Fixes #1625 issue by caching the field list and validation state in a…

…ddition to the existing layoutInfo and fieldOrder caches.

src/com/sun/jna/Structure.java

@@ -157,6 +157,8 @@ private static class NativeStringTracking {
     protected static final int CALCULATE_SIZE = -1;
     static final Map<Class<?>, LayoutInfo> layoutInfo = new WeakHashMap<>();
     static final Map<Class<?>, List<String>> fieldOrder = new WeakHashMap<>();
+    static final Map<Class<?>, List<Field>> fieldList = new WeakHashMap<>();
+    static final Map<Class<?>, Boolean> validationMap = new WeakHashMap<>();
 
     // This field is accessed by native code
     private Pointer memory;
@@ -1015,22 +1017,26 @@ protected void sortFields(List<Field> fields, List<String> names) {
      * this {@link Structure} class.
      */
     protected List<Field> getFieldList() {
-        List<Field> flist = new ArrayList<>();
-        for (Class<?> cls = getClass();
-             !cls.equals(Structure.class);
-             cls = cls.getSuperclass()) {
-            List<Field> classFields = new ArrayList<>();
-            Field[] fields = cls.getDeclaredFields();
-            for (int i=0;i < fields.length;i++) {
-                int modifiers = fields[i].getModifiers();
-                if (Modifier.isStatic(modifiers) || !Modifier.isPublic(modifiers)) {
-                    continue;
+        synchronized (fieldList) {
+            return fieldList.computeIfAbsent(getClass(), (c) -> {
+                List<Field> flist = new ArrayList<>();
+                List<Field> classFields = new ArrayList<>();
+                for (Class<?> cls = getClass();
+                     !cls.equals(Structure.class);
+                     cls = cls.getSuperclass()) {
+                    for (Field field : cls.getDeclaredFields()) {
+                        int modifiers = field.getModifiers();
+                        if (Modifier.isStatic(modifiers) || !Modifier.isPublic(modifiers)) {
+                            continue;
+                        }
+                        classFields.add(field);
+                    }
+                    flist.addAll(0, classFields);
+                    classFields.clear();
                 }
-                classFields.add(fields[i]);
-            }
-            flist.addAll(0, classFields);
+                return flist;
+            });
         }
-        return flist;
     }
 
     /** Cache field order per-class.
@@ -1039,12 +1045,7 @@ protected List<Field> getFieldList() {
     private List<String> fieldOrder() {
         Class<?> clazz = getClass();
         synchronized(fieldOrder) {
-            List<String> list = fieldOrder.get(clazz);
-            if (list == null) {
-                list = getFieldOrder();
-                fieldOrder.put(clazz, list);
-            }
-            return list;
+            return fieldOrder.computeIfAbsent(clazz, (c) -> getFieldOrder());
         }
     }
 
@@ -1250,9 +1251,13 @@ private void validateField(String name, Class<?> type) {
 
     /** ensure all fields are of valid type. */
     private void validateFields() {
-        List<Field> fields = getFieldList();
-        for (Field f : fields) {
-            validateField(f.getName(), f.getType());
+        synchronized (validationMap) {
+            validationMap.computeIfAbsent(getClass(), (cls) -> {
+                for (Field f : getFieldList()) {
+                    validateField(f.getName(), f.getType());
+                }
+                return true;
+            });
         }
     }