Use app classloader instead of platform classloader as parent

[SirYwell]

When using the platform classloader as parent, service loaders fail to look up services that are defined in a jdk module.

As an example, consider the following onEnable of a plugin:

    @Override
    public void onEnable() {
        System.out.println(RandomGenerator.getDefault());
    }

this will result in an exception:

[12:41:12 ERROR]: Error occurred while enabling Paper-Test-Plugin v1.0.0-SNAPSHOT (Is it up to date?)
java.lang.IllegalArgumentException: No implementation of the random number generator algorithm "L32X64MixRandom" is available
        at java.util.random.RandomGeneratorFactory.findProvider(RandomGeneratorFactory.java:229) ~[?:?]
        at java.util.random.RandomGeneratorFactory.of(RandomGeneratorFactory.java:257) ~[?:?]
        at java.util.random.RandomGenerator.of(RandomGenerator.java:124) ~[?:?]
        at java.util.random.RandomGenerator.getDefault(RandomGenerator.java:139) ~[?:?]
        at io.papermc.testplugin.TestPlugin.onEnable(TestPlugin.java:12) ~[test-plugin-1.0.0-SNAPSHOT.jar:?]
        at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:287) ~[paper-api-1.20.4-R0.1-SNAPSHOT.jar:?]
        at io.papermc.paper.plugin.manager.PaperPluginInstanceManager.enablePlugin(PaperPluginInstanceManager.java:188) ~[paper-1.20.4.jar:git-Paper-436]
        at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.enablePlugin(PaperPluginManagerImpl.java:104) ~[paper-1.20.4.jar:git-Paper-436]
        at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:507) ~[paper-api-1.20.4-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.craftbukkit.v1_20_R3.CraftServer.enablePlugin(CraftServer.java:639) ~[paper-1.20.4.jar:git-Paper-436]
        at org.bukkit.craftbukkit.v1_20_R3.CraftServer.enablePlugins(CraftServer.java:550) ~[paper-1.20.4.jar:git-Paper-436]
        at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:275) ~[paper-1.20.4.jar:git-Paper-436]
        at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1131) ~[paper-1.20.4.jar:git-Paper-436]
        at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:319) ~[paper-1.20.4.jar:git-Paper-436]
        at java.lang.Thread.run(Thread.java:1583) ~[?:?]

This is because the service loader doesn't find the proper implementations with the platform classloader being the direct parent of the context classloader. With this change, the service loader can properly look up the providers in the jdk.random module.

The behavior prior and after the change is the same on Java 17 and Java 21 respectively.

Some alternatives:

• Plugins set the context classloader to the app classloader before accessing code that loads services from a jdk module
• Plugins use reflections to load such services
• Instead of using a URLClassLoader, use a custom ModuleLayer and load the jars as modules
• Preload known services before calling the main method of the server

[yannicklamprecht]

Likely related bug. PaperMC/Paper#10114

[mcmdev]

I'm attempting to use a custom Log4J appender for my server by adding the appender to the classpath. This is currently not possible. Merging this PR would also solve that issue.

[salatmaster]

This is also relevant for me, I can’t add log4j-layout-template-json