diff --git a/core/src/main/java/hudson/ExtensionFinder.java b/core/src/main/java/hudson/ExtensionFinder.java
index 92e524e04d..62234e0692 100644
--- a/core/src/main/java/hudson/ExtensionFinder.java
+++ b/core/src/main/java/hudson/ExtensionFinder.java
@@ -486,12 +486,6 @@ public abstract class ExtensionFinder implements ExtensionPoint {
                     return;
                 }
                 try {
-                    ClassLoader ecl = c.getClassLoader();
-                    if (ecl != null) { // Not bootstrap classloader
-                        Method m = ClassLoader.class.getDeclaredMethod("resolveClass", Class.class);
-                        m.setAccessible(true);
-                        m.invoke(ecl, c);
-                    }
                     for (Class<?> cc = c; cc != Object.class && cc != null; cc = cc.getSuperclass()) {
                         /*
                          * See {@link com.google.inject.spi.InjectionPoint#getInjectionPoints(TypeLiteral, boolean, Errors)}
diff --git a/core/src/main/java/hudson/PluginFirstClassLoader.java b/core/src/main/java/hudson/PluginFirstClassLoader.java
index dde635d24d..51ef789da0 100644
--- a/core/src/main/java/hudson/PluginFirstClassLoader.java
+++ b/core/src/main/java/hudson/PluginFirstClassLoader.java
@@ -84,7 +84,7 @@ public class PluginFirstClassLoader
     }
 
     @Override
-    protected Enumeration findResources( String name )
+    public Enumeration findResources( String name )
         throws IOException
     {
         return super.findResources( name );
diff --git a/core/src/main/java/jenkins/ClassLoaderReflectionToolkit.java b/core/src/main/java/jenkins/ClassLoaderReflectionToolkit.java
index 052fc7bd8f..06a61b879b 100644
--- a/core/src/main/java/jenkins/ClassLoaderReflectionToolkit.java
+++ b/core/src/main/java/jenkins/ClassLoaderReflectionToolkit.java
@@ -4,41 +4,29 @@ import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.net.URL;
+import java.net.URLClassLoader;
 import java.util.Enumeration;
 import edu.umd.cs.findbugs.annotations.CheckForNull;
 import edu.umd.cs.findbugs.annotations.NonNull;
+import jenkins.util.AntClassLoader;
+import jenkins.util.AntWithFindResourceClassLoader;
 
 /**
  * Reflective access to various {@link ClassLoader} methods which are otherwise {@code protected}.
+ * <p>
+ * Initially tries to access methods using known classloaders in use that expose the methods
+ * to prevent illegal reflective access errors on Java 11+
+ * Then falls back to accessing the {@link ClassLoader} methods.
+ * <p>
+ * All reflection method initialisation is delayed until first use so that we don't access the methods if we don't need to.
+ * <p>
+ * Note: Currently there is no known production use-case for the fallback case of accessing these methods via reflection:
+ * the {@code JenkinsRule} tests use a different classloader,
+ * but once that is made consistent with production Jenkins we can re-evaluate the fallback code.
  */
 @SuppressWarnings({"unchecked", "rawtypes"})
 public class ClassLoaderReflectionToolkit {
 
-    private static final Method FIND_CLASS, FIND_LOADED_CLASS, FIND_RESOURCE, FIND_RESOURCES, GET_CLASS_LOADING_LOCK;
-
-    static {
-        try {
-            FIND_CLASS = ClassLoader.class.getDeclaredMethod("findClass",String.class);
-            FIND_CLASS.setAccessible(true);
-            FIND_LOADED_CLASS = ClassLoader.class.getDeclaredMethod("findLoadedClass",String.class);
-            FIND_LOADED_CLASS.setAccessible(true);
-            FIND_RESOURCE = ClassLoader.class.getDeclaredMethod("findResource",String.class);
-            FIND_RESOURCE.setAccessible(true);
-            FIND_RESOURCES = ClassLoader.class.getDeclaredMethod("findResources",String.class);
-            FIND_RESOURCES.setAccessible(true);
-        } catch (NoSuchMethodException e) {
-            throw new AssertionError(e);
-        }
-        Method gCLL;
-        try {
-            gCLL = ClassLoader.class.getDeclaredMethod("getClassLoadingLock", String.class);
-            gCLL.setAccessible(true);
-        } catch (NoSuchMethodException x) {
-            throw new AssertionError(x);
-        }
-        GET_CLASS_LOADING_LOCK = gCLL;
-    }
-
     private static <T extends Exception> Object invoke(Method method, Class<T> exception, Object obj, Object... args) throws T {
         try {
             return method.invoke(obj, args);
@@ -59,7 +47,25 @@ public class ClassLoaderReflectionToolkit {
     }
 
