[FIXED JENKINS-15869] When running on Java 5, must still load...

[FIXED JENKINS-15869] When running on Java 5, must still load DiskSpaceMonitor/TemporarySpaceMonitor, even though they will show N/A.
Returning null from an @Extension method does not seem to work reliably, and is not supported by SezPoz.
If desirable to hide a monitor column in $server/computer/, add a boolean isEnabled() method interpretable in Jelly.

changelog.html

@@ -61,6 +61,9 @@ Upcoming changes</a>
   <li class=bug>
     Build records were broken if timezone was changed while running.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-15816">issue 15816</a>)
+  <li class='major bug'>
+    Linkage errors trying to run Jenkins on Java 5.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-15869">issue 15869</a>)
   <li class=bug>
     Symlink detection refinement on Java 7.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-13202">issue 13202</a>)

core/src/main/java/hudson/node_monitors/DiskSpaceMonitor.java

@@ -59,9 +59,10 @@ public class DiskSpaceMonitor extends AbstractDiskSpaceMonitor {
         return Jenkins.getInstance().hasPermission(Jenkins.ADMINISTER) ? super.getColumnCaption() : null;
     }
 
+    @Extension
     public static final DiskSpaceMonitorDescriptor DESCRIPTOR = new DiskSpaceMonitorDescriptor() {
         public String getDisplayName() {
-            return Messages.DiskSpaceMonitor_DisplayName();
+            return Functions.isMustangOrAbove() ? Messages.DiskSpaceMonitor_DisplayName() : Messages.DiskSpaceMonitor_DisplayName5();
         }
 
         protected DiskSpace getFreeSpace(Computer c) throws IOException, InterruptedException {
@@ -72,9 +73,4 @@ public class DiskSpaceMonitor extends AbstractDiskSpaceMonitor {
         }
     };
 
-    @Extension
-    public static DiskSpaceMonitorDescriptor install() {
-        if(Functions.isMustangOrAbove())    return DESCRIPTOR;
-        return null;
-    }
 }

core/src/main/java/hudson/node_monitors/TemporarySpaceMonitor.java

@@ -61,9 +61,10 @@ public class TemporarySpaceMonitor extends AbstractDiskSpaceMonitor {
         return Jenkins.getInstance().hasPermission(Jenkins.ADMINISTER) ? super.getColumnCaption() : null;
     }
 
+    @Extension
     public static final DiskSpaceMonitorDescriptor DESCRIPTOR = new DiskSpaceMonitorDescriptor() {
         public String getDisplayName() {
-            return Messages.TemporarySpaceMonitor_DisplayName();
+            return Functions.isMustangOrAbove() ? Messages.TemporarySpaceMonitor_DisplayName() : Messages.TemporarySpaceMonitor_DisplayName5();
         }
 
         protected DiskSpace getFreeSpace(Computer c) throws IOException, InterruptedException {
@@ -74,12 +75,6 @@ public class TemporarySpaceMonitor extends AbstractDiskSpaceMonitor {
         }
     };
 
-    @Extension
-    public static DiskSpaceMonitorDescriptor install() {
-        if(Functions.isMustangOrAbove())    return DESCRIPTOR;
-        return null;
-    }
-    
     protected static final class GetTempSpace implements FileCallable<DiskSpace> {
         @IgnoreJRERequirement
         public DiskSpace invoke(File f, VirtualChannel channel) throws IOException {

core/src/main/resources/hudson/node_monitors/Messages.properties

@@ -25,8 +25,10 @@ ClockMonitor.DisplayName=Clock Difference
 DiskSpaceMonitor.MarkedOffline=Making {0} offline temporarily due to the lack of disk space
 DiskSpaceMonitor.MarkedOnline=Putting {0} back online as there is enough disk space again
 DiskSpaceMonitor.DisplayName=Free Disk Space
+DiskSpaceMonitor.DisplayName5=Free Disk Space (Java 6+)
 ResponseTimeMonitor.DisplayName=Response Time
 ResponseTimeMonitor.MarkedOffline=Making {0} offline because it''s not responding
 ResponseTimeMonitor.TimeOut=Time out for last {0} try
 SwapSpaceMonitor.DisplayName=Free Swap Space
-TemporarySpaceMonitor.DisplayName=Free Temp Space
\ No newline at end of file
+TemporarySpaceMonitor.DisplayName=Free Temp Space
+TemporarySpaceMonitor.DisplayName5=Free Temp Space (Java 6+)