native code compilers : Java Glossary

Java Glossary : native code compilers

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Most compilers generate JVM byte codes. Native Code Compilers will optionally generate native *.exe files. JITs create native code at execute time, but don't create *.exe files. The Microsoft SDK for Java includes a tool called jexegen which turns your .class file(s) into an .exe file. It doesn't compile them or convert them to machine code; it just stuffs the contents of the .class files into the .exe file along with some code to pass those bytecodes to the MS JVM to be interpreted. The resulting exe files need access to the Microsoft JVM to work, unlike true native exe files which are standalone and written in machine code. download it as part of the Microsoft SDK for Java 4. There is a similar product called Java2exe.

Here are eight ways to get what acts like a Java executable file:

Use a native compiler. This is the only method that gives you a true machine code executable. You may still have to distribute some DLLs or the JRE along with your executable.
Set up a shortcut with the appropriate java.exe command to load the class.
Bundle your classes in a jar with the Main-Class attribute. Set up an association for *.jar to make it executable.
Write yourself a tiny C program that exec's a Java command to load your class. Assuming the jar file Foo.jar with main class Bar, here how to making a Foo.exe tiny kickoff program. Compile the following program foo.c to foo.exe:
If you want to play really strange games, concatenate the jar file onto the end of your foo.exe kicker, and put the EXE file on the classpath where you would normally need to place foo.jar. This way you can distribute but a single file, albeit uncompressed. Reputedly java.exe will happily treat this combo as a standard jar. Executing the foo.exe just ignores the jar tacked on the end. e.g. change the foo.c code to read
then
```
rename foo.exe littlefoo.exe
copy /b littlefoo.exe + foo.jar foo.exe
```
You only need distribute foo.exe.
Use the InstallAnywhere NOW installer (or other installer). It uses a standard platform-dependent EXE kickoff file for a standard interpreted platform-independent Java application packaged in a jar file.
Use Microsoft's Jexegen bundler. It only works with the Microsoft JVM. Webgain Café also works this way as does J2Exe.
Distribute using Java Web Start. Users can click on menu or desktop items to kick off pure java Programs.
Use exe4J bundle up your jar file and a copy of the JRE into one big installable exe file. Your program runs in the standard Sun Hotspot JIT.

What are the advantages of a native compiler?

Execution speed
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 1.4.1 Hotspot.
Load speed
A natively compiled program will load and start execution much faster than one using Java 1.4.1 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.
Self-Contained Distribution Package
Java, or even Java Web Start requires the customer to pre-install the JRE. You are legally not permitted to bundle that JRE with your app. The customer has to go get it 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.
Distribution size
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.
Security
Native apps, especially optimised apps, are much harder to decompile (reverse engineer).
Prejudice
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.
Code Sharing
Compilers like Jet, bundle classes into DLLs. This means if you have two JVMs running, you need only one copy of the DLL in RAM. This makes more efficient use of RAM.

What are the disadvantages of a native compiler:

Single Platform
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.
Cost
The compiler is an extra cost item.
Complexity
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 DLLs on your client site.
Learning Curve
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 in to 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.
You can use native compilers with Java Web Start Weblets, but only after unpacking jars, bypassing security restrictions and execing the native exe.

Most native code compilers are for Windows, though TowerJ creates native code for various Unices. Supercede has been sold to the Jove people, leaving the floor to Jet, BulletTrain, IBM Visual Age and JBuilder, logo Excelsior JET, GNU Java Compiler for Linux; Jove, Webgain Café, TowerJ and gcj.