# 问题代码

private static final DateFormat df = new SimpleDateFormat("yyyy年MM月");

public String convertDate( Date date ){

  String date = df.format(date);

  return date;

}

# 官方解释

Call to static DateFormat

As the JavaDoc states, DateFormats are inherently unsafe for multithreaded use. The detector has found a call to an instance of DateFormat that has been obtained via a static field. This looks suspicous.
For more information on this see Sun Bug #6231579 and Sun Bug #6178997.

Sun Bug #6231579

Sun Bug #6178997

DateFormat 不是线程安全的