Make deployment class loader resettable and coordinate the reset via a BuildItem

core/deployment/src/main/java/io/quarkus/deployment/builditem/AdditionalClassLoaderResourcesBuildItem.java

@@ -0,0 +1,19 @@
+package io.quarkus.deployment.builditem;
+
+import java.util.Map;
+
+import io.quarkus.builder.item.MultiBuildItem;
+
+public final class AdditionalClassLoaderResourcesBuildItem extends MultiBuildItem {
+
+    final Map<String, byte[]> resources;
+
+    public AdditionalClassLoaderResourcesBuildItem(Map<String, byte[]> resources) {
+        this.resources = resources;
+    }
+
+    public Map<String, byte[]> getResources() {
+        return resources;
+    }
+
+}

core/deployment/src/main/java/io/quarkus/deployment/steps/AdditionalClassLoaderResourcesBuildStep.java

@@ -0,0 +1,45 @@
+package io.quarkus.deployment.steps;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+
+import io.quarkus.bootstrap.classloading.QuarkusClassLoader;
+import io.quarkus.deployment.annotations.BuildProducer;
+import io.quarkus.deployment.annotations.BuildStep;
+import io.quarkus.deployment.builditem.AdditionalClassLoaderResourcesBuildItem;
+import io.quarkus.deployment.builditem.AdditionalIndexedClassesBuildItem;
+
+public class AdditionalClassLoaderResourcesBuildStep {
+
+    @BuildStep
+    void appendAdditionalClassloaderResources(BuildProducer<AdditionalIndexedClassesBuildItem> producer,
+            List<AdditionalClassLoaderResourcesBuildItem> additionalResources) {
+
+        if (!additionalResources.isEmpty()) {
+            QuarkusClassLoader cl = (QuarkusClassLoader) Thread.currentThread().getContextClassLoader();
+
+            Map<String, byte[]> collected = new LinkedHashMap<String, byte[]>();
+            List<String> additionalClassesToIndex = new ArrayList<String>();
+            for (AdditionalClassLoaderResourcesBuildItem item : additionalResources) {
+
+                for (Entry<String, byte[]> entry : item.getResources().entrySet()) {
+                    additionalClassesToIndex.add(entry.getKey());
+
+                    collected.put(entry.getKey(), entry.getValue());
+                    // add it also as resources to allow index to work properly
+                    collected.put(entry.getKey().replace('.', '/') + ".class", entry.getValue());
+
+                }
+            }
+
+            cl.reset(collected, Collections.emptyMap());
+            // produce the AdditionalIndexedClassesBuildItem so this build step
+            // is actually invoked and allow to directly index all the classes
+            producer.produce(new AdditionalIndexedClassesBuildItem(additionalClassesToIndex.stream().toArray(String[]::new)));
+        }
+    }
+}

independent-projects/bootstrap/core/src/main/java/io/quarkus/bootstrap/app/CuratedApplication.java

@@ -256,6 +256,8 @@ public QuarkusClassLoader createDeploymentClassLoader() {
             builder.addElement(ClassPathElement.fromPath(root));
         }
 
+        builder.setResettableElement(new MemoryClassPathElement(Collections.emptyMap()));
+
         //additional user class path elements first
         for (AdditionalDependency i : quarkusBootstrap.getAdditionalApplicationArchives()) {
             for (Path root : i.getArchivePath()) {