Child-first mod class loading

core/src/mindustry/core/Platform.java

@@ -21,9 +21,29 @@ import static mindustry.Vars.*;
 
 public interface Platform{
 
-    /** Dynamically creates a class loader for a jar file. */
+    /** Dynamically creates a class loader for a jar file. This loader must be child-first. */
     default ClassLoader loadJar(Fi jar, ClassLoader parent) throws Exception{
-        return new URLClassLoader(new URL[]{jar.file().toURI().toURL()}, parent);
+        return new URLClassLoader(new URL[]{jar.file().toURI().toURL()}, parent){
+            @Override
+            protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException{
+                //check for loaded state
+                Class<?> loadedClass = findLoadedClass(name);
+                if(loadedClass == null){
+                    try{
+                        //try to load own class first
+                        loadedClass = findClass(name);
+                    }catch(ClassNotFoundException e){
+                        //use parent if not found
+                        loadedClass = super.loadClass(name, resolve);
+                    }
+                }
+
+                if(resolve){
+                    resolveClass(loadedClass);
+                }
+                return loadedClass;
+            }
+        };
     }
 
     /** Steam: Update lobby visibility.*/

core/src/mindustry/mod/ModClassLoader.java

@@ -11,25 +11,23 @@ public class ModClassLoader extends ClassLoader{
     }
 
     @Override
-    protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException{
-        //always try the superclass first
-        try{
-            return super.loadClass(name, resolve);
-        }catch(ClassNotFoundException error){
-            //a child may try to delegate class loading to its parent, which is *this class loader* - do not let that happen
-            if(inChild) throw error;
-            int size = children.size;
-            //if it doesn't exist in the main class loader, try all the children
-            for(int i = 0; i < size; i++){
-                try{
-                    inChild = true;
-                    var out = children.get(i).loadClass(name);
-                    inChild = false;
-                    return out;
-                }catch(ClassNotFoundException ignored){
-                }
+    protected Class<?> findClass(String name) throws ClassNotFoundException{
+        //a child may try to delegate class loading to its parent, which is *this class loader* - do not let that happen
+        if(inChild) throw new ClassNotFoundException(name);
+
+        ClassNotFoundException last = null;
+        int size = children.size;
+        //if it doesn't exist in the main class loader, try all the children
+        for(int i = 0; i < size; i++){
+            try{
+                inChild = true;
+                var out = children.get(i).loadClass(name);
+                inChild = false;
+                return out;
+            }catch(ClassNotFoundException e){
+                last = e;
             }
-            throw error;
         }
+        throw (last == null ? new ClassNotFoundException(name) : last);
     }
 }