The basic idea was that the current Java Runtime Environment is simply too big, and most programs require only a small subset of the functionality.
From Ethan Nicholas's Blog
Building a minimal JRE
The first thing I have to do is establish that this project is feasible. Remember that even though it has been approved, it could alway be un-approved (disapproved?) at any point in the future if things aren't looking good. So I figured I would start out by creating a simple, stripped-down JRE installer that contained only the functionality necessary to run System.out.println("Hello world!"), to get an estimate of the size reduction we could expect.
Stripping out the unecessary classes is easy -- you just run Java with the -verbose option to get a list of all of the classes it loads while running the Hello World program. Those classes are all that we need to include in rt.jar.The real problem is the rest of the functionality. My devel build of the Java 6 JRE contains 683 files totalling 119MB. Many of them are not necessary to run Hello World, but which ones? Determining which files were truly necessary and which weren't could be a tough job, so I made my computer determine it for me.
I wrote a simple program which would iterate through all of the files in the JRE. It would remove a file and then attempt to run the Hello World program using this stripped-down JRE. If the test succeeded, the file was evidently unnecessary. If the test failed, the file was deemed necessary and restored. After going through all of the files in this fashion, I was left with an extremely minimal JRE that could run Hello World and... well, that's about it. But it at least provided a starting point. Building a working installer from this JRE was itself a challenge, because several of the files that weren't necessary to run Hello World were still necessary to successful install the JRE, but I persevered and now have a fully functioning, minimal JRE.
I built two JREs using this methodology: one with a program that prints "Hello World" to System.out, and one with a program that displays an empty
java.awt.Frame. Here are the results:
Java 6 Runtime Environment: 15.5MB
"Hello World" JRE: 2.6MB
java.awt.Frame JRE: 3.5MB