Move some initialization steps (especially reflection) to Plugin's static initialization

src/main/java/de/thetaphi/forbiddenapis/gradle/ForbiddenApisPlugin.java

@@ -20,10 +20,15 @@
 import groovy.lang.GroovyCodeSource;
 import groovy.util.DelegatingScript;
 
+import java.lang.reflect.Field;
+import java.lang.reflect.Modifier;
 import java.net.URL;
 import java.nio.charset.StandardCharsets;
 import java.security.AccessController;
 import java.security.PrivilegedAction;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 
 import org.codehaus.groovy.control.CompilerConfiguration;
 import org.codehaus.groovy.control.customizers.ImportCustomizer;
@@ -61,6 +66,12 @@ public class ForbiddenApisPlugin implements Plugin<Project> {
   /** Minimum Gradle version this plugin requires to run (v3.2). */
   public static final GradleVersion MIN_GRADLE_VERSION = GradleVersion.version("3.2");
 
+  /** True, if this version of Gradle supports task avoidance API (&gt;=v4.9). */
+  public static final boolean TASK_AVOIDANCE_AVAILABLE = GradleVersion.current().compareTo(GradleVersion.version("4.9")) >= 0;
+
+  /** All properties that our ForbiddenApisExtension provides. Used by plugin init script to create convention mapping. */
+  public static final List<String> FORBIDDEN_APIS_EXTENSION_PROPS = determineExtensionProps();
+
   /** Java Package that contains the Gradle Daemon (needed to detect it on startup). */
   private static final String GRADLE_DAEMON_PACKAGE = "org.gradle.launcher.daemon.";
 
@@ -97,6 +108,17 @@ public Class<? extends DelegatingScript> run() {
     });
   }
 
+  private static List<String> determineExtensionProps() {
+    final List<String> props = new ArrayList<>();
+    for (final Field f : CheckForbiddenApisExtension.class.getDeclaredFields()) {
+      final int mods = f.getModifiers();
+      if (Modifier.isPublic(mods) && !f.isSynthetic() && !Modifier.isStatic(mods)) {
+        props.add(f.getName());
+      }
+    }
+    return Collections.unmodifiableList(props);
+  }
+
   private static boolean isGradleDaemon() {
     // see: http://stackoverflow.com/questions/23265217/how-to-know-whether-you-are-running-inside-a-gradle-daemon
     if (System.getProperty("sun.java.command", "").startsWith(GRADLE_DAEMON_PACKAGE)) {

src/main/resources/de/thetaphi/forbiddenapis/gradle/plugin-init.groovy

@@ -20,16 +20,10 @@ import java.lang.reflect.Modifier;
 import org.gradle.api.plugins.JavaBasePlugin;
 import org.gradle.util.GradleVersion;
 
-final boolean TASK_AVOIDANCE_AVAILABLE = GradleVersion.current() >= GradleVersion.version("4.9");
-
 project.plugins.apply(JavaBasePlugin.class);
 
 // create Extension for defaults:
 def extension = project.extensions.create(FORBIDDEN_APIS_EXTENSION_NAME, CheckForbiddenApisExtension.class, project);
-def extensionProps = CheckForbiddenApisExtension.class.declaredFields.findAll{ f -> 
-  int mods = f.modifiers;
-  return Modifier.isPublic(mods) && !f.synthetic && !Modifier.isStatic(mods)
-}*.name;
 
 // Create a convenience task for all checks (this does not conflict with extension, as it has higher priority in DSL):
 def forbiddenTask = TASK_AVOIDANCE_AVAILABLE ? project.tasks.register(FORBIDDEN_APIS_TASK_NAME) : project.tasks.create(FORBIDDEN_APIS_TASK_NAME)
@@ -48,7 +42,7 @@ project.sourceSets.all{ sourceSet ->
     dependsOn(sourceSet.output);
     outputs.upToDateWhen { true }
     conventionMapping.with{
-      extensionProps.each{ key ->
+      FORBIDDEN_APIS_EXTENSION_PROPS.each{ key ->
         map(key, { extension[key] });
       }
       classesDirs = { sourceSet.output.hasProperty('classesDirs') ? sourceSet.output.classesDirs : project.files(sourceSet.output.classesDir) }