Add optional artefact index reader for startup scanning

grails-core/src/main/groovy/grails/boot/config/ApplicationArtefactScanner.groovy

@@ -60,8 +60,14 @@ final class ApplicationArtefactScanner {
      * @return The classes that constitute the Grails application
      */
     static Collection<Class> scanApplicationClasses(Class<?> applicationClass, Collection<String> packageNames) {
-        Collection<Class> classes = new HashSet<>()
-        classes.addAll(new ClassPathScanner().scan(applicationClass, packageNames))
+        Collection<Class> classes = new LinkedHashSet<>()
+        Collection<Class> indexedClasses = ArtefactIndexReader.read(applicationClass, packageNames)
+        if (indexedClasses == null) {
+            classes.addAll(new ClassPathScanner().scan(applicationClass, packageNames))
+        }
+        else {
+            classes.addAll(indexedClasses)
+        }
         classes.addAll(loadTransformedClasses(applicationClass.classLoader))
         return classes
     }

grails-core/src/main/groovy/grails/boot/config/ArtefactIndexReader.java

@@ -0,0 +1,83 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one
+ *  or more contributor license agreements.  See the NOTICE file
+ *  distributed with this work for additional information
+ *  regarding copyright ownership.  The ASF licenses this file
+ *  to you under the Apache License, Version 2.0 (the
+ *  "License"); you may not use this file except in compliance
+ *  with the License.  You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing,
+ *  software distributed under the License is distributed on an
+ *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ *  KIND, either express or implied.  See the License for the
+ *  specific language governing permissions and limitations
+ *  under the License.
+ */
+package grails.boot.config;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.net.URL;
+import java.nio.charset.StandardCharsets;
+import java.util.Collection;
+import java.util.LinkedHashSet;
+import java.util.Set;
+
+import grails.io.IOUtils;
+
+/**
+ * Reads the optional application artefact index.
+ *
+ * <p>The index is UTF-8 text at {@value #RESOURCE_NAME}, with one fully qualified
+ * class name per nonempty line. Any unreadable or unresolvable entry rejects the
+ * complete index so callers can use their normal classpath scan.</p>
+ */
+final class ArtefactIndexReader {
+
+    static final String RESOURCE_NAME = "META-INF/grails/artefacts.idx";
+
+    private ArtefactIndexReader() {
+    }
+
+    static Collection<Class> read(Class<?> applicationClass, Collection<String> packageNames) {
+        try {
+            URL resource = new URL(IOUtils.findRootResource(applicationClass), RESOURCE_NAME);
+            Set<Class> classes = new LinkedHashSet<>();
+            return readResource(resource, applicationClass.getClassLoader(), packageNames, classes) ? classes : null;
+        } catch (IOException ignored) {
+            return null;
+        }
+    }
+
+    private static boolean readResource(URL resource, ClassLoader classLoader, Collection<String> packageNames, Set<Class> classes) {
+        try (InputStream inputStream = resource.openStream();
+             BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))) {
+            String className;
+            while ((className = reader.readLine()) != null) {
+                if (className.isEmpty()) {
+                    return false;
+                }
+                if (isInPackage(className, packageNames)) {
+                    classes.add(classLoader.loadClass(className));
+                }
+            }
+            return true;
+        } catch (IOException | ClassNotFoundException | LinkageError ignored) {
+            return false;
+        }
+    }
+
+    private static boolean isInPackage(String className, Collection<String> packageNames) {
+        for (String packageName : packageNames) {
+            if (packageName != null && (packageName.isEmpty() ? !className.contains(".") : className.startsWith(packageName + "."))) {
+                return true;
+            }
+        }
+        return false;
+    }
+}