Class loading | Together Java | Page 1

dawn dove Oct 24, 2024, 1:53 PM

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I have a large project for which I have an API. The API is standalone and is shaded into my main project. I then load any addons with this code:

        try {
            URLClassLoader loader = new URLClassLoader(new URL[] { file.toURI().toURL() }, Helix.class.getClassLoader());
            try (loader) {
                YmlConfig yml = YmlConfig.parse(loader.getResourceAsStream("addon.yml"));
                if (!yml.contains("addon")) {
                    throw new AddonException("no 'addon' section for addon '" + file.getPath() + "'");
                }

                Class<?> main = loader.loadClass(yml.get("addon").getString("main"));
                return new Addon((HelixAddon) Reflections.getInstance(main));
            }
        } catch (Throwable e) {
            throw new AddonException(e);
        }

Though while loading this addon I get the following error

package com.sniskus.helix.addon;

import com.sniskus.helix.api.HelixAddon;
import com.sniskus.helix.api.attribute.Domain;
import com.sniskus.helix.api.attribute.element.Function;

public class Test implements HelixAddon {

    @Override
    public void onLoad() {
        HelixApi.getApi().getDomain(this).register(new Function<Void, Void>() {
                    // code (i've hit character limit lol)
        });
    }
}

error: ```
[15:46:53 WARN]: java.lang.NoClassDefFoundError: com/sniskus/helix/addon/Test$1
[15:46:53 WARN]: at com.sniskus.helix.addon.Test.onLoad(Test.java:34)
[15:46:53 WARN]: at Helix-3.2.1.jar//com.sniskus.helix.api.impl.ApiImpl.lambda$0(ApiImpl.java:31)
[15:46:53 WARN]: at java.base/java.util.HashMap$KeySet.forEach(HashMap.java:1016)
[15:46:53 WARN]: at Helix-3.2.1.jar//com.sniskus.helix.api.impl.ApiImpl.loadAddons(ApiImpl.java:29)
[15:46:53 WARN]: at Helix-3.2.1.jar//com.sniskus.helix.Helix.onPluginEnable(Helix.java:115)
[15:46:53 WARN]: at Yggdrasil-2.1.1.jar//com.sniskus.yggdrasil.plugin.YPlugin.onEnable(YPlugin.java:40)
[15:46:53 WARN]: at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:288)
[15:46:53 WARN]: at io.papermc.paper.plugin.manager.PaperPluginInstanceManager.enablePlugin(PaperPluginInstanceManager.java:202)
[15:46:53 WARN]: at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.enablePlugin(PaperPluginManagerImpl.java:109)
[15:46:53 WARN]: at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:520)
[15:46:53 WARN]: at org.bukkit.craftbukkit.CraftServer.enablePlugin(CraftServer.java:640)
[15:46:53 WARN]: at org.bukkit.craftbukkit.CraftServer.enablePlugins(CraftServer.java:589)
[15:46:53 WARN]: at net.minecraft.server.MinecraftServer.loadWorld0(MinecraftServer.java:753)
[15:46:53 WARN]: at net.minecraft.server.MinecraftServer.loadLevel(MinecraftServer.java:515)
[15:46:53 WARN]: at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:329)
[15:46:53 WARN]: at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1214)
[15:46:53 WARN]: at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:329)
[15:46:53 WARN]: at java.base/java.lang.Thread.run(Thread.java:1583)
[15:46:53 WARN]: Caused by: java.lang.ClassNotFoundException: com.sniskus.helix.addon.Test$1
[15:46:53 WARN]: at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:445)
[15:46:53 WARN]: at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:593)
[15:46:53 WARN]: at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
[15:46:53 WARN]: ... 18 more

stuck raptorBOT Oct 24, 2024, 1:53 PM

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dawn dove I have a large project for which I have an API. The API is standalone and is sha...

Detected code, here are some useful tools:

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<@&987246652869971988> please have a look, thanks.

dawn dove Oct 24, 2024, 1:55 PM

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The Function and HelixAddon are both in the same project. And since HelixAddon apparently exists I can't figure out why Function does not...

onyx socket Oct 24, 2024, 2:11 PM

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dawn dove The `Function` and `HelixAddon` are both in the same project. And since `HelixA...

Because inner classes get compiled into separate .class files. Your code loads just the main class.

dawn dove Oct 24, 2024, 2:13 PM

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Ah ok. So I how’d I solve it? Do I have to load all classes in the addon manually?

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That’s seems unresonable.

onyx socket Oct 24, 2024, 2:14 PM

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Where does the file variable point to?

dawn dove Oct 24, 2024, 2:14 PM

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To the addon .jar.

onyx socket Oct 24, 2024, 2:14 PM

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And all related classes are contained within it, right?

dawn dove Oct 24, 2024, 2:14 PM

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Correct.

onyx socket Oct 24, 2024, 2:15 PM

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Use URLClassLoader#loadClass(Class<?>, boolean) overload.

dawn dove Oct 24, 2024, 2:16 PM

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Where ’cls’ is what? The main class?

onyx socket Oct 24, 2024, 2:16 PM

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Just pass true to the loader.loadClass.

