[JENKINS-15816] Working around javac bug triggered by d84eca45.

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

@@ -240,8 +240,8 @@ public abstract class Run <JobT extends Job<JobT,RunT>,RunT extends Run<JobT,Run
     private volatile transient RunExecution runner;
 
     private static final SimpleDateFormat CANONICAL_ID_FORMATTER = new SimpleDateFormat("yyyy-MM-dd_HH-mm-ss");
-    protected static final ThreadLocal<SimpleDateFormat> ID_FORMATTER =
-            new ThreadLocal<SimpleDateFormat>() {
+    protected static final ThreadLocal<SimpleDateFormat> ID_FORMATTER = new IDFormatterProvider();
+    private static final class IDFormatterProvider extends ThreadLocal<SimpleDateFormat> {
                 @Override
                 protected SimpleDateFormat initialValue() {
                     synchronized (CANONICAL_ID_FORMATTER) {