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Java Kernel Unmasked

Posted by enicholas on May 24, 2007 at 8:40 AM PDT

In my last entry, I briefly introduced the major features of the upcoming Consumer JRE. I'd like to now go into details on my pet project, code-named Java Kernel.


As previously mentioned, the idea is to create a 'minimal' JRE which has enough code to run System.out.println("Hello world!") and... well, that's about it. Every class or native library that isn't strictly necessary to boot up the JVM is excluded.

This minimal JRE has a few tricks up its sleeve, of course. It can detect when you try to access a class, such as javax.swing.JFrame, which isn't currently installed. It will then go download and install a "bundle" containing the required functionality. As far as your program can tell, nothing unusual happened -- it requested javax.swing.JFrame, it got javax.swing.JFrame. The only real difference is that (due to the required download) the classload took longer than usual.

User Interface

Naturally, we display a progress dialog for any downloads taking a meaningful amount of time. If you use a freshly-installed Kernel JRE to run a Java program, you'll see a dialog telling you that a few components are being downloaded, and then the program window will pop up and life will continue as normal.

You usually won't see any other progress dialogs -- most programs download everything they need before the main window shows up. Even with the ones that don't, Swing and AWT are by far the biggest bundles you will end up downloading, and both of them will be there before the main window appears. The other bundles are mostly quite small and won't involve an objectionable delay (and, of course, if the delay is short enough we don't pop up a dialog at all).

Other than this, the Kernel JRE looks and feels exactly like any other JRE.


The Kernel JRE is currently divided into a hundred or so different bundles. These bundles generally follow package boundaries -- if you touch any class in (say) java.rmi, the entire java.rmi package will be downloaded. This means you'll end up downloading more classes than strictly necessary to run your program, but the alternative, downloading classes one-by-one, would be ridiculously slow due to all of the individual HTTP requests involved. We are trying to strike the proper balance between reducing the number of bytes downloaded and reducing the number of HTTP requests made.

Some bundles involve more than one package. javax.swing, for example, is entirely useless without javax.swing.event and several other packages. Since they are so tightly interconnected, they are packaged together into a single bundle. A few bundles don't cleanly follow package lines. In java.awt, for example, it makes sense to separate out the subset of AWT used by Swing programs. A Swing program isn't likely to touch AWT components like java.awt.Button, so we have a separate bundle (internally named java_awt_core) which includes only the AWT classes that a typical Swing program would use.

Still not small enough...

We've got other space-saving tricks, as well. Take a look at one of the core, absolutely essential files in Java 6: jvm.dll. This is (obviously) the JVM itself, needed to run all Java code. It's 2.3MB. And that doesn't include any classes, launchers, the installer, the Java Plug-In, Java Web Start, or any of the other essential JRE features. When you're trying to deliver an entire JRE in under 2MB, the fact that one of the required files is 2.3MB puts you at a pretty severe disadvantage.

Compression helps, obviously, but it takes more than a good compressor to squeeze things down this small. Java Kernel has its own version of jvm.dll, which omits a lot of optional features like JVMTI and additional garbage collectors. The current prototype's jvm.dll is a much more svelte 1.1MB. And when the Kernel JRE finishes downloading itself in the background, it will swap in the good old full client JVM, so you won't be without these optional features for long.

Background Downloading

The Kernel JRE will continue to download its missing bundles in the background, whether they were specifically requested or not. Over a broadband connection, this will only take a couple of minutes, so the window of time during which you might run into missing bundles is brief.

After the last bundle is downloaded, the Kernel JRE will reassemble itself into an exact replica of the "normal" JRE. All of the disparate bundles will be repackaged into a unified rt.jar file, the Kernel JVM mentioned above will be replaced with the traditional client JVM, and so forth. A "finished" Kernel JRE will be byte-for-byte identical to a "normal" offline JRE.

But what if I want to pre-download everything I need?

The single most frequently asked question is "Can I force the Kernel JRE to go ahead and download everything I need, so that there are no pauses or download progress dialogs while my program is running?"

I mentioned during my JavaOne session that we were well aware of the need for this, and working on a solution, but that we weren't ready to discuss it yet. I'm pleased to announce that the plans for this have been finalized (well, as final as anything gets in the software industry...) and I can reveal them now.

The JDK will include a tool which allows you to assemble a "custom bundle" containing all of the classes and files needed by your particular program. You determine the entire set of JRE classes needed by your program (for instance by running java -verbose or by using a static analyzer) and then use this list to create the bundle.

(Command names and options likely to change)

> java -verbose -jar MyProgram.jar > class_list.txt
> jkernel -create -classes class_list.txt

You can then install this bundle into a freshly installed Kernel JRE:

> jkernel -install

You can run the jkernel -install command as part of your program's installation or startup. With a custom bundle installed, you can rely upon the absolute minimum set of classes and files needed to support your program, and thus get the smallest possible download size.

This isn't yet optimal for applets or web start programs, as (unlike standalone programs) they don't have the ability to install the bundle before they start to execute, and thus before any bundles are automatically downloaded. Ideally I'd like the ability to simply specify "And my program needs this custom bundle, also" in the applet tag or JNLP file somewhere -- the only question is whether we'll be able to get this into the first release or not.


Remember how the Java 6 jvm.dll is 2.3MB by itself?

The Kernel JRE's installer includes jvm.dll, the other native files and hundreds of classes needed to boot the JVM, the Java Plug-In, Java Web Start, java.exe, javaw.exe, javaws.exe, the installation code, and various support libraries needed to support the installer (such as unpack200).

And it's only 1.9MB.

If you build a custom bundle containing the classes required to run a typical Swing program, it comes out to about 1.5MB, for a total download of around 3.4MB for the JRE + custom bundle. Bigger programs might use as much as 4MB-5MB of the total JRE size, but it would be rare to exceed that.

Compared to the current JRE's size of somewhere between 10MB and 15MB, depending on how you measure it, hopefully you will agree that this is quite an improvement.

So, I'm sure you've got lots of questions for me. Shoot.

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