Merge pull request #1200 from paopaofish/master

xxl-job-core/src/main/java/com/xxl/job/core/util/DateUtil.java

     private static final String DATE_FORMAT = "yyyy-MM-dd";
     private static final String DATETIME_FORMAT = "yyyy-MM-dd HH:mm:ss";
 
-    private static ThreadLocal<Map<String, DateFormat>> dateFormatThreadLocal = new ThreadLocal<Map<String, DateFormat>>();
+    private static final ThreadLocal<Map<String, DateFormat>> dateFormatThreadLocal = new ThreadLocal<Map<String, DateFormat>>();
     private static DateFormat getDateFormat(String pattern) {
         if (pattern==null || pattern.trim().length()==0) {
             throw new IllegalArgumentException("pattern cannot be empty.");