Java: Launching Single-File Source-Code Programs

Single-File Source-Code Program Execution

In JDK 11, Java introduced the ability to launch a single-file source-code program with the java launcher, without first needing to explicitly compile the source code. This works by the java launcher automatically invoking the compiler and storing the compiled code in memory. This can be a great way to learn how to use Java or explore new features within the Java API, without having to go through the cruft of compiling and then executing code. There are several ways to use this feature, as well as some limitations and things to keep in mind.

Executing Your First Single-File Source-Code Program

To execute a single-file source-code program, the first class defined in the source file must contain public static void main(String[]) like in HelloWorld below:

public class HelloWorld {

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

}

From the command line, HelloWorld can be launched with (accepting the name of the file is also HelloWorld.java):

$ java HelloWorld.java

Passing in Arguments

Arguments can also be passed in like with a normally compiled class, so in the below:

public class HelloJava {

    public static void main(String[] args) {
        System.out.println("Hello " + args[0]);
    }

}

Passing in an argument can be done like this:

$ java HelloJava.java World!

Multiple Classes in Same File

Multiple classes can be defined within the same source file if needed for encapsulation purposes, like in this example:

public class MultipleClassesInSameFile {
    public static void main(String[] args) {
 
        System.out.println(GenerateMessage.generateMessage());
        System.out.println(AnotherMessage.generateAnotherMessage());
    }
}
 
class GenerateMessage {
    static String generateMessage() {
        return "Here is one message";
    }
}
 
class AnotherMessage {
    static String generateAnotherMessage() {
        return "Here is another message";
    }
}

When executed:

$ java MultipleClassesInSameFile.java

Will output:

Here is one message
Here is another message

Reference JDK Classes and Non-JDK Classes

A class that is part of the core JDK does not need to be added to the classpath to be executed. So this example, referencing the Scanner and MatchResult classes, can be executed simply with the java launcher:

import java.util.Scanner;
import java.util.regex.MatchResult;

public class ScannerExample {

    public static void main(String... args) {
        String wordsAndNumbers = """
                Longing rusted furnace
                daybreak 17 benign 
                9 homecoming 1 
                freight car
                """;

        try (Scanner scanner = new Scanner(wordsAndNumbers)) {
            scanner.findAll("benign").map(MatchResult::group).forEach(System.out::println);
        }
    }
}

To launch:

$ java ScannerExample.java

However the below example referencing RandomUtils, part of the Apache Commons Lang, will need to have the commons-lang.jar added to the classpath at launch:

import org.apache.commons.lang3.RandomUtils;

public class ReferenceNonJDKClass {

    public static void main(String[] args) {
        System.out.println(RandomUtils.nextInt());
    }

}

To launch:

java -cp /path/to/commons-lang3-3.12.0.jar ReferenceNonJDKClass.java

Executing as a Shebang File

On a Unix-like operating system, a single-file source-code application can also be launched as a shebang file like a script.
Within the a java source file, as the first line in the file add path/to/java/home --source <version> like in the below example:

#!/path/to/your/bin/java --source 16
 
public class HelloJava {
 
    public static void main(String[] args) {
        System.out.println("Hello " + args[0]);
    }
}

The file cannot have .java as its file extension, and must also be executable chmod +x. With that, it can be launched with:

./HelloJava