     private static Object getClassLoadingLock(ClassLoader cl, String name) {
-        return invoke(GET_CLASS_LOADING_LOCK, RuntimeException.class, cl, name);
+        if (cl instanceof AntWithFindResourceClassLoader) {
+            return ((AntWithFindResourceClassLoader) cl).getClassLoadingLock(name);
+        }
+        return invoke(GetClassLoadingLock.GET_CLASS_LOADING_LOCK, RuntimeException.class, cl, name);
+    }
+
+    private static class GetClassLoadingLock {
+        private static final Method GET_CLASS_LOADING_LOCK;
+
+        static {
+            Method gCLL;
+            try {
+                gCLL = ClassLoader.class.getDeclaredMethod("getClassLoadingLock", String.class);
+                gCLL.setAccessible(true);
+            } catch (NoSuchMethodException x) {
+                throw new AssertionError(x);
+            }
+            GET_CLASS_LOADING_LOCK = gCLL;
+        }
     }
 
     /**
@@ -68,7 +74,27 @@ public class ClassLoaderReflectionToolkit {
      */
     public static @CheckForNull Class<?> _findLoadedClass(ClassLoader cl, String name) {
         synchronized (getClassLoadingLock(cl, name)) {
-            return (Class) invoke(FIND_LOADED_CLASS, RuntimeException.class, cl, name);
+            Class<?> c;
+            if (cl instanceof AntWithFindResourceClassLoader) {
+                c = ((AntWithFindResourceClassLoader) cl).findLoadedClass2(name);
+            } else {
+                c = (Class) invoke(FindLoadedClass.FIND_LOADED_CLASS, RuntimeException.class, cl, name);
+            }
+
+            return c;
+        }
+    }
+
+    private static class FindLoadedClass {
+        private static final Method FIND_LOADED_CLASS;
+
+        static {
+            try {
+                FIND_LOADED_CLASS = ClassLoader.class.getDeclaredMethod("findLoadedClass", String.class);
+            } catch (NoSuchMethodException e) {
+                throw new AssertionError(e);
+            }
+            FIND_LOADED_CLASS.setAccessible(true);
         }
     }
 
@@ -77,17 +103,57 @@ public class ClassLoaderReflectionToolkit {
      * @since 1.553
      */
     public static @NonNull Class<?> _findClass(ClassLoader cl, String name) throws ClassNotFoundException {
+        if (cl instanceof AntClassLoader) {
+            return ((AntClassLoader) cl).findClass(name);
+        }
+
         synchronized (getClassLoadingLock(cl, name)) {
-            return (Class) invoke(FIND_CLASS, ClassNotFoundException.class, cl, name);
+            return (Class) invoke(FindClass.FIND_CLASS, ClassNotFoundException.class, cl, name);
         }
     }
 
+    private static class FindClass {
+        private static final Method FIND_CLASS;
+
+        static {
+            try {
+                FIND_CLASS = ClassLoader.class.getDeclaredMethod("findClass",String.class);
+            } catch (NoSuchMethodException e) {
+                throw new AssertionError(e);
+            }
+            FIND_CLASS.setAccessible(true);
+        }
+    }
+
+
     /**
      * Calls {@link ClassLoader#findResource}.
      * @since 1.553
      */
     public static @CheckForNull URL _findResource(ClassLoader cl, String name) {
-        return (URL) invoke(FIND_RESOURCE, RuntimeException.class, cl, name);
+        URL url;
+        if (cl instanceof AntWithFindResourceClassLoader) {
+            url = ((AntWithFindResourceClassLoader) cl).findResource(name);
+        } else if (cl instanceof URLClassLoader) {
+            url = ((URLClassLoader) cl).findResource(name);
+        } else {
+            url = (URL) invoke(FindResource.FIND_RESOURCE, RuntimeException.class, cl, name);
+        }
+
+        return url;
+    }
+
+    private static class FindResource {
+        private static final Method FIND_RESOURCE;
+
+        static {
+            try {
+                FIND_RESOURCE = ClassLoader.class.getDeclaredMethod("findResource", String.class);
+            } catch (NoSuchMethodException e) {
+                throw new AssertionError(e);
+            }
+            FIND_RESOURCE.setAccessible(true);
+        }
     }
 
