Is Java's TimeZone thread-safe?

Question

I wanted my application to just have one TimeZone object which will be used by many SimpleDateFormat and Calendar objects from the other places concurrently. This is to avoid having to always do TimeZone.getTimeZone(ID).

I know SimpleDateFormat and Calendar classes are not thread safe, which is why I configure one thread to always create new instances of them. But what about TimeZone? It is not clear to me whether I can do the following safely:

final TimeZone tz = TimeZone.getTimeZone("GMT");
...
//Thread 1.
Thread t1 = new Thread(Runnable(){
    public void run()
    {
        Calendar cal = Calendar.getInstance(tz);
        ...
        SimpleDateFormat sdf = new SimpleDateFormat();
        sdf.setTimeZone(tz);
        ...
    }
});
t1.start();
...
//Thread 2.
Thread t2 = new Thread(Runnable(){
    public void run()
    {
        Calendar cal = Calendar.getInstance(tz);
        ...
        SimpleDateFormat sdf = new SimpleDateFormat();
        sdf.setTimeZone(tz);
        ...
    }
});
t2.start();
...

Thanks!

Answer 1

I looked once at the source code for it and came to a conclusion that it was not.

Look at JodaTime timezone class. It says in javadoc that it's thread-safe and immutable.

Answer 2

It is not.

And you don't need it. You code uses it as:

final TimeZone tz = TimeZone.getTimeZone("GMT");
 ......
// thread 1 SimpleDateFormat instance
sdf.setTimeZone(tz);

// thread 2 SimpleDateFormat instance
sdf.setTimeZone(tz);

You have two threads using the same time zone, but since you are not modifying it you don't need it to be thread safe.

The only thing that you can modify is the ID, but even then you're ok, for the rest of the attributes are read-only

The only way you can get into trouble is by changing the id and caching your self the timezone, if you get it always from the TimeZone.getTimeZone() you are safe too, because that method is thread safe.