native compiler : Java Glossary
- native compiler
aka AOT (Ahead Of Time). Compilers that compile to native code before they
are shipped to the customer. More usually called native compilers. Most compilers
generate JVM (Java Virtual Machine) byte codes. Native Code Compilers generate native *.exe files. JITs (Just In Times)
native code at execute time, but don’t create *.exe Examples are JET and gcj. There are eight ways to get what acts like a *.exe file without using a true native compiler using some sort of executable wrapper.
Advantages of Native Compilation
What are the advantages of a native compiler?
Visit any of the vendor sites, such as JET for independent benchmarks. The improvement
is even more marked for old clunker machines that your customers may use. Native compilations tend to me more
scalable, doing well as you add more threads. Depending on just what you are doing, you may get over twice
the speed of Java Hotspot.
A natively compiled program will load and start execution much faster than one using Java JVM. Customers
resent slow load times even more than slow execution times. They have nothing to do but sit there and wait.
JITed Java is precluded from many applications because of its extraordinarily slow load time. Natively
compiled Java opens the doors to all the traditional C applications.Java version 1.5 or later load time are vastly improved
and so the need to convert jars to native code is much less pressing than in past.
Self-Contained Distribution Package
Java, or even Java Web Start requires the customer to pre-install the JRE (Java Runtime Environment). You may not be legally permitted
to bundle that JRE with your app. You are most certainly not permitted to strip out the parts your app does
not need. The customer has to go get and install the JRE for himself. Native apps can be distributed in
familiar ways as totally self-contained packages. On the other hand, once Java Web Start is installed, the
end user can install new apps with a single click.
Java class files are quite tiny, but they need the entire JRE to make them work. If the customer has Java
already installed, then the class files packed in a jar are exceedingly compact. However, if he does not have
Java installed, that then the native distribution will be smaller than distributing tradiditional class files
plus a JRE.
Native apps, especially optimised apps, are much harder to decompile (reverse engineer).
The customer need not know your app is written in Java. Some customers are
Microsoft fanatics who follow the party line that Java is slow or evil.
Compilers like JET, bundle classes into DLLs (Dynamic Link Libraries). This means if you have two JVMs (Java Virtual Machines)
you need only one copy of the DLL (Dynamic Link Library) in RAM (Random Access Memory). This makes more efficient use of RAM.
Disadvantages of Native Compilation
What are the disadvantages of a native compiler:
Your code works on only one platform. Even if you buy a multiplatform compiler, you need to generate
different versions, one for each platform, and different install scripts for each platform. It is even more
import than ever to test the code on each platform.
The compiler is an extra cost item.
Distribution is more complex with DLLs
and EXEs and an install script. (JET comes with
an install generator, so you don’t necessarily have to buy Installshield or equivalent.)
DLL version hell
In place of the problem of mixed generations of JRE, you have the problem of mixed generations of support
on your client site.
Native compilation is more complex than standard compilation. You need to learn new skills to master it, e.g.
dealing with a mixture of static and dynamic class loading. It is quite simple. The only mildly complicated
thing is dealing with dynamic class loading.
You can’t use native compilers on Applets. In theory you could compile your
app as a DLL and make pure java hooks into it. You would have to design your app as a dll, sign it, arrange
for installation of the DLL. The game is hardly worth the candle.
Java Web Start
You can use native compilers with Java Web Start Weblets, but only after
unpacking jars, bypassing security restrictions and execing the native exe.
Most AOT native code projects have been abandoned. TowerJ and Webgain Café are defunct. Supercede was
sold to Jove, who went out of the business. I have heard nothing of
BulletTrain. I don’t think IBM (International Business Machines) with Visual Age or JBuilder
may have bundlers, but not native code generators.
So left standing are:
Excelsior JET, and gcj.