     /**
@@ -95,7 +161,27 @@ public class ClassLoaderReflectionToolkit {
      * @since 1.553
      */
     public static @NonNull Enumeration<URL> _findResources(ClassLoader cl, String name) throws IOException {
-        return (Enumeration<URL>) invoke(FIND_RESOURCES, IOException.class, cl, name);
+        Enumeration<URL> urls;
+        if (cl instanceof AntWithFindResourceClassLoader) {
+            urls = ((AntWithFindResourceClassLoader) cl).findResources(name);
+        } else {
+            urls = (Enumeration<URL>) invoke(FindResources.FIND_RESOURCES, IOException.class, cl, name);
+        }
+
+        return urls;
+    }
+
+    private static class FindResources {
+        private static final Method FIND_RESOURCES;
+
+        static {
+            try {
+                FIND_RESOURCES = ClassLoader.class.getDeclaredMethod("findResources", String.class);
+            } catch (NoSuchMethodException e) {
+                throw new AssertionError(e);
+            }
+            FIND_RESOURCES.setAccessible(true);
+        }
     }
 
     /** @deprecated unsafe */
@@ -105,7 +191,7 @@ public class ClassLoaderReflectionToolkit {
     @Deprecated
     public Class findLoadedClass(ClassLoader cl, String name) throws InvocationTargetException {
         try {
-            return (Class)FIND_LOADED_CLASS.invoke(cl,name);
+            return (Class)FindLoadedClass.FIND_LOADED_CLASS.invoke(cl,name);
         } catch (IllegalAccessException e) {
             throw new Error(e);
         }
@@ -115,7 +201,7 @@ public class ClassLoaderReflectionToolkit {
     @Deprecated
     public Class findClass(ClassLoader cl, String name) throws InvocationTargetException {
         try {
-            return (Class)FIND_CLASS.invoke(cl,name);
+            return (Class)FindClass.FIND_CLASS.invoke(cl,name);
         } catch (IllegalAccessException e) {
             throw new Error(e);
         }
@@ -125,7 +211,7 @@ public class ClassLoaderReflectionToolkit {
     @Deprecated
     public URL findResource(ClassLoader cl, String name) throws InvocationTargetException {
         try {
-            return (URL)FIND_RESOURCE.invoke(cl,name);
+            return (URL)FindResource.FIND_RESOURCE.invoke(cl,name);
         } catch (IllegalAccessException e) {
             throw new Error(e);
         }
@@ -135,7 +221,7 @@ public class ClassLoaderReflectionToolkit {
     @Deprecated
     public Enumeration<URL> findResources(ClassLoader cl, String name) throws InvocationTargetException {
         try {
-            return (Enumeration<URL>)FIND_RESOURCES.invoke(cl,name);
+            return (Enumeration<URL>)FindResources.FIND_RESOURCES.invoke(cl,name);
         } catch (IllegalAccessException e) {
             throw new Error(e);
         }
diff --git a/core/src/main/java/jenkins/model/AssetManager.java b/core/src/main/java/jenkins/model/AssetManager.java
index a688f0f718..5d317f2b2a 100644
--- a/core/src/main/java/jenkins/model/AssetManager.java
+++ b/core/src/main/java/jenkins/model/AssetManager.java
@@ -2,8 +2,10 @@ package jenkins.model;
 
 import hudson.Extension;
 import hudson.model.UnprotectedRootAction;
+
 import java.util.concurrent.TimeUnit;
 
