Java 11 upcoming features – Launch Single-File source programs

Jaikiran PaiJuly 17th, 2018Last Updated: July 17th, 2018

1 31 3 minutes read

Java 11 is nearing completion and it’s entered the rampdown phase. It almost feels like a few weeks back that Java 9 was released and here we are, within a few months of Java 11 being released. Given the new release process and timelines for Java, this will become a common thing. Whether that’s a good thing or not, we’ll keep it aside.

The changes coming in Java 11 are listed here. These are some nice enhancements and features coming in this release. Two of them that I’m really excited about are:

– HTTP client (standard) http://openjdk.java.net/jeps/321 which will bring in HTTP client APIs as part of the Java language.

– Launch Single-File Source-Code Programs http://openjdk.java.net/jeps/330

In this article, I will go through the “Launch Single-File Source-Code Programs” feature. What this enhancement proposes to accomplish is to make it easy for running Java code which consists of a single file with the “main()” method in it.

Imagine you have a simple HelloWorld program as follows in a file HelloWorld.java under org/myapp directory:

package org.myapp;


public class HelloWorld {
    
    public static void main(String[] args) throws Exception {
        System.out.println("Hello World!");
    }
}

Right now, without the proposed feature, in order to run this program, the user has to first compile it using javac command:

javac org/myapp/HelloWorld.java

Once that succesfully compiles, you then run the java command to execute the program:

java org.myapp.HelloWorld

So it’s a 2 step process. It looks trivial even for beginners, but it can still be made simpler not just for beginners but even developers who regularly work with Java.

Once Java 11 gets released (or if you want to try it now, you can get the early access builds from http://jdk.java.net/11/) we can run the above program as follows (as a single command):

java org/myapp/HelloWorld.java

Notice the difference here:

1. one, we no longer use the javac command to explicitly compile the source

2. The java command is now passed the path to the source file (org/myapp/HelloWorld.java) instead previously where we used to pass it the fully-qualified classname.

This difference is minor but important, since the java command now “understands” that it now has to internally do whatever it’s necessary (like compiling the source) when it’s passed a file path whose file name ends with the .java extension. Of course, such a file is expected to contain regular/valid Java code with a top level class exposing the “public static void main(String[])” method.

Furthermore, just like your regular Java programs you can continue to pass application specific arguments to the program as before. For example, for a calculator program which looks below, in a org/myapp/Calculator.java file:

package org.myapp;


public class Calculator {
 
 public static void main(final String[] args) throws Exception {
  final int sum = Integer.parseInt(args[0]) +  Integer.parseInt(args[1]);
  System.out.println(args[0] + " + " + args[1] + " = " + sum);
 }
}

you can pass the program arguments as follows:

java org/myapp/Calculator.java 2 4

where 2 and 4 are passed as the program arguments and you would see the output as follows:

2 + 4 = 6

This feature also adds support for “shebang” files, files which are expected to hava valid Java code plus a “shebang”. Personally, I’m not too fond of this specific aspect of the feature. However, the good thing is, the JDK team took feedback from the community and made this additional aspect of a feature, non-intrusive (for tools/commands which already deal with Java source files) and something that some of us can ignore if we don’t want to use it. The details of when/how to use the “shebang” files for this feature are explained in the linked JEP-330.

So far, although Java 11 hasn’t been released, I have been using the early access builds and extensively using this feature for some of my regular work which sometimes involves coming up with short programs that help reproduce an issue. I usually don’t use IDEs for things like these, so it’s been a welcome enhancement to be able to issue a single command against such files and have them executed.

Published on Java Code Geeks with permission by Jaikiran Pai, partner at our JCG program. See the original article here: Java 11 upcoming features – Launch Single-File source programs

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