What is the role of the OS when JVM executes a Java application? And why do we need the OS?

Question

I have done some reading on the internet and some people say that Java application is executed by the java virtual machine (JVM). The word \"execute\" confuses me a little bit.

Answer 1

It seems to me that the JVM has "by-passed" the OS and directly instruct the CPU to execute the application. If so, why do we need the OS?

All C/C++ binaries (not just the JVM) run directly on the CPU. Once running, these programs can call into more machine code provided by the operating system to do useful things like reading files, starting threads, or using the network.

The JVM translates a Java program into instructions that run on the CPU. Behind the scenes, though, Java's threads, file i/o, and network sockets (to name a few) all contain instructions that call into the code provided by the operating system for threads/files/etc. This is one of the reasons you still need the OS.

Since it is already machine codes, do we need the JVM any more?

The JVM provides features that you don't get from the JIT compiler. At the end of the day the JVM is just running a lot of machine code, but not all of that machine code comes from the JIT (or from the interpreter). Some of that machine code does garbage collection, for example. That's why you need the JVM.