fixing up the semantics and adding a test.

core/src/main/java/hudson/matrix/MatrixBuild.java

@@ -29,15 +29,16 @@ import hudson.matrix.listeners.MatrixBuildListener;
 import hudson.model.AbstractBuild;
 import hudson.model.AbstractProject;
 import hudson.model.BuildListener;
+import hudson.model.Cause.UpstreamCause;
 import hudson.model.Executor;
 import hudson.model.Fingerprint;
-import jenkins.model.Jenkins;
-import hudson.model.JobProperty;
 import hudson.model.ParametersAction;
 import hudson.model.Queue;
 import hudson.model.Result;
-import hudson.model.Cause.UpstreamCause;
-import hudson.tasks.Publisher;
+import jenkins.model.Jenkins;
+import org.kohsuke.stapler.StaplerRequest;
+import org.kohsuke.stapler.StaplerResponse;
+import org.kohsuke.stapler.export.Exported;
 
 import java.io.File;
 import java.io.IOException;
@@ -45,13 +46,10 @@ import java.io.PrintStream;
 import java.util.ArrayList;
 import java.util.Calendar;
 import java.util.Collection;
+import java.util.Comparator;
 import java.util.HashSet;
 import java.util.List;
-
 import java.util.TreeSet;
-import org.kohsuke.stapler.StaplerRequest;
-import org.kohsuke.stapler.StaplerResponse;
-import org.kohsuke.stapler.export.Exported;
 
 /**
  * Build of {@link MatrixProject}.
@@ -263,9 +261,9 @@ public class MatrixBuild extends AbstractBuild<MatrixProject,MatrixBuild> {
                     return Result.FAILURE;
 
             MatrixConfigurationSorter sorter = p.getSorter();
-            if(p.getSorter()!=null){
-               touchStoneConfigurations = new TreeSet<MatrixConfiguration>(touchStoneConfigurations);
-               delayedConfigurations = new TreeSet<MatrixConfiguration>(delayedConfigurations);
+            if (sorter != null) {
+                touchStoneConfigurations = createTreeSet(touchStoneConfigurations, sorter);
+                delayedConfigurations    = createTreeSet(delayedConfigurations,sorter);
             }
 
             try {
@@ -401,6 +399,12 @@ public class MatrixBuild extends AbstractBuild<MatrixProject,MatrixBuild> {
         }
     }
 
+    private <T> TreeSet<T> createTreeSet(Collection<T> items, Comparator<T> sorter) {
+        TreeSet<T> r = new TreeSet<T>(sorter);
+        r.addAll(items);
+        return r;
+    }
+
     /**
      * A private exception to help maintain the correct control flow after extracting the 'waitForCompletion' method
      */

core/src/main/java/hudson/matrix/MatrixConfiguration.java

@@ -56,7 +56,7 @@ import java.util.Map;
  *
  * @author Kohsuke Kawaguchi
  */
-public class MatrixConfiguration extends Project<MatrixConfiguration,MatrixRun> implements SCMedItem, NonBlockingTask, Comparable<MatrixConfiguration> {
+public class MatrixConfiguration extends Project<MatrixConfiguration,MatrixRun> implements SCMedItem, NonBlockingTask {
     /**
      * The actual value combination.
      */
@@ -290,19 +290,6 @@ public class MatrixConfiguration extends Project<MatrixConfiguration,MatrixRun>
         throw new UnsupportedOperationException();
     }
 
-    public int compareTo(MatrixConfiguration that) {
-        MatrixConfigurationSorter sorter = getParent().getSorter();
-        try {
-            if (sorter != null) {
-                return getParent().getSorter().compare(this, that);
-            } else {
-                return getDisplayName().compareTo(((MatrixConfiguration) that).getDisplayName());
-            }
-        } catch (Exception e) {
-            throw new IllegalArgumentException(e);
-        }
-    }
-
     /**
      * Returns true if this configuration is a configuration
      * currently in use today (as opposed to the ones that are

core/src/main/java/hudson/matrix/MatrixProject.java

@@ -154,7 +154,10 @@ public class MatrixProject extends AbstractProject<MatrixProject,MatrixBuild> im
         super(parent, name);
     }
 
-    public MatrixConfigurationSorter getSorter(){
+    /**
+     * @return can be null (to indicate that the configurations should be left to their natural order.)
+     */
+    public MatrixConfigurationSorter getSorter() {
         return sorter;
     }
     
@@ -586,12 +589,18 @@ public class MatrixProject extends AbstractProject<MatrixProject,MatrixBuild> im
         checkAxisNames(newAxes);
         this.axes = new AxisList(newAxes.toList());
         
