In this article by Vincent van der Leun, the author of the book, Introduction to JVM Languages, you will learn the history of the JVM and five important languages that run on the JVM.

(For more resources related to this topic, see here.)

While many other programming languages have come in and gone out of the spotlight, Java always managed to return to impressive spots, either near to, and lately even on, the top of the list of the most used languages in the world. It didn't take language designers long to realize that they as well could run their languages on the JVM—the virtual machine that powers Java applications—and take advantage of its performance, features, and extensive class library. In this article, we will take a look at common JVM use cases and various JVM languages.

The JVM was designed from the ground up to run anywhere. Its initial goal was to run on set-top boxes, but when Sun Microsystems found out the market was not ready in the mid '90s, they decided to bring the platform to desktop computers as well. To make all those use cases possible, Sun invented their own binary executable format and called it Java bytecode. To run programs compiled to Java bytecode, a Java Virtual Machine implementation must be installed on the system. The most popular JVM implementations nowadays are Oracle's free but partially proprietary implementation and the fully open source OpenJDK project (Oracle's Java runtime is largely based on OpenJDK).

This article covers the following subjects:

Popular JVM use cases
Java language
Scala language
Clojure language
Kotlin language
Groovy

The Java platform as published by Google on Android phones and tablets is not covered in this article. One of the reasons is that the Java version used on Android is still based on the Java 6 SE platform from 2006. However, some of the languages covered in this article can be used with Android. Kotlin, in particular, is a very popular choice for modern Android development.

Popular use cases

Since the JVM platform was designed with a lot of different use cases in mind, it will be no surprise that the JVM can be a very viable choice for very different scenarios. We will briefly look at the following use cases:

Web applications
Big data
Internet of Things (IoT)

Web applications

With its focus on performance, the JVM is a very popular choice for web applications. When built correctly, applications can scale really well if needed across many different servers.

The JVM is a well-understood platform, meaning that it is predictable and many tools are available to debug and profile problematic applications. Because of its open nature, the monitoring of JVM internals is also very well possible. For web applications that have to serve thousands of users concurrently, this is an important advantage.

The JVM already plays a huge role in the cloud. Popular examples of companies that use the JVM for core parts of their cloud-based services include Twitter (famously using Scala), Amazon, Spotify, and Netflix. But the actual list is much larger.

Big data

Big data is a hot topic. When data is regarded too big for traditional databases to be analyzed, one can set up multiple clusters of servers that will process the data. Analyzing the data in this context can, for example, be searching for something specific, looking for patterns, and calculating statistics.

This data could have been obtained from data collected from web servers (that, for example, logged visitor's clicks), output obtained from external sensors at a manufacturer plant, legacy servers that have been producing log files over many years, and so forth. Data sizes can vary wildly as well, but often, will take up multiple terabytes in total.

Two popular technologies in the big data arena are:

Apache Hadoop (provides storage of data and takes care of data distribution to other servers)
Apache Spark (uses Hadoop to stream data and makes it possible to analyze the incoming data)

Both Hadoop and Spark are for the most part written in Java. While both offer interfaces for a lot of programming languages and platforms, it will not be a surprise that the JVM is among them.

The functional programming paradigm focuses on creating code that can run safely on multiple CPU cores, so languages that are fully specialized in this style, such as Scala or Clojure, are very appropriate candidates to be used with either Spark or Hadoop.

Internet of Things - IoT

Portable devices that feature internet connectivity are very common these days. Since Java was created with the idea of running on embedded devices from the beginning, the JVM is, yet again, at an advantage here.

For memory constrained systems, Oracle offers Java Micro Edition Embedded platform. It is meant for commercial IoT devices that do not require a standard graphical or console-based user interface.

For devices that can spare more memory, the Java SE Embedded edition is available. The Java SE Embedded version is very close to the Java Standard Edition discussed in this article. When running a full Linux environment, it can be used to provide desktop GUIs for full user interaction. Java SE Embedded is installed by default on Raspbian, the standard Linux distribution of the popular Raspberry Pi low-cost, credit card-sized computers.

Both Java ME Embedded and Java SE Embedded can access the General Purpose input/output (GPIO) pins on the Raspberry Pi, which means that sensors and other peripherals connected to these ports can be accessed by Java code.

Java

Java is the language that started it all. Source code written in Java is generally easy to read and comprehend. It started out as a relatively simple language to learn. As more and more features were added to the language over the years, its complexity increased somewhat. The good news is that beginners don't have to worry about the more advanced topics too much, until they are ready to learn them.

Programmers that want to choose a different JVM language from Java can still benefit from learning the Java syntax, especially once they start using libraries or frameworks that provide Javadocs as API documentation. Javadocs is a tool that generates HTML documentation based on special comments in the source code. Many libraries and frameworks provide the HTML documents generated by Javadocs as part of their documentation.

While Java is not considered a pure Object Orientated Programming (OOP) language because of its support for primitive types, it is still a serious OOP language. Java is known for its verbosity, it has strict requirements for its syntax. A typical Java class looks like this:

package com.example;

import java.util.Date;

public class JavaDemo {
  private Date dueDate = new Date();

  public void getDueDate(Date dueDate) {
    this.dueDate = dueDate;
  }

  public Date getValue() {
    return this.dueDate;
  }
}

A real-world example would implement some other important additional methods that were omitted for readability.

Note that declaring the dueDate variable, the Date class name has to be specified twice; first, when declaring the variable type and the second time, when instantiating an object of this class.

Scala

Scala is a rather unique language. It has a strong support for functional programming, while also being a pure object orientated programming language at the same time.

