Simple Source-Code Program Execution

In JDK 11, Java introduced the ability to launch a single-file source-code program with the java launcher, now multi-file as of JDK 22, 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:

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

Passing in Arguments

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

Passing in an argument can be done like this:

Multiple Classes in Same File

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

When executed:

Will output:

Launching a Multi-File Program

Since the launch of JDK 22, which included JEP 458: Launch Multi-File Source-Code Programs, the java launcher can now launch a multi-file source code program. This provides a more gradual transition from trivial single-file programs to more complex programs.

The java launcher will now automatically locate, compile, and load multi source-file programs so as long as the location of the source files match the package structure of the classes being imported. Take this example:

The classes model.Person and service.PersonService would need to exist in the sub-directories model and service, respectively, of where the Main class is located. If they are present, the java launcher will compile and execute them, along with the Main class.

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:

To launch:

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:

To launch:

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 a java source file, as the first line in the file add path/to/java/home --source <version> like in the below example:

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

If you do not pass any argument, you will get an ArrayIndexOutOfBoundsException.