remove Ambiguity.

mybatis-plus-core/src/main/java/com/baomidou/mybatisplus/core/MybatisConfiguration.java

@@ -405,6 +405,7 @@ public class MybatisConfiguration extends Configuration {
         private static final long serialVersionUID = -4950446264854982944L;
         private final String name;
         private BiFunction<V, V, String> conflictMessageProducer;
+        private final Object AMBIGUITY_INSTANCE = new Object();
 
         public StrictMap(String name, int initialCapacity, float loadFactor) {
             super(initialCapacity, loadFactor);
@@ -453,7 +454,7 @@ public class MybatisConfiguration extends Configuration {
                     if (super.get(shortKey) == null) {
                         super.put(shortKey, value);
                     } else {
-                        super.put(shortKey, (V) new StrictMap.Ambiguity(shortKey));
+                        super.put(shortKey, (V) AMBIGUITY_INSTANCE);
                     }
                 }
             }
@@ -465,7 +466,6 @@ public class MybatisConfiguration extends Configuration {
             if (key == null) {
                 return false;
             }
-
             return super.get(key) != null;
         }
 
@@ -475,25 +475,13 @@ public class MybatisConfiguration extends Configuration {
             if (value == null) {
                 throw new IllegalArgumentException(name + " does not contain value for " + key);
             }
-            if (useGeneratedShortKey && value instanceof StrictMap.Ambiguity) {
-                throw new IllegalArgumentException(((StrictMap.Ambiguity) value).getSubject() + " is ambiguous in " + name
+            if (useGeneratedShortKey && AMBIGUITY_INSTANCE == value) {
+                throw new IllegalArgumentException(key + " is ambiguous in " + name
                     + " (try using the full name including the namespace, or rename one of the entries)");
             }
             return value;
         }
 
-        protected class Ambiguity {
-            private final String subject;
-
-            public Ambiguity(String subject) {
-                this.subject = subject;
-            }
-
-            public String getSubject() {
-                return subject;
-            }
-        }
-
         private String getShortName(String key) {
             final String[] keyParts = key.split("\\.");
             return keyParts[keyParts.length - 1];