CODEWITHSUNDEEP
javajvmjit
Anurupa_Dey
Anurupa_Dey

Reputation: 113

JVM : Bytecode Confusion and JIT

So here's a question that came to my mind when I was studying Java. We know (please correct me if I am wrong!) that the Bytecode runs atop JVM. So does the JVM convert the Bytecode to the native machine code it's(JVM) written for? If that is so, isn't it less secure?

Also what exactly is a just-in-time compiler? It compiles when it is asked to do so...I studied some resources, but still didn't get the just-in-time part clear.

Thanks for any help !

Upvotes: 1

Views: 452

Answers (2)

vladimir
vladimir

Reputation: 2765

So does the JVM convert the Byte-code to the native machine code it's(JVM) written for?

All JVM implementation I have seen so far are converting byte-code to the native machine code VM is written for. Although I can't see how and why doing otherwise would be useful.

Also what exactly is a just-in-time compiler?

It's simply process of converting byte code to native code in run-time. Although for performance improvement it's being done by VM in parallel with your program execution. It also usually including compiled native code caching and some other techniques of performance improvement.

If that is so, isn't it less secure?

Well, to some very small degree it is. VERY VERY SMALL DEGREE. There're some security-related modifications to different OSes eliminating JIT compilation. For example, grsecurity Linux kernel patch is in fact doing JIT impossible (actually doing impossible to execute JIT-compiled code). And another fact is that similar memory protection mechanism (writable memory pages can't be executable) is implemented in iOS which makes impossible to do any JIT compilation in user mode.

Upvotes: 1

Ingo
Ingo

Reputation: 36339

So does the JVM convert the Bytecode to the native machine code it's(JVM) written for?

No, not necessarily. Though, nowadays it is state of the art to do so by default.

If that is so, isn't it less secure?

Less secure than what? Just because one can do insecure operations in machine code (like dereferencing an unitialized pointer or accessing unallocated memory) does not mean the JIT generates such insecure code.

Also what exactly is a just-in-time compiler?

It's that part of the JVM that converts bytecode to native machine code. The name "just in time" means that the code is compiled (in a separate thread) while it is executed. Once completely compiled, the JVM takes notice that certain methods are compiled and can be invoked on the machine level.

Upvotes: 1

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