While a lot more can be said about functional programming, in a nutshell, functional programming is about writing code in such a way that existing variables are not modified while the program is running. Values are specified as function parameters and output is generated based on their parameters. Functions are required to return the same output when specifying the same parameters on each call. A class is supposed to not hold internal states that can change over time. When data changes, a new copy of the object must be returned and all existing copies of the data must be left alone. When following the rules of functional programming, which requires a specific mindset of programmers, the code is safe to be executed on multiple threads on different CPU cores simultaneously.

The Scala installation offers two ways of running Scala code. It offers an interactive shell where code can directly be entered and is run right away. This program can also be used to run Scala source code directly without manually first compiling it. Also offered is scalac, a traditional compiler that compiles Scala source code to Java bytecode and compiles to files with the .class extension.

Scala comes with its own Scala Standard Library. It complements the Java Class Library that is bundled with the Java Runtime Environment (JRE) and installed as part of the Java Developers Kit (JDK). It contains classes that are optimized to work with Scala's language features. Among many other things, it implements its own collection classes, while still offering compatibility with Java's collections.

Scala's equivalent of the code shown in the Java section would be something like the following:

package com.example

import java.util.Date

class ScalaDemo(var dueDate: Date) {

}

Scala will generate the getter and setter methods automatically. Note that this class does not follow the rules of functional programming as the dueDate variable is mutable (it can be changed at any time). It would be better to define the class like this:

class ScalaDemo(val dueDate: Date) {

}

By defining dueDate with the val keyword instead of the var keyword, the variable has become immutable. Now Scala only generates a getter method and the dueDate can only be set when creating an instance of ScalaDemo class. It will never change during the lifetime of the object.

Clojure

Clojure is a language that is rather different from the other languages covered in this article. It is a language largely inspired by the Lisp programming language, which originally dates from the late 1950s. Lisp stayed relevant by keeping up to date with technology and times. Today, Common Lisp and Scheme are arguably the two most popular Lisp dialects in use today and Clojure is influenced by both.

Unlike Java and Scala, Clojure is a dynamic language. Variables do not have fixed types and when compiling, no type checking is performed by the compiler. When a variable is passed to a function that it is not compatible with the code in the function, an exception will be thrown at run time. Also noteworthy is that Clojure is not an object orientated language, unlike all other languages in this article. Clojure still offers interoperability with Java and the JVM as it can create instances of objects and can also generate class files that other languages on the JVM can use to run bytecode compiled by Clojure.

Instead of demonstrating how to generate a class in Clojure, let's write a function in Clojure that would consume a javademo instance and print its dueDate:

(defn consume-javademo-instance [d] (println (.getDueDate d)))

This looks rather different from the other source code in this article. Code in Clojure is written by adding code to a list. Each open parenthesis and the corresponding closing parenthesis in the preceding code starts and ends a new list. The first entry in the list is the function that will be called, while the other entries of that list are its parameters. By nesting the lists, complex evaluations can be written. The defn macro defines a new function that will be called consume-javademo-instance. It takes one parameter, called d. This parameter should be the javademo instance. The list that follows is the body of the function, which prints the value of the getDueDate function of the passed javademo instance in the variable, d.

Kotlin

Like Java and Scala, Kotlin, is a statically typed language. Kotlin is mainly focused on object orientated programming but supports procedural programming as well, so the usage of classes and objects is not required. Kotlin's syntax is not compatible with Java; the code in Kotlin is much less verbose. It still offers a very strong compatibility with Java and the JVM platform. The Kotlin equivalent of the Java code would be as follows:

import java.util.Date

data class KotlinDemo(var dueDate: Date)

One of the more noticeable features of Kotlin is its type system, especially its handling of null references. In many programming languages, a reference type variable can hold a null reference, which means that a reference literally points to nothing. When accessing members of such null reference on the JVM, the dreaded NullPointerException is thrown. When declaring variables in the normal way, Kotlin does not allow references to be assigned to null. If you want a variable that can be null, you'll have to add the question mark (?)to its definition:

var thisDateCanBeNull: Date? = Date()

When you now access the variable, you'll have to let the compiler know that you are aware that the variable can be null:

if (thisDateCanBeNull != null)
  println("${thisDateCanBeNull.toString()}")

Without the if check, the code would refuse to compile.

Groovy

Groovy was an early alternative language for the JVM. It offers, for a large degree, Java syntax compatibility, but the code in Groovy can be much more compact because many source code elements that are required in Java are optional in Groovy. Like Clojure and mainstream languages such as Python, Groovy is a dynamic language (with a twist, as we will discuss next).

Unusually, while Groovy is a dynamic language (types do not have to be specified when defining variables), it still offers optional static compilation of classes. Since statically compiled code usually performs better than dynamic code, this can be used when the performance is important for a particular class. You'll give up some convenience when switching to static compilation, though.

Some other differences with Java is that Groovy supports operator overloading. Because Groovy is a dynamic language, it offers some tricks that would be very hard to implement with Java. It comes with a huge library of support classes, including many wrapper classes that make working with the Java Class Library a much more enjoyable experience.

A JavaDemo equivalent in Groovy would look as follows:

@Canonical
class GroovyDemo {
  Date dueDate
}

The @Canonical annotation is not necessary but recommended because it will generate some support methods automatically that are used often and required in many use cases. Even without it, Groovy will automatically generate the getter and setter methods that we had to define manually in Java.

Summary

We started by looking at the history of the Java Virtual Machine and studied some important use cases of the Java Virtual Machine: web applications, big data, and IoT (Internet of Things). We then looked at five important languages that run on the JVM: Java (a very readable, but also very verbose statically typed language), Scala (both a strong functional and OOP programming language), Clojure (a non-OOP functional programming language inspired by Lisp and Haskell), Kotlin (a statically typed language, that protects the programmer from very common NullPointerException errors), and Groovy (a dynamic language with static compiler support that offers a ton of features).