make $class and kind mutually exclusive

Use kind when descriptor.id != descriptor.clazz.name and $clazz otherwise.

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

@@ -916,7 +916,8 @@ public abstract class Descriptor<T extends Describable<T>> implements Saveable {
                 }
                 if (d == null) {
                   kind = jo.getString("$class");
-                  d = findByClassName(descriptors, kind);
+                  d = findByDescribableClassName(descriptors, kind);
+                  if (d == null) d = findByClassName(descriptors, kind);
                 }
                 if (d != null) {
                     items.add(d.newInstance(req, jo));
@@ -952,6 +953,17 @@ public abstract class Descriptor<T extends Describable<T>> implements Saveable {
         return null;
     }
 
+    /**
+     * Finds a descriptor from a collection by the class name of the Describable it describes.
+     */
+    public static @CheckForNull <T extends Descriptor> T findByDescribableClassName(Collection<? extends T> list, String className) {
+        for (T d : list) {
+            if(d.clazz.getName().equals(className))
+                return d;
+        }
+        return null;
+    }
+
     /**
      * Finds a descriptor from a collection by its class name or ID.
      * @deprecated choose between {@link #findById(java.util.Collection, String)} or {@link #findByClassName(java.util.Collection, String)}

core/src/main/java/hudson/tools/DownloadFromUrlInstaller.java

@@ -122,7 +122,7 @@ public abstract class DownloadFromUrlInstaller extends ToolInstaller {
         }
 
         protected Downloadable createDownloadable() {
-            return new Downloadable(getId());
+            return new Downloadable(getDownloadableId());
         }
 
         /**
@@ -130,7 +130,7 @@ public abstract class DownloadFromUrlInstaller extends ToolInstaller {
          * <p>
          * By default we use the fully-qualified class name of the {@link DownloadFromUrlInstaller} subtype.
          */
-        public String getId() {
+        public String getDownloadableId() {
             return clazz.getName().replace('$','.');
         }
 
@@ -144,7 +144,7 @@ public abstract class DownloadFromUrlInstaller extends ToolInstaller {
          * @return never null.
          */
         public List<? extends Installable> getInstallables() throws IOException {
-            JSONObject d = Downloadable.get(getId()).getData();
+            JSONObject d = Downloadable.get(getDownloadableId()).getData();
             if(d==null)     return Collections.emptyList();
             return Arrays.asList(((InstallableList)JSONObject.toBean(d,InstallableList.class)).list);
         }

core/src/main/resources/lib/form/class-entry.jelly

@@ -36,11 +36,15 @@ THE SOFTWARE.
   </st:documentation>
   <j:set var="clazz" value="${attrs.clazz ?: attrs.descriptor.clazz.name}" />
   <f:invisibleEntry>
-    <j:if test="${attrs.descriptor != null}">
-      <input type="hidden" name="kind" value="${attrs.descriptor.id}" />
-    </j:if>
-    <!-- Legacy: Remove once plugins have been staged onto $class -->
-    <input type="hidden" name="stapler-class" value="${clazz}" />
-    <input type="hidden" name="$class" value="${clazz}" />
+    <j:choose>
+      <j:when test="${descriptor != null and descriptor.id != clazz}">
+        <input type="hidden" name="kind" value="${attrs.descriptor.id}" />
+      </j:when>
+      <j:otherwise>
+        <!-- Legacy: Remove once plugins have been staged onto $class -->
+        <input type="hidden" name="stapler-class" value="${clazz}" />
+        <input type="hidden" name="$class" value="${clazz}" />
+      </j:otherwise>
+    </j:choose>
   </f:invisibleEntry>
 </j:jelly>