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The second argument decides whether the class should be resolved or not. Resolution step is also responsible for ensuring all dependent classes are loaded as well

dawn dove Oct 24, 2024, 2:16 PM

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Ah ok, got it! I’ll give it a go!

rugged ivy Oct 24, 2024, 2:19 PM

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dawn dove I have a large project for which I have an API. The API is standalone and is sha...

May I ask why you do it dynamically, at runtime
Also dont catch Throwable please

onyx socket Oct 24, 2024, 2:21 PM

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A fair question btw ☝️

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Maybe using an SPI would be sufficient for your task?

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Instead of manually loading jar files, you can simply add them to classpath. java.util.ServiceLoader will do the rest for you. Read its javadoc for more details

dawn dove Oct 24, 2024, 2:24 PM

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I don’t really know how else to go about doing it? Not familiar with SPI?
Also I know catching throwables is a big no no, but if an error is thrown in the constructor of the addon my entire software will crash etc.. I usually don’t catch throwables but since I don’t know what the addons do or what they throw I catch it all to save my plugin.

onyx socket Oct 24, 2024, 2:25 PM

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I don’t really know how else to go about doing it? Not familiar with SPI?
https://www.baeldung.com/java-spi

Baeldung

Java Service Provider Interface | Baeldung

Have a look at the components of the Java Service Provider Interface with a practical use case.

onyx socket Oct 24, 2024, 2:26 PM

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dawn dove I don’t really know how else to go about doing it? Not familiar with SPI? Also I...

Some Errors are extended from Throwable, including ones you must never catch. Catching just Exception is not enough?

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TBH you must never catch any Errors

dawn dove Oct 24, 2024, 2:28 PM

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onyx socket Some `Error`s are extended from `Throwable`, including ones you must never catch...

I'm aware error extend throwable. Ig in theory catching exception would suffice, but again, if an error does occur that would screw up my entire software.

dawn dove Oct 24, 2024, 2:28 PM

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onyx socket > I don’t really know how else to go about doing it? Not familiar with SPI? http...

Thanks! I'll look into it :)

dawn dove Oct 24, 2024, 2:36 PM

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onyx socket Use `URLClassLoader#loadClass(Class<?>, boolean)` overload.

The method URLClassLoader#loadClass(Class<?>, boolean) is URLClassLoader#loadClass(String, boolean) and is protected.

lucid edge Oct 24, 2024, 2:38 PM

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dawn dove I have a large project for which I have an API. The API is standalone and is sha...

im guessing its because the jar isnt being loaded directly into AppClassLoader(or whatever classloader paper/spigot uses), you would need to do some reflection trickery for that

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or not maybe

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hmm

onyx socket Oct 24, 2024, 2:45 PM

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dawn dove The method `URLClassLoader#loadClass(Class<?>, boolean)` is `URLClassLoader#load...

You're right. I was looking at completely different method, incorrectly assuming it was the correct one. But I'm on the right track, I think.

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I originally wanted to suggest using Class#forName, but since you were using a custom class loader, I thought it won't work. But it actually has an overload that does exactly what is required: Class#forName(String name, boolean initialize, ClassLoader loader).

dawn dove Oct 24, 2024, 2:50 PM

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omfg I'm dumb. I initially had that but came across another class error so tried ClassLoader#loadClass. Thank you :)

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Problem solved!

onyx socket Oct 24, 2024, 2:56 PM

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Haha yeah :^)

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You learn something new every day. You're welcome

dawn dove Oct 24, 2024, 2:59 PM

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Hold on. I'm back 💀

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I messed around with SPIs a bit to see if I could use it, by mistake i disabled my Addon loading code so the addon was not loaded and consequently no exception was thrown lol. I'm still stuck xd

rugged ivy Oct 24, 2024, 4:03 PM

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dawn dove I don’t really know how else to go about doing it? Not familiar with SPI? Also I...

so ? why do you need to do it dynamically ?

dawn dove Oct 24, 2024, 4:17 PM

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rugged ivy so ? why do you need to do it dynamically ?

I have a folder to put addon .jars in. They have to be loaded after my software has initialized all registries etc. so that they can hook properly,

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I don’t really understand your question but think that was it?

dawn dove Oct 24, 2024, 7:06 PM

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rugged ivy so ? why do you need to do it dynamically ?

I looked into the SPI some more and got it working. So much cleaner than the previous solution! Thank you!!

stuck raptorBOT Oct 24, 2024, 7:06 PM

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Closed the thread.

#Class loading