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Partially constructed object / multithreading

I use joda due to its good reputation for multithreading. This makes great distances to make multi-threaded date processing efficient, for example, making all Date / Time / DateTime objects immutable.

But here is a situation where I’m not sure that Joda is really doing the right thing. Perhaps this is so, but I am very interested to see the explanation.

When the call toString () DateTime is called Joda does the following:

/* org.joda.time.base.AbstractInstant */
public String toString() {
    return ISODateTimeFormat.dateTime().print(this);
}

All formatting files are thread safe (they are immutable), but what about formatter-factory:

private static DateTimeFormatter dt;

/*  org.joda.time.format.ISODateTimeFormat */
public static DateTimeFormatter dateTime() {
    if (dt == null) {
        dt = new DateTimeFormatterBuilder()
            .append(date())
            .append(tTime())
            .toFormatter();
    }
    return dt;
}

This is a common pattern in single-threaded applications, but it is known to be prone to errors in a multi-threaded environment.

I see the following dangers:

  • Race condition at zero check → worst case: two objects are created.

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