+import jenkins.ClassLoaderReflectionToolkit;
 import org.apache.commons.lang.StringUtils;
 import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.StaplerRequest;
@@ -14,8 +16,6 @@ import edu.umd.cs.findbugs.annotations.NonNull;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletResponse;
 import java.io.IOException;
-import java.lang.reflect.InvocationTargetException;
-import java.lang.reflect.Method;
 import java.net.URL;
 import java.util.Enumeration;
 
@@ -83,41 +83,26 @@ public class AssetManager implements UnprotectedRootAction {
             return null;
         }
 
-        try {
-            if (path.contains("..")) // crude avoidance of directory traversal attack
-                throw new IllegalArgumentException(path);
+        if (path.contains("..")) // crude avoidance of directory traversal attack
+            throw new IllegalArgumentException(path);
 
-            String name;
-            if (path.charAt(0) == '/') {
-                name = "assets" + path;
-            } else {
-                name = "assets/" + path;
-            }
-
-            ClassLoader cl = Jenkins.class.getClassLoader();
-            URL url = (URL) $findResource.invoke(cl, name);
-            if (url==null) {
-                // pick the last one, which is the one closest to the leaf of the classloader tree.
-                Enumeration<URL> e = cl.getResources(name);
-                while (e.hasMoreElements()) {
-                    url = e.nextElement();
-                }
-            }
-            return url;
-        } catch (InvocationTargetException|IllegalAccessException e) {
-            throw new Error(e);
+        String name;
+        if (path.charAt(0) == '/') {
+            name = "assets" + path;
+        } else {
+            name = "assets/" + path;
         }
+
+        ClassLoader cl = Jenkins.class.getClassLoader();
+        URL url = ClassLoaderReflectionToolkit._findResource(cl, name);
+        if (url == null) {
+            // pick the last one, which is the one closest to the leaf of the classloader tree.
+            Enumeration<URL> e = cl.getResources(name);
+            while (e.hasMoreElements()) {
+                url = e.nextElement();
+            }
+        }
+        return url;
     }
 
-    private static final Method $findResource = init();
-
-    private static Method init() {
-        try {
-            Method m = ClassLoader.class.getDeclaredMethod("findResource", String.class);
-            m.setAccessible(true);
-            return m;
-        } catch (NoSuchMethodException e) {
-            throw (Error)new NoSuchMethodError().initCause(e);
-        }
-    }
 }
diff --git a/core/src/main/java/jenkins/util/AntClassLoader.java b/core/src/main/java/jenkins/util/AntClassLoader.java
index 7843abb69a..656678a7e8 100644
--- a/core/src/main/java/jenkins/util/AntClassLoader.java
+++ b/core/src/main/java/jenkins/util/AntClassLoader.java
@@ -960,7 +960,7 @@ public class AntClassLoader extends ClassLoader implements SubBuildListener {
      * @return an enumeration of URLs for the resources
      * @exception IOException if I/O errors occurs (can't happen)
      */
-    protected Enumeration/*<URL>*/ findResources(String name) throws IOException {
+    public Enumeration/*<URL>*/ findResources(String name) throws IOException {
         return findResources(name, true);
     }
 
diff --git a/core/src/main/java/jenkins/util/AntWithFindResourceClassLoader.java b/core/src/main/java/jenkins/util/AntWithFindResourceClassLoader.java
index 7b70413a48..6f45892bfa 100644
--- a/core/src/main/java/jenkins/util/AntWithFindResourceClassLoader.java
+++ b/core/src/main/java/jenkins/util/AntWithFindResourceClassLoader.java
@@ -37,10 +37,25 @@ public class AntWithFindResourceClassLoader extends AntClassLoader implements Cl
     }
 
     @Override
-    protected URL findResource(String name) {
+    public URL findResource(String name) {
         // try and load from this loader if the parent either didn't find
         // it or wasn't consulted.
         return getUrl(pathComponents, name);
     }
 
+    /**
+     * Public version of {@link ClassLoader#findLoadedClass(String)}
+     */
+    public Class<?> findLoadedClass2(String name) {
+        return findLoadedClass(name);
+    }
+    
+    /**
+     * Public version of {@link ClassLoader#getClassLoadingLock(String)}
+     */
+    @Override
+    public Object getClassLoadingLock(String className) {
+        return super.getClassLoadingLock(className);
+    }
+
 }