The reason for JVM existence

Question

I am educating myself in the theory of programming languages and I wonder, why exactly do we need a Java Virtual Machine or any virtual machine at all for that matter? What are the fundamental reasons?

Is it solely for making it multi-platform? If so, why cannot we just have a platform independent language and different compilers for different platforms?

Answer 1

Virtual machines are an important abstraction used to make language development and implementation easier.

A large number of languages use virtual machines of some kind to allow them to be executed. Dynamic scripting languages such as Ruby and Python are interpreted at runtime on simple virtual machine. The advantage of this is that if the interpreter can be re-compiled to run on an given environment then then language itself can be used on that environment too.

Other languages such as Java can be compiled ahead of time to bytecode which is then either interpreted or just-in-time (JIT) compiled for execution. In this model only the virtual machine itself, and not the compiler, needs to be ported to any given environment to run code there. Java used this to advantage by allowing applications to be embedded in web pages.

Even outside these more dynamic languages virtual machines are used to abstract away from the details of the underlaying hardware. For example the Low-Level Virtual Machine (llvm) compiler is engineered in such a way that it first compiles C, C++, Objective-C or whatever into instructions to run on virtual machine architecture and then translates this to real machine code. This translation can be done straight away, just as a traditional C or C++ compiler would, or at runtime using JIT compilation.

These different types of virtual machines are working at different levels of abstraction. The llvm virtual machine is, as it's name suggests, at a very low level. It abstracts the different peculiarities of CPU architectures such as how to load and store floating point numbers, if things should be passed around on the stack or in registers, and so on. Virtual machines for languages such as Python however are abstracting over operating system APIs and similar things.

Answer 2

Is it for platform portability? Yes. You already know most of the obvious features of JVM and its advantages and others have already given splendid responses.

Here I'll add the human side of the advantage Virtual Machines provide. It is primarily for ease of development and reach.

Consider C as an example of platform independent language with specific compilers for specific Operating Systems. One can code in C on Linux as well as Windows. But, you'll require an additional library header file conio.h to run your same program on a Windows system.

Now, if massive million lined source code programs and application suites were required to be recompiled on every system (with diverse hardware and software) will require them to recompile the same code over and over on each and every compiler. This may leave out some systems as possible targets, if the developers missed compiling for that system.

This actually happens in the game industry where certain games are just not compiled and build for certain systems (like most high end games are not made for Linux). The game studios are forced to compile each time for every target machine they want, like Wii, PS3, PS4, PC, XBOX etc.

It's a waste of time, effort, resources and sanity (specially when you're dealing with super massive heterogeneous file types and source codes, which take massive amounts of time to compile).

In short; It is to reduce menial repetitive recompilation of the same source code for every system, so that we programmers can focus on things worthy of our time. [Or we're just lazy ;)]

Addendum:

According to Larry Wall, the original author of the Perl programming language, there are three great virtues of a programmer; Laziness, Impatience and Hubris. Link

Answer 3

In their 1996 whitepaper The Java Language Environment, the Java team at Sun states the following design goals for the Java Language:

The design requirements of the Java TM programming language are driven by the nature of the computing environments in which software must be deployed.

The massive growth of the Internet and the World-Wide Web leads us to a completely new way of looking at development and distribution of software. To live in the world of electronic commerce and distribution, Java technology must enable the development of secure, high performance, and highly robust applications on multiple platforms in heterogeneous, distributed networks.

Operating on multiple platforms in heterogeneous networks invalidates the traditional schemes of binary distribution, release, upgrade, patch, and so on. To survive in this jungle, the Java programming language must be architecture neutral, portable, and dynamically adaptable.

The system that emerged to meet these needs is simple, so it can be easily programmed by most developers; familiar, so that current developers can easily learn the Java programming language; object oriented, to take advantage of modern software development methodologies and to fit into distributed client-server applications; multithreaded, for high performance in applications that need to perform multiple concurrent activities, such as multimedia; and interpreted, for maximum portability and dynamic capabilities.

A bit further down, they address the reasons for using an interpreter in greater detail:

The Java interpreter can execute Java bytecodes directly on any machine to which the interpreter and run-time system have been ported. In an interpreted platform such as Java technology-based system, the link phase of a program is simple, incremental, and lightweight. You benefit from much faster development cycles--prototyping, experimentation, and rapid development are the normal case, versus the traditional heavyweight compile, link, and test cycles.

While the Java Compiler is strict in its compile-time static checking, the language and run-time system are dynamic in their linking stages. Classes are linked only as needed. New code modules can be linked in on demand from a variety of sources, even from sources across a network. In the case of the HotJava Browser and similar applications, interactive executable code can be loaded from anywhere, which enables transparent updating of applications. The result is on-line services that constantly evolve; they can remain innovative and fresh, draw more customers, and spur the growth of electronic commerce on the Internet.

Answer 4

why cannot we just have a platform independent language and different compilers for different platforms?

Well. What if I write a Linear Search program (in any language..) on a 16 bit machine, compile it using a 16 - bit compiler and then try running it on a 32 - bit machine. Will it behave in the same way?.

Imagine products that have millions of lines of code. Do you think that nothing in that million line code will break because of change in machine architecture ?

Now,

Virtual Machines : These are basically software written to convert instructions into the "machine understanding / OS understanding language". They sit on top of your OS and make calls to it i.e, make the OS understand what your application wants.

JVM : is a kind of virtual Machine in which is used for Java. When you write and compile a java program, it will be in an " almost -machine independent" state. This is called as byte code. You can take it to another machine and run / interpret it.

Answer 5

Java virtual machine (JVM) is a platform or a sandbox to run the byte code. Byte code has special instruction sets and operation which can only be identified by JVM.

This is the same case with any virtual machines where it expects specific set of operations.