Because the resource requirements for Java Standard Edition are by no means insignificant, developers interested in utilizing Java on smaller platforms have traditionally opted for one of the Java Micro Edition configurations.   It should therefore come as no surprise that some of the Standard Edition functionality has to be sacrificed in these constrained Java ME environments.  However, as the trend towards more capable devices continues, it becomes reasonable to once again consider the advantages of utilizing Java SE.  Unfortunately, with a static footprint that could easily exceed 100MB, Java SE may still be too large to swallow for a lot of applications.

It is duly noted that the criticism leveled at Sun for exacting too much control over the Java platform has been widespread.  Like it or not though, one benefit of Sun's initial stewardship has been that Java SE has remained a standard platform, and threats to splinter it have thus far been reasonably thwarted.  Accordingly, in order to avoid incompatibilities, there are restrictions spelled out in the Java SE Licensing Agreement which prohibit modification of the Java SE binary distribution.

That  being said, there are a list of optional files, specified by the Java Runtime Environment's README file which can be removed, without ramification, to lessen the footprint.  They include:

• Deployment tools (e.g. Java Web Start, Java plugin)
• IDL and RMI tools (e.g. rmid, tnameserv)
  
• Security tools (e.g. policytool, keytool)
• orbd
• Localization (charsets.jar)

In addition, further space optimization can be achieved by compressing the class library files contained in the rt.jar file.  By default, Java SE ships this jar file uncompressed.  The tradeoff here is space vs. performance, i.e. the classloader must expend cycles to uncompress the Java classes as they are being loaded.

An Example

So let's download a sample JRE from java.sun.com and see how it's footprint can be minimized.   For this example, we'll use Java SE 1.5.0 Update 13 for Linux x86.

After installation, the JRE is approximately 88MB

   jimc@jalopy:/tmp> du -sk ./jre1.5.0_13/
   88185   ./jre1.5.0_13/

Here's what it looks like after removal of the optional files

   jimc@jalopy:/tmp> cd jre1.5.0_13/

   jimc@jalopy:/tmp/jre1.5.0_13> /bin/rm -rf lib/charsets.jar lib/ext/sunjce_provider.jar \\
   lib/ext/localedata.jar lib/ext/ldapsec.jar lib/ext/dnsns.jar bin/rmid \\
   bin/rmiregistry bin/tnameserv bin/keytool bin/kinit bin/klist bin/ktab \\
   bin/policytool bin/orbd bin/servertool bin/javaws, lib/javaws/ and lib/javaws.jar

   jimc@jalopy:/tmp/jre1.5.0_13> cd ..
   jimc@jalopy:/tmp> du -sk ./jre1.5.0_13/
   77227   ./jre1.5.0_13/

And after rt.jar has been compressed

   jimc@jalopy:/tmp> mkdir rtjar
   jimc@jalopy:/tmp> cd rtjar/

   jimc@jalopy:/tmp/rtjar> jar -xf /tmp/jre1.5.0_13/lib/rt.jar
   jimc@jalopy:/tmp/rtjar> zip -q -r /tmp/rt .
   jimc@jalopy:/tmp/rtjar> mv /tmp/rt.zip /tmp/jre1.5.0_13/lib/rt.jar

   jimc@jalopy:/tmp/rtjar> du -sk /tmp/jre1.5.0_13/
   59358   /tmp/jre1.5.0_13/  

Conclusion

In many cases, you can lop off about a third of the Java Runtime Environment footprint with no ill effects.  In a future post, we'll discuss how Sun has further reduced Java SE significantly, not only from the point of view of static footprint, but also from a RAM usage perspective too.  For a preview you can check out Sun's Java SE Embedded technology.

Index