minor cleanup for JsonMappingException, serializability, based on sug…

…gestions

src/main/java/com/fasterxml/jackson/databind/JsonMappingException.java

@@ -5,6 +5,7 @@
 import java.io.Serializable;
 import java.util.*;
 
+import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.core.*;
 import com.fasterxml.jackson.databind.util.ClassUtil;
 
@@ -44,14 +45,10 @@ public class JsonMappingException
      */
     public static class Reference implements Serializable
     {
-        private static final long serialVersionUID = 1L;
+        private static final long serialVersionUID = 2L; // changes between 2.7 and 2.8
 
-        /**
-         * Object through which reference was resolved. Can be either
-         * actual instance (usually the case for serialization), or
-         * Class (usually the case for deserialization).
-         */
-        protected Object _from;
+        // transient since 2.8
+        protected transient Object _from;
 
         /**
          * Name of field (for beans) or key (for Maps) that is part
@@ -65,7 +62,7 @@ public static class Reference implements Serializable
          * Index within a {@link Collection} instance that contained
          * the reference; used if index is relevant and available.
          * If either not applicable, or not available, -1 is used to
-         * denote "not known".
+         * denote "not known" (or not relevant).
          */
         protected int _index = -1;
 
@@ -75,9 +72,9 @@ public static class Reference implements Serializable
          * non-serializable (and has to be dropped) but we still want to pass
          * actual description along.
          *
-         * @since 2.7.4
+         * @since 2.8
          */
-        protected String _asString;
+        protected String _desc;
 
         /**
          * Default constructor for deserialization/sub-classing purposes
@@ -99,23 +96,28 @@ public Reference(Object from, int index) {
             _index = index;
         }
 
-        private Reference(Reference src, String asString, Object newFrom) {
-            _asString = asString;
-            _from = newFrom;
-            _fieldName = src._fieldName;
-            _index = src._index;
-        }
-
-        public void setFrom(Object o) { _from = o; }
-        public void setFieldName(String n) { _fieldName = n; }
-        public void setIndex(int ix) { _index = ix; }
+        // Setters to let Jackson deserialize instances, but not to be called from outside
+        void setFieldName(String n) { _fieldName = n; }
+        void setIndex(int ix) { _index = ix; }
+        void setDescription(String d) { _desc = d; }
 
+        /**
+         * Object through which reference was resolved. Can be either
+         * actual instance (usually the case for serialization), or
+         * Class (usually the case for deserialization).
+         *<p>
+         * Note that this value must be `transient` to allow serializability (as
+         * often such Object is NOT serializable; or, in case of `Class`, may
+         * not available at the point of deserialization). As such will return
+         * `null` if instance has been passed using JDK serialization.
+         */
+        @JsonIgnore
         public Object getFrom() { return _from; }
+
         public String getFieldName() { return _fieldName; }
         public int getIndex() { return _index; }
-
-        @Override public String toString() {
-            if (_asString == null) {
+        public String getDescription() {
+            if (_desc == null) {
                 StringBuilder sb = new StringBuilder();
 
                 if (_from == null) { // can this ever occur?
@@ -144,22 +146,26 @@ private Reference(Reference src, String asString, Object newFrom) {
                     sb.append('?');
                 }
                 sb.append(']');
-                _asString = sb.toString();
+                _desc = sb.toString();
             }
-            return _asString;
+            return _desc;
+        }
+
+        @Override
+        public String toString() {
+            return getDescription();
         }
 
         /**
          * May need some cleaning here, given that `from` may or may not be serializable.
          *
-         * since 2.7.4
+         * since 2.8
          */
         Object writeReplace() {
-            // as per [databind#1195], reference may cause trouble, if non-serializable
-            // instance (either directly or transitively); and even use of Class would often
-            // be problematic. Because of this, clear up `_from` always, but ensure that
-            // description is preserved
-            return new Reference(this, toString(), null);
+            // as per [databind#1195], need to ensure description is not null, since
+            // `_from` is transient
+            getDescription();
+            return this;
         }
     }