generalized the reconfigure concept.

core/src/main/java/hudson/model/ReconfigurableDescribable.java

+/*
+ * The MIT License
+ *
+ * Copyright (c) 2011, CloudBees, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package hudson.model;
+
+import hudson.model.Descriptor.FormException;
+import hudson.slaves.NodeProperty;
+import net.sf.json.JSONObject;
+import org.kohsuke.stapler.StaplerRequest;
+
+/**
+ * Marks modern {@link Describable}s that allow the current instances to pass information down to the next
+ * instance when the configuration is submitted.
+ *
+ * <p>
+ * As the generics signature implies, it isn't up to individual {@link Describable} implementation to
+ * implement this. Rather, it is up to the designer of an extension point to mark the extension point
+ * as {@link ReconfigurableDescribable}, as it requires coordination at the owner of the extension point.
+ *
+ * <h1>Use Cases</h1>
+ * <h2>Invisible Property</h2>
+ * <p>
+ * This mechanism can be used to create an entirely invisible {@link Describable}, which is handy
+ * for {@link NodeProperty}, {@link JobProperty}, etc. To do so, define a descriptor with null
+ * {@linkplain Descriptor#getDisplayName() display name} and empty config.jelly to prevent it from
+ * showing up in the config UI, then implement {@link #reconfigure(StaplerRequest, JSONObject)}
+ * and simply return {@code this}.
+ *
+ * <h2>Passing some values without going through clients</h2>
+ * <p>
+ * Sometimes your {@link Describable} object may have some expensive objects that you might want to
+ * hand over to the next instance. This hook lets you do that.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.406
+ */
+public interface ReconfigurableDescribable<T extends ReconfigurableDescribable<T>> extends Describable<T> {
+
+    /**
+     * When a parent/owner object of a Describable gets a config form submission and instances are
+     * recreated, this method is invoked on the existing instance (meaning the 'this' reference
+     * points to the existing instance) to create a new instance to be added to the parent/owner object.
+     *
+     * <p>
+     * The default implementation of this should be the following:
+     * <pre>
+     * return form==null ? null : getDescriptor().newInstance(req, form);
+     * </pre>
+     *
+     * @param req
+     *      The current HTTP request being processed.
+     * @param form
+     *      JSON fragment that corresponds to this describable object.
+     *      If the newly submitted form doesn't include a fragment for this describable
+     *      (meaning the user has de-selected your descriptor), then this argument is null.
+     *
+     * @return
+     *      The new instance. To not to create an instance of a describable, return null.
+     */
+    T reconfigure(StaplerRequest req, JSONObject form) throws FormException;
+}

core/src/main/java/hudson/model/ViewProperty.java

@@ -50,7 +50,7 @@ import java.util.List;
  * @author Stephen Connolly
  * @since 1.406
  */
-public class ViewProperty implements Describable<ViewProperty>, ExtensionPoint {
+public class ViewProperty implements ReconfigurableDescribable<ViewProperty>, ExtensionPoint {
     /**
      * The view object that owns this property.
      * This value will be set by the core code.
@@ -71,6 +71,6 @@ public class ViewProperty implements Describable<ViewProperty>, ExtensionPoint {
     }
 
     public ViewProperty reconfigure(StaplerRequest req, JSONObject form) throws Descriptor.FormException {
-    	return getDescriptor().newInstance(req, form);
+    	return form==null ? null : getDescriptor().newInstance(req, form);
     }
 }

core/src/main/java/hudson/util/DescribableList.java

@@ -36,8 +36,10 @@ import hudson.model.DependencyGraph;
 import hudson.model.Describable;
 import hudson.model.Descriptor;
 import hudson.model.Descriptor.FormException;
+import hudson.model.ReconfigurableDescribable;
 import hudson.model.Saveable;
 import net.sf.json.JSONObject;
+import org.apache.avalon.framework.configuration.Reconfigurable;
 import org.kohsuke.stapler.StaplerRequest;
 
 import java.io.IOException;
@@ -141,11 +143,23 @@ public class DescribableList<T extends Describable<T>, D extends Descriptor<T>>
         List<T> newList = new ArrayList<T>();
 
         for (Descriptor<T> d : descriptors) {
+            T existing = get((D)d);
             String name = d.getJsonSafeClassName();
-            if (json.has(name)) {
-                T instance = d.newInstance(req, json.getJSONObject(name));
-                newList.add(instance);
+            JSONObject o = json.optJSONObject(name);
+
+            T instance = null;
+            if (o!=null) {
+                if (existing instanceof ReconfigurableDescribable)
+                    instance = (T)((ReconfigurableDescribable)existing).reconfigure(req,o);
+                else
+                    instance = d.newInstance(req, o);
+            } else {
+                if (existing instanceof ReconfigurableDescribable)
+                    instance = (T)((ReconfigurableDescribable)existing).reconfigure(req,null);
             }
+
+            if (instance!=null)
+                newList.add(instance);
         }
 
         replaceBy(newList);