Why is Java platform independent?

Question

When I used C++ programs, I needed Turbo C complier; and when I have a Java program, I need to have JVM. Still C++ isn't platform independent, but Java is! If any Java program require a JVM running in order to execute, why does Java is said to be Platform Independent?

Answer 1

No software is really "independent". Eventually, your program has to call the underlying OS in order to make some basic operations, like allocating memory, create new threads etc.

The way to achieve an executable which is "cross platform" is to create specific executable for each OS. Common practice is to write different code for each OS, and then "hide" it in a cross platform interface and compile the relevant code to the relevant OS. For example, std::thread is "cross-platform" to the user who uses this class, but behind the scenes it will call different functions based on the OS which was specified on compile time (such as CreateThread on Windows, but pthread_create on *nix OS's).

So basically, the JVM is a C/C++ executable that was written with different set of functions for each OS, and was compiled separately for each OS. A JVM executable which works on Linux, will not work on Windows, and vice-versa.

That JVM compiles .class files to machine code based on the OS and CPU it currently operating on, so that's why Java programs can "run anywhere".

But basically, it's a lie. It's like saying a human being can live on Mars.... if he lives inside a sealed spaceship with proper temperature, water, food, air and sunlight supply

So Java program can run anywhere.... if the JVM already is installed and running on the computer.

Answer 2

Firstly, I'd like to link to this question which has a lot of good information.

https://softwareengineering.stackexchange.com/questions/85175/what-is-the-exact-meaning-of-platform-independence

In the question above are comments about what it means to be "Platform independent," but the one thing I wanted to mention, but is summed up nicely here is

You're right, platform independence means that the same program works on any platform (operating system) without needing any modification.

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The code we write is known as "Write once, run anywhere" or as someone else said "run once, test everywhere."

Our Java code SHOULD run everywhere, but sometimes there are little native bugs that cause issues i.e., someone was having issues with printing on Mac OSX with JavaFX-8 Printing, while it works fine on Windows. There was also a bug report on this to fix this "Mac specific Java issue."

So.... For the most part, the underlying JavaSE code SHOULD work across all platforms.....

** however, if you do have an Application running on multiple computers and work with the File System, you will have to do checks to understand which OS you are working with i.e., (System.getProperty("os.name").contains("Windows")); **

more info on that here https://docs.oracle.com/javase/tutorial/essential/environment/sysprop.html

There is also another thing to note.

Certain components do not work cross platform, i.e., JavaSE vs JavaEE. JSP/JSF is what is used in JavaEE, on the web, that is specific for running code on a server, and in a webpage, but cannot be used in the Desktop (to my knowledge).

However, JavaSE has GUIs such as Swing, and JavaFX, which also cannot work on the web, either in the Client, or in the Server.

Android has it's own set of commands and things it can/cannot do, and other "Platforms" have specifics to it as well.

Overall, the underlying Java Architecture is what is used across all platforms, where certain "Java Specifics" are used in certain platforms i.e., JSP.

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So what does Java do differently?

In the case of Java the application runs in a Java Virtual Machine which itself isn't platform independent. This has to be the interface between the actual machine (operating system) and the Java code you've written.

I am not really knowledgeable that much of the JVM, but it seems that each JVM is specifically tailored to each "Platform" (Which is why there are soo many versions to install), and does a lot of heavy lifting in the background, whereas C you might have to do this all yourself (not really sure how it works), for each OS.

Want a JVM for Windows? Np... Linux? Np.... That new car you just bought with all that fancy tech in it? JVM for that.... Or how about that new parking meter where you just parked your car? Yeah, there's one for that too...

For example, here is an Answer from this site on how Java is converted to Dalvik for Android.

https://stackoverflow.com/a/24570735/3599960

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Hope this helps and makes sense.

Answer 3

C or C++ program gets compiled into native-code which is close to the metal where metal is the OS (earlier it could be hardware also in DOS era). There is no further re-compilation necessary to run the executable on target platform. But, a developer must build executables for all platforms he/she indents the program should run on.

It doesn't just mean different OSes, but bit-ness of particular OS. For example a 64-bit binary (EXE) cannot run on a 32-bit OS (vice-versa is possible, mostly, however). Here Java/.NET and other platform virtualization engine are boon for the developers - they just build once (for example "AnyCPU" for C# module), and don't need to give multiple binaries (EXE files). The runtime installed on given OS would re-compiler (JIT - Just In Time compilation).

I'm not sure about Java, but a.NET program can be compiled into specific platform. It is also possible by the .NET engine to JIT compile the intermediate (or byte-code in Java) into native format just once (and keep the EXE for direct run). The advantage is that .NET JIT compiler can take advantage of current hardware and latest CPU instructions, which C++ program cannot (it won't have JIT/re-compilation).

Answer 4

Java is operating-system independent because it runs on the Java platform (the JVM): The mantra is "write once, run anywhere" because you write your code using the JDK API, compile that once, and it runs on any operating system that has a JVM available. You write your code, wrap it up in a jar, and that jar runs wherever you want to use it, within reasonable bounds. The job of the JDK and JVM are to abstract away the differences in environments.

In contrast, particularly back when Java was created, writing C or C++ for multiple operating systems was a big pain and usually required additional toolkits (of course, the JDK and JVM are a toolkit of sorts), and even today still requires at least recompiling for the target system.

There's nothing magic about Java's OS-independence. It would be entirely possible to build the same thing for C or C++: Compile to an intermediary form, provide a runtime that knows how to interpret or recompile that intermediary form for different environments and provide a library that abstracts away environmental differences. Java just...did that, with its own spin on the language. Later, so did the .Net platform.