Java Runtime Without src.zip?

Question

Using Netbeans 18 and JDK 20, I've built a project that includes a Java runtime. But I notice the resulting JRE includes Java's src.zip file. Is this really needed? The application seems to run fine without it, and I'd love to shrink the distribution by src.zip's 53MB.

Answer 1

DISCLAIMER: Understand, this is a risky step, because taking away ANY files from the installation/shipping package might lead to runtime errors.

In your case, for example, someone could, during runtime, try to load source files from the src.zip file, manipulate the file, then try to compile it during runtime, and then load it via Reflection.

So there is always edge cases, ESPECIALLY when using Reflection and (custom) Class Loaders.

But as long as you stick to the basics, chances are near zero of running into those problems.

Now, to your question:

Necessary files for Java Runtime:

Actually, Java under Windows run with far less files:

Java 8u231 and before (still runs on WinXP):

bin/
    client/
        jvm.dll
        Xusage.txt
    java.dll
    java.exe
    msvcr100.dll
    verify.dll
    zip.dll
lib/
    i386/
        jvm.cfg
    rt.jar

From there on, all subsequent versions

• run with the server files (no matter if the JVM is put in server or client mode).
• from Java 9+ on, the lib/modules file replaces the lib/rt.jar

So these are the needed files from there on out (JDK 20.0.2 runtime files):

bin/
    server/
        jvm.dll
    java.dll
    java.exe
    jimage.dll
    jli.dll
lib/
    jvm.cfg
    modules

BUT, this is without class file verification, so you have to run your app with bin\java.exe -Xverify:none HelloWorld and get a warning, because it's deprecated.

To include file verification, all files you need are (JDK 20.0.2 runtime):

bin/
    server/
        jvm.dll
    java.dll
    java.exe
    jimage.dll
    jli.dll
    verify.dll
lib/
    jvm.cfg
    modules

Further optimization

Okay, so far, so good.

BUT: we can further cut down on the rt.jar (resp modules file), by running the app with java <app_name> --verbose:class. This goes even deeper than the java modules approach, because it is not module/package focused, but class focused.

(Make sure you run and re-run to cover all branches of your code)

JRE8u231 32bit

I have written a 'compiler' that takes the resulting list of classes, packages them into the rt.jar, and when compressed (7zip self-extracting, ultra compressed), my whole JRE8u231 package is 6 MB in size.

Why JRE8u231 32bit? Because it's the last Oracle Java release that runs on Windows XP without further magic. I'm using it as a beachhead to autmatically identify, download, and set up custom Java RT 'installations' and jvm configs from my server.

Later Java versions (9+)

Not sure if Your Netbeans packaging already considers the modules (using jlink), but it already might be doing that, reducing the size of the modules file from 131 MB to something smaller.

The modules file is the spiritual successor of rt.jar, but with the java modules system included. It can by analyzed by using the jimage tool: bin/jimage.exe list lib/modules.

I have not yet cracked the modules file format for the later Java versions, but once I get to that, I'll reduce the size of the modules file even more.

Until then, the current RT size (modules file is untouched) is 28 MB, but only including a HellWorld app.