+        runSequentially = json.optBoolean("runSequentially");
+
         // set sorter if any sorter is chosen
+        if (runSequentially) {
             MatrixConfigurationSorter s = req.bindJSON(MatrixConfigurationSorter.class,json.optJSONObject("sorter"));
-        s.validate(this);
+            if (s!=null)    s.validate(this);
+            if (s instanceof NoopMatrixConfigurationSorter) s=null;
             setSorter(s);
+        } else {
+            setSorter(null);
+        }
 
-        runSequentially = json.has("runSequentially");
 
         buildWrappers.rebuild(req, json, BuildWrappers.getFor(this));
         builders.rebuildHetero(req, json, Builder.all(), "builder");

core/src/main/java/hudson/matrix/NoopMatrixConfigurationSorter.java

+package hudson.matrix;
+
+import hudson.Extension;
+import hudson.util.FormValidation;
+import org.kohsuke.stapler.DataBoundConstructor;
+
+/**
+ * Place holder for default "do not sort" {@link MatrixConfigurationSorter}.
+ *
+ * @author Kohsuke Kawaguchi
+ */
+public class NoopMatrixConfigurationSorter extends MatrixConfigurationSorter {
+    @DataBoundConstructor
+    public NoopMatrixConfigurationSorter() {
+    }
+
+    @Override
+    public void validate(MatrixProject p) throws FormValidation {
+        // nothing
+    }
+
+    public int compare(MatrixConfiguration o1, MatrixConfiguration o2) {
+        return o1.getDisplayName().compareTo(o2.getDisplayName());
+    }
+
+    @Extension(ordinal=100) // this is the default
+    public static class DescriptorImpl extends MatrixConfigurationSorterDescriptor {
+        @Override
+        public String getDisplayName() {
+            return "Doesn't care";
+        }
+    }
+}

core/src/main/resources/hudson/matrix/MatrixConfigurationSorter/config.groovy

+// default is empty
\ No newline at end of file

core/src/main/resources/hudson/matrix/MatrixProject/configure-entries.jelly

@@ -59,11 +59,11 @@ THE SOFTWARE.
                      addCaption="${%Add axis}"/>
     </f:block>
 
-    <j:if test="${(descriptor.sorterDescriptors().size() > 0)}">
-      <f:dropdownDescriptorSelector title="${%Execution order of builds}" field="sorter" />
+    <f:optionalBlock field="runSequentially" title="${%Run each configuration sequentially}" inline="true">
+      <j:if test="${descriptor.sorterDescriptors.size() gt 1}">
+        <f:dropdownDescriptorSelector title="${%Execution order of builds}" field="sorter"/>
       </j:if>
-
-    <f:optionalBlock field="runSequentially" title="${%Run each configuration sequentially}"/>
+    </f:optionalBlock>
 
     <f:optionalBlock name="hasCombinationFilter" title="${%Combination Filter}" checked="${!empty(it.combinationFilter)}"
         help="/help/matrix/combinationfilter.html">

test/src/test/java/hudson/matrix/MatrixConfigurationSorterTest.java

+package hudson.matrix;
+
+import hudson.util.FormValidation;
+import org.jvnet.hudson.test.HudsonTestCase;
+import org.jvnet.hudson.test.TestExtension;
+import org.kohsuke.stapler.DataBoundConstructor;
+
+/**
+ * @author Kohsuke Kawaguchi
+ */
+public class MatrixConfigurationSorterTest extends HudsonTestCase {
+    public void testConfigRoundtrip() throws Exception {
+        MatrixProject p = createMatrixProject();
+        configRoundtrip(p);
+        assertNull(p.getSorter());
+
+        SorterImpl before = new SorterImpl();
+        p.setSorter(before);
+        p.setRunSequentially(true);
+        configRoundtrip(p);
+        Object after = p.getSorter();
+        assertNotSame(before,after);
+        assertSame(before.getClass(),after.getClass());
+    }
+
+    public static class SorterImpl extends MatrixConfigurationSorter {
+        @DataBoundConstructor
+        public SorterImpl() {}
+
+        @Override
+        public void validate(MatrixProject p) throws FormValidation {
+        }
+
+        public int compare(MatrixConfiguration o1, MatrixConfiguration o2) {
+            return o1.getDisplayName().compareTo(o2.getDisplayName());
+        }
+
+        @TestExtension
+        public static class DescriptorImpl extends MatrixConfigurationSorterDescriptor {
+            @Override
+            public String getDisplayName() {
+                return "Test Sorter";
+            }
+        }
+    }
+}