JobHandler中开启子线程时，支持子线程输出执行日志并通过Rolling查看。(From：kangxinghua@github)

xxl-job-core/src/main/java/com/xxl/job/core/log/XxlJobFileAppender.java

 public class XxlJobFileAppender {
 	private static Logger logger = LoggerFactory.getLogger(XxlJobFileAppender.class);
 	
-	// for JobThread (support log for child thread)
+	// for JobThread (support log for child thread of job handler)
 	//public static ThreadLocal<String> contextHolder = new ThreadLocal<String>();
 	public static InheritableThreadLocal<String> contextHolder = new InheritableThreadLocal<String>();
 	public static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");