What happens when you run a Java program?

Direct Answer

When java HelloWorld runs a traditional class, the launcher starts a JVM, locates the initial class through the configured class path or module path, loads and links it, initializes it, invokes its accepted main entry point, loads dependencies as needed, and executes bytecode using that JVM's runtime strategy.

Deep Dive & Architecture

Java execution phases and their failures

“Could not find or load main class” means the launcher could not locate or load the requested initial class with the supplied class/module path. An unsupported class-file version means the runtime is older than the class-file format it was asked to accept. VerifyError points to verification, while a linkage error can reveal an incompatible class or missing member. NoClassDefFoundError during execution can mean a needed definition is unavailable or that a previous initialization attempt failed.

Initialization happens before visible main work

Before ordinary use of the initial class, the JVM initializes it as required. Static field initializers and static blocks run in class-initialization order. A failed static initializer prevents normal entry, and later uses can observe the recorded initialization failure. A long or circular initializer can make startup appear hung even though the launcher found the class correctly.

Arguments have two owners

In java -cp out Hello Ravi, -cp out configures the launcher, Hello names the initial class, and Ravi becomes an application argument. Mixing their order can change whether the launcher or main receives a value. Dependencies may then be loaded and initialized on demand as code reaches them.

Termination is also a lifecycle

The process normally remains alive while non-daemon threads are active. Shutdown hooks run during orderly shutdown, but abrupt process termination, an operating-system failure, or forced halt can skip them. Durable work needs a database, file, queue, or other external contract rather than an assumption that a final in-memory callback always runs.

Code Example

class HelloWorld {
    static {
        System.out.println("class initialized");
    }

    public static void main(String[] args) {
        String name = args.length == 0 ? "world" : args[0];
        System.out.println("Hello, " + name);
    }
}

Compile with javac -d out HelloWorld.java, then run java -cp out HelloWorld Ravi.