add an extra unit test

core/src/main/java/com/opensymphony/xwork2/ognl/OgnlUtil.java

@@ -102,7 +102,8 @@ public class OgnlUtil {
 
     @Inject(value = XWorkConstants.OGNL_EXCLUDED_CLASSES, required = false)
     public void setExcludedClasses(String commaDelimitedClasses) {
-        Set<Class<?>> excludedClasses = this.excludedClasses;
+        Set<Class<?>> excludedClasses = new HashSet<>();
+        excludedClasses.addAll(this.excludedClasses);
         excludedClasses.addAll(parseExcludedClasses(commaDelimitedClasses));
         this.excludedClasses = Collections.unmodifiableSet(excludedClasses);
     }
@@ -124,7 +125,8 @@ public class OgnlUtil {
 
     @Inject(value = XWorkConstants.OGNL_EXCLUDED_PACKAGE_NAME_PATTERNS, required = false)
     public void setExcludedPackageNamePatterns(String commaDelimitedPackagePatterns) {
-        Set<Pattern> excludedPackageNamePatterns = this.excludedPackageNamePatterns;
+        Set<Pattern> excludedPackageNamePatterns = new HashSet<>();
+        excludedPackageNamePatterns.addAll(this.excludedPackageNamePatterns);
         excludedPackageNamePatterns.addAll(parseExcludedPackageNamePatterns(commaDelimitedPackagePatterns));
         this.excludedPackageNamePatterns = Collections.unmodifiableSet(excludedPackageNamePatterns);
     }
@@ -142,7 +144,8 @@ public class OgnlUtil {
 
     @Inject(value = XWorkConstants.OGNL_EXCLUDED_PACKAGE_NAMES, required = false)
     public void setExcludedPackageNames(String commaDelimitedPackageNames) {
-        Set<String> excludedPackageNames = this.excludedPackageNames;
+        Set<String> excludedPackageNames = new HashSet<>();
+        excludedPackageNames.addAll(this.excludedPackageNames);
         excludedPackageNames.addAll(parseExcludedPackageNames(commaDelimitedPackageNames));
         this.excludedPackageNames = Collections.unmodifiableSet(excludedPackageNames);
     }

core/src/test/java/com/opensymphony/xwork2/ognl/OgnlUtilStrutsTest.java

@@ -0,0 +1,56 @@
+/*
+ * 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
+ *
+ *  http://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 com.opensymphony.xwork2.ognl;
+
+import org.apache.struts2.StrutsInternalTestCase;
+
+public class OgnlUtilStrutsTest extends StrutsInternalTestCase {
+    
+    private OgnlUtil ognlUtil;
+    
+    @Override
+    public void setUp() throws Exception {
+        super.setUp();
+        ognlUtil = container.getInstance(OgnlUtil.class);
+    }
+    
+    public void testDefaultExcludes() {
+        ognlUtil.setExcludedClasses("");
+        ognlUtil.setExcludedPackageNames("");
+        ognlUtil.setExcludedPackageNamePatterns("");
+        assertTrue(ognlUtil.getExcludedClasses().size() > 0);
+        assertTrue(ognlUtil.getExcludedPackageNames().size() > 0);
+
+        try {
+            ognlUtil.getExcludedClasses().clear();
+        } catch (Exception ex){
+            assertTrue(ex instanceof UnsupportedOperationException);
+        }
+        try {
+            ognlUtil.getExcludedPackageNames().clear();
+        } catch (Exception ex){
+            assertTrue(ex instanceof UnsupportedOperationException);
+        }
+        try {
+            ognlUtil.getExcludedPackageNamePatterns().clear();
+        } catch (Exception ex){
+            assertTrue(ex instanceof UnsupportedOperationException);
+        }
+    }
+}