[FIXED] What is considered as Java core module?

Issue

I'm reading through JDK9 release notes and found a small lack of information.

In one of the paragraphs, there is written:

The classes in many non-core modules are now defined to the platform class loader rather than the boot class loader. This may impact code that creates class loaders with null as the parent class loader and assumes that all platform classes are visible to the parent class loader.

Based on this paragraph I tried to find out what are core and non-core modules in Java, but can not find any answer.

I also tried to run a simple program to check classloaders of classes from different modules, but so far sometimes it returned null (which means boot classloader) and sometimes it returned PlatformClassLoader.

I do not want to check all modules manualy. Is there available some list of modules which are considered as core and non-core?

Thank you.

Solution

It is easy to check all modules manually, using ModuleLayer.boot().modules().
For completeness, you should also add the --add-modules=ALL-MODULE-PATH VM option on the command line:

public class ModuleCL {
    public static void main(String[] args) {
        Map<ClassLoader, Set<Module>> moduleMap = new HashMap<>();
        for (var m : ModuleLayer.boot().modules()) {
            ClassLoader cl = m.getClassLoader();
            Set<Module> modules = moduleMap.computeIfAbsent(cl, __ -> new HashSet<>());
            modules.add(m);
        }
        for (var e : moduleMap.entrySet()) {
            System.out.println(e.getKey());
            for (var m : e.getValue()) {
                System.out.println("    " + m);
            }
        }
    }
}

Which outputs on (my machine) with Java 18:

null
    module java.rmi
    module java.xml
    module java.datatransfer
    module jdk.nio.mapmode
    module jdk.jfr
    module jdk.naming.rmi
    module java.naming
    module java.management.rmi
    module jdk.net
    module jdk.management.jfr
    module java.management
    module java.logging
    module jdk.sctp
    module java.security.sasl
    module jdk.management.agent
    module java.instrument
    module jdk.unsupported
    module java.base
    module java.desktop
    module java.prefs
    module jdk.management
jdk.internal.loader.ClassLoaders$PlatformClassLoader@2f2c9b19
    module java.net.http
    module java.transaction.xa
    module jdk.dynalink
    module java.scripting
    module jdk.crypto.mscapi
    module jdk.crypto.ec
    module jdk.localedata
    module jdk.security.jgss
    module jdk.jsobject
    module java.security.jgss
    module java.sql.rowset
    module jdk.accessibility
    module jdk.zipfs
    module java.xml.crypto
    module java.sql
    module jdk.naming.dns
    module jdk.charsets
    module java.smartcardio
    module java.compiler
    module jdk.security.auth
    module jdk.xml.dom
    module jdk.httpserver
    module jdk.crypto.cryptoki
jdk.internal.loader.ClassLoaders$AppClassLoader@1d44bcfa
    module jdk.attach
    module jdk.internal.le
    module jdk.jpackage
    module jdk.internal.opt
    module jdk.jdeps
    module jdk.compiler
    module jdk.jartool
    module jdk.javadoc
    module jdk.internal.ed
    module jdk.jlink
    module jdk.internal.jvmstat
    module jdk.editpad
    module jdk.random
    module jdk.jdwp.agent
    module jdk.jshell
    module jdk.unsupported.desktop
    module jdk.jconsole
    module jdk.jstatd
    module jdk.jdi

This file defines the mapping of the different modules to the different loaders

Answered By - Johannes Kuhn

Answer Checked By - Senaida (PHPFixing Volunteer)