IS there a way to turn off JIT compiler and is the

Question

What does it mean for a java program to be JIT'ed and does it make the execution a lot more faster or are there bytecodes which are not JIT'ed?

Answer 1

From the doc,

-Xint

Runs the application in interpreted-only mode. Compilation to native code is disabled, and all bytecode is executed by the interpreter.

The java.compiler system property is known by Compiler class before Java 9. In Java 9 it's marked as @Deprecated.

Answer 2

For IBM the correct option is -Xnojit or -Xint

Answer 3

There is two ways to disable the JIT

-Djava.compiler=NONE 

or this will almost never compile anything

-XX:CompileThreshold=2000000000

or on IBM JVM

-nojit

Disabling the JIT can slow down your code a lot e.g. 50x but not always. If you spend most of your time doing IO or GUI updates you might find it makes little difference.

Answer 4

In HotSpot VM JIT compilation can be disabled with the option -XX:-UseCompiler which skips compiler threads initialization.

It is true by default:

product(bool, UseCompiler, true,                                          
        "Use Just-In-Time compilation")