Why do we need other JVM languages

Question

I see here that there are a load of languages aside from Java that run on the JVM. I'm a bit confused about the whole concept of other languages running in the JVM. So:

What is the advantage in having other languages for the JVM?

What is required (in high level terms) to write a language/compiler for the JVM?

How do you write/compile/run code in a language (other than Java) in the JVM?

---

EDIT: There were 3 follow up questions (originally comments) that were answered in the accepted answer. They are reprinted here for legibility:

How would an app written in, say, JPython, interact with a Java app?

Also, Can that JPython application use any of the JDK functions/objects??

What if it was Jaskell code, would the fact that it is a functional language not make it incompatible with the JDK?

Answer 1

To address your three questions separately:

What is the advantage in having other languages for the JVM?

There are two factors here. (1) Why have a language other than Java for the JVM, and (2) why have another language run on the JVM, instead of a different runtime?

1. Other languages can satisfy other needs. For example, Java has no built-in support for closures, a feature that is often very useful.
2. A language that runs on the JVM is bytecode compatible with any other language that runs on the JVM, meaning that code written in one language can interact with a library written in another language.

What is required (in high level terms) to write a language/compiler for the JVM?

The JVM reads bytecode (.class) files to obtain the instructions it needs to perform. Thus any language that is to be run on the JVM needs to be compiled to bytecode adhering to the Sun specification. This process is similar to compiling to native code, except that instead of compiling to instructions understood by the CPU, the code is compiled to instructions that are interpreted by the JVM.

How do you write/compile/run code in a language (other than Java) in the JVM?

Very much in the same way you write/compile/run code in Java. To get your feet wet, I'd recommend looking at Scala, which runs flawlessly on the JVM.

Answering your follow up questions:

How would an app written in, say, JPython, interact with a Java app?

This depends on the implementation's choice of bridging the language gap. In your example, Jython project has a straightforward means of doing this (see here):

from java.net import URL
u = URL('http://jython.org')

Also, can that JPython application use any of the JDK functions/objects?

Yes, see above.

What if it was Jaskell code, would the fact that it is a functional language not make it incompatible with the JDK?

No. Scala (link above) for example implements functional features while maintaining compatibility with Java. For example:

object Timer {
  def oncePerSecond(callback: () => unit) {
    while (true) { callback(); Thread sleep 1000 }
  }
  def timeFlies() {
    println("time flies like an arrow...")
  }
  def main(args: Array[String]) {
    oncePerSecond(timeFlies)
  }
}

Answer 2

You need other languages on the JVM for the same reason you need multiple programming languages in general: Different languages are better as solving different problems ... static typing vs. dynamic typing, strict vs. lazy ... Declarative, Imperative, Object Oriented ... etc.

In general, writing a "compiler" for another language to run on the JVM (or on the .Net CLR) is essentially a matter of compiling that language into java bytecode (or in the case of .Net, IL) instead of to assembly/machine language.

That said, a lot of the extra languages that are being written for JVM aren't compiled, but rather interpreted scripting languages...

Answer 3

Turning this on its head, consider you want to design a new language and you want it to run in a managed runtime with a JIT and GC. Then consider that you could:

(a) write you own managed runtime (VM) and tackle all sorts of technically difficult issues that will doubtless lead to many bugs, bad performance, improper threading and a great deal of portability effort

or

(b) compile your language into bytecode that can run on the Java VM which is already quite mature, fast and supported on a number of platforms (sometimes with more than one choice of vendor impementation).

Given that the JavaVM bytecode is not tied so closely to the Java language as to unduly restrict the type of language you can implement, it has been a popular target environment for languages that want to run in a VM.

Answer 4

Java is a fairly verbose programming language that is getting outdated very quickly with all of the new fancy languages/frameworks coming out in the past 5 years. To support all the fancy syntax that people want in a language AND preserve backwards compatibility it makes more sense to add more languages to the runtime.

Another benefit is it lets you run some web frameworks written in Ruby ala JRuby (aka Rails), or Grails(Groovy on Railys essentially), etc. on a proven hosting platform that likely already is in production at many companies, rather than having to using that not nearly as tried and tested Ruby hosting environments.

To compile the other languages you are just converting to Java byte code.