What is the mechanism of java and javac?

Question

I just picked up my Ubuntu machine after a long time for some java related work and found that I have java already installed but not javac.

I made a Test.java file with a main method and a simple print statement. I wrote this in my terminal:

java Test.java

I expected that without javac this shouldn't compile and run but it printed the output on my console. I then installed a JDK to enable the javac and ran this:

javac Test.java

This created a Test.class file. Still to run the Test class I need to type java Test.java and on typing java Test it throws java.lang.NoClassDefFoundError.

Can someone please explain to me what's happening in the background of these commands? Edit: Here are the contents of my Test.java:

package Learning;

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("Hello World!");
    }

}

Note also that if you're using an IDE to create the code, the `.class` file is made by default. — Hovercraft Full Of Eels, Jul 25 '19 at 18:36
@HovercraftFullOfEels If you want I can add the terminal results, I also didn't expect to directly run a java file. — prakasht, Jul 25 '19 at 18:38
@prakasht When you create a class `Test` in the package `Learning` you have to execute the `java` command with `java Learning.Test` as this is the fully qualified name of the class you wrote (and compiled). — Progman, Jul 25 '19 at 18:47

GhostCat · Accepted Answer · 2019-07-26T12:14:23.390

What you experience here is a new feature, added for Java 11:

In Java SE 11, you get the option to launch a single source code file directly, without intermediate compilation. Just for your convenience, so that newbies like you don't have to run javac + java (of course, leaving them confused why that is).

Quoted from here. For more details, see the corresponding JEP 330.

So: if you have a single self-contained .java file ... then the java binary recognizes that, compiles it, and directly runs it (when using Java 11 or newer).

But keep in mind: it is just that, a way to quickly run a single class. It isn't meant to replace the "real" way of doing things.

In general, you still use javac first, and then java. Or, more real world: you use a built system where you describe what to build, and then the build system invokes javac for you behind the covers.

score 0 · Answer 2 · answered Aug 02 '22 at 05:28

GhostCat's answer is good, but here are a few additions taken from a longer post about this same behavior.

Can someone please explain to me what's happening in the background of these commands?

What you ran into – where you can use "java" (not "javac") to compile and run a program in one command – JEP 330: Launch Single-File Source-Code Programs – was designed to make it easier for "early stages of learning Java, and when writing small utility programs".

It's definitely not meant to replace anything – use it if convenient, but nothing changes with the normal steps of "compile to .class file" and "run JVM using .class files or JARs".

There's some good info in the JEP 330 link, but also in the java command itself, namely that one option (among four total) is to provide a single filename (which is what you did)

To launch a single source-file program:
java [options] source-file [args ...]

Further, there's a nice summary in Using Source-File Mode to Launch Single-File Source-Code Programs:

In source-file mode, the effect is as though the source file is compiled into memory, and the first class found in the source file is executed. Any arguments placed after the name of the source file in the original command line are passed to the compiled class when it is executed.

What is the mechanism of java and javac?

2 Answers2