such as supplying additional arguments based on other logic, etc.

I'll note that while "-" as a filename is a fairly strong convention, it isn't strictly necessary; /dev/stdin can be used on many systems as a filename that represents stdin. As you note this prevents the script's stdin from being read by the Java program. Instead, one can use a here-document with a file descriptor and then use /dev/fd/n as the "source file" to be compiled and run.

This is system-specific, of course, but these facilities should be widely available. I tested this on a Mac using Jon's patch from a couple weeks ago, [1] and it works fine. I'm sure Linux has the /dev/fd stuff as well. Also, the --source option must be used to coax the launcher to compile a file that doesn't end in ".java".

This is a little clunky, but it's no worse than the bits of magic you'd have to put at the top of a perl script in order to run it as a shell script.

Here's an example:

#!/bin/bash

exec java --source 11 /dev/fd/3 "$@" 3<<EOF

public class Hello { 

 public static void main(String[] args) { 

     String name = System.console().readLine("Please enter your name: "); 

     System.console().printf("Hello, %s!%n", name); 

 }

}

EOF

$ ./Hello.sh Please enter your name: Stuart Hello, Stuart!

s'marks

[1] http://mail.openjdk.java.net/pipermail/compiler-dev/2018-May/011874.html

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