fixes walking over internal JDK11 fields

ehcache-core/src/main/java/net/sf/ehcache/pool/sizeof/ObjectGraphWalker.java

@@ -273,7 +273,12 @@ private static Collection<Field> getAllFields(Class<?> refClass) {
                         LOG.error("Security settings prevent Ehcache from accessing the subgraph beneath '{}'" +
                                 " - cache sizes may be underestimated as a result", field, e);
                         continue;
+                    } catch (RuntimeException e) {
+                        LOG.warn("The JVM is preventing Ehcache from accessing the subgraph beneath '{}'" +
+                                " - cache sizes may be underestimated as a result", field, e);
+                        continue;
                     }
+
                     fields.add(field);
                 }
             }

ehcache-core/src/test/java/net/sf/ehcache/pool/sizeof/ObjectGraphWalkerTest.java

@@ -6,6 +6,8 @@
 import org.junit.BeforeClass;
 import org.junit.Test;
 
+import java.net.URL;
+import java.net.URLClassLoader;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
@@ -63,6 +65,7 @@ public void increment(String value) {
     assertThat(map.isEmpty(), is(true));
 
     assertThat(walker.walk(MAX_SIZEOF_DEPTH, false, new SomeInnerClass()), is(14L));
+
     assertThat(map.remove("java.util.concurrent.locks.ReentrantReadWriteLock$Sync$ThreadLocalHoldCounter"), is(1L));
 
     assertThat(map.remove(SomeInnerClass.class.getName()), is(1L));
@@ -91,6 +94,30 @@ public class SomeInnerClass {
     private final Object   four       = new ReentrantReadWriteLock();
     private final Object[] anArray    = new Object[]{new Object(), new Object(), new Object(), one, two, two, three, four, value};
     private final int[]    anIntArray = new int[]   {1, 2, 1300                                                                 };
+  }
+
+  @Test
+  public void testWalksAGraphWithInacessibleFieldsWithoutThrowingException() {
+    class ClassWithNestedInacessibleFields {
+      /**
+       * Certain fields of JDK11 classes are internal and throw new kind of exception upon reflection access
+       *
+       * java.lang.reflect.InaccessibleObjectException: Unable to make field final jdk.internal.loader.URLClassPath jdk.internal.loader.ClassLoaders$AppClassLoader.ucp accessible: module java.base does not "opens jdk.internal.loader"
+       */
+      public final URLClassLoader objectWithInaccessibleFields = new URLClassLoader(new URL[]{});
+    }
+
+    ObjectGraphWalker walker = new ObjectGraphWalker(
+        new ObjectGraphWalker.Visitor() {
+          @Override
+          public long visit(Object object) {
+            return 0;
+          }
+        }, new PassThroughFilter()
+    );
 
+    walker.walk(MAX_SIZEOF_DEPTH, false, new ClassWithNestedInacessibleFields());
   }
+
+
 }