为什么有人需要线程安全的SimpleDateFormat对象？

Question

I was looking for the usage of ThreadLocal and landed on this popular page When and how should I use a ThreadLocal variable?

The accepted, highest voted answer says

One possible (and common) use is when you have some object that is not thread-safe, but you want to avoid synchronizing access to that object (I'm looking at you, SimpleDateFormat).

And the core part of the code is

 return new SimpleDateFormat("yyyyMMdd HHmm");

which won't change or be affected by conncurrent execution, or would it?

Can you please highlight how this could be a issue? And why would we need a thread safe object here?

In other occurrence, I have come across a similar usage with java.security.MessageDigest; , which is also a puzzler to me. It would be great if anyone could explain the reasons behind this, with some helpful code if possible.

Answer 1

SimpleDateFormat扩展了具有setter方法的DateFormat，因此一个线程可以更改SimpleDateFormat实例的属性，而其他线程可以使用它并假设更早的属性，或者更糟糕的是，在执行过程中更改属性会导致内部不一致的结果。

Answer 2

Well, take the first line in format(Date, StringBuffer, FieldDelegate) :

calendar.setTime(date);

calendar there is an instance member, so that's obviously not thread-safe there. Firstly there's a date race (since setTime is not synchronized), but even more glaringly, someone could come through and set the calendar's time to something else part-way through the function ( calendar 's value is accessed in subFormat , which format calls).