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Please bear with my attempt to inform you a little regarding the execution of Java code.

Javas code is compiled into byte code, which Javas virtual machine executes. This output is an

intermediate language (IL) that is neither human readable nor machine-executable. Only the virtual

machine understands it. To execute this output, the virtual machine uses a Just in Time (JIT) compiler

to interpret the byte code so is Java an interpreted language as some people describe it, or is Java

a compiled language? The answer: Java is both. Its source code is compiled with a static compiler

into byte code and then, the JIT interprets the compilers output.

From the previous description, one might be tempted to conclude that the JIT hurts performance, as it

compiles during execution, a design that would seem to cause Java to run slowly. This conclusion,

while intuitive, is incorrect. The JIT actually performs quite well. At run time, it compiles byte code

into machine code, which is later run by the CPU. When Java executes, it executes machine code just

as native code does so it should run discounting the JITs compilation costs as fast as native

code does.

However, C/C++ is generally regarded as providing better performance than Java does. The on-the-fly

nature of Javas execution lends credence to this thinking. Compiling a piece of code into machine

code takes a non-trivial amount of time. If a language executes similar machine instructions but does

not need compilation prior to the execution of them, then one can assume that the language in

question runs faster than Java does even when Javas machine code output is maximally

optimized. However, Java isnt maximally optimized, but C++ is not maximally optimized either. No

compiler can match the performance of a top-notch assembly programmer who is an expert at

manipulating the targeted hardware so both Java and C++ can never be maximally optimized.

Making matters worse, both languages have inherent issues. Java must bear the reality that at least

some of its code has to be compiled every time that it runs, and C++ is, when it must be generic,

often not fully tailored to its hardware, as Java always is. Even with the aforementioned handicap, one

still expects Javas compilation cost to give C++ an edge in every scenario outside of very poorly

coded or compiled C++.

However, appearances are deceiving; the JVM is a very intelligent environment and the JIT an

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intelligence compiler. For example, the JVM knows which methods are called frequently, as it

maintains a count of each methods calls. When this count exceeds a certain threshold, the methods

machine code is kept, so that the JIT does not have to compile the method when it is called again a

decision that greatly reduces the cost of translating byte code into machine code. Furthering this

performance improvement, the JIT optimizes the most frequently used code. The JIT collects statistics

to determine which parts of code fit this mold. Intelligent optimization of this ilk can, as Research

indicates, yield big gains. Most research states that 80% of execution time is spent executing 20% of

the code (hot code) so optimizing these sections of code can yield sizeable performance benefits.

The JIT optimizes hot code. It executes these sections in a highly optimized way directly on the

operating system. To achieve this goal, the JIT uses statistics to identify the hot code. In fact, Java

performs even more optimization than efficiently executing hot code. It identifies the code that it

executes the most and recompiles to run more efficiently than it previously did. These optimizations

allow Java to achieve, in certain situations, better performance than C++ gives, without the need to

resort to dangerous compiler settings or to dangerous optimization techniques.

Published at DZone with permission of its author, A. N. M. Bazlur Rahman. (source)

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

Tags: compilation execution Java java jit performance Tips and Tricks

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Advait Trivedi replied on Thu, 2014/09/25 - 11:18pm

Hi Rahman,

Well written article.. short and simple.

Login or register to post comments

A. N. M. Bazlur... replied on Sun, 2014/10/19 - 12:33am

Thanks

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