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Compiling a program to bytecode instead of native code enables a certain level of portability, so long a fitting Virtual Machine exists.
But I'm kinda wondering, why delay the compilation? Why not simply compile the byte code when installing an application?
And if that is done, why not adopt it to languages that directly compile to native code? Compile them to an intermediate format, distribute a "JIT" compiler with the installer and compile it on the target machine.
The only thing I can think of is runtime optimization. That's about the only major thing that can't be done at installation time. Thoughts?
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The JIT compiler is enabled by default, and is activated when a Java method is called. The JIT compiler compiles the bytecodes of that method into native machine code, compiling it "just in time" to run. When a method has been compiled, the JVM calls the compiled code of that method directly instead of interpreting it.
The advantages of a JIT are due to the fact, that since the compilation takes place in run time, a JIT compiler has access to dynamic runtime information enabling it to make better optimizations (such as inlining functions).
The Just-In-Time (JIT) compiler is a component of the runtime environment that improves the performance of Java™ applications by compiling bytecodes to native machine code at run time.
Just In Time Compiler (JIT) : The above loop code runs for 10 times if the value of i is 0. It is not necessary to compile the bytecode for 10 times again and again as the same instruction is going to execute for 10 times.
Often it is precompiled. Consider, for example, precompiling .NET code with NGEN.
One reason for not precompiling everything would be extensibility. Consider those languages which allow use of reflection to load additional code at runtime.
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