DateFormat 线程不安全

    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

    public static void main(String[] args) throws ParseException{
        for (int i = 0; i < 2; i++) {
            Thread t = new Thread(new TestRunnable(), "t-" + i);
            t.start();
        }
    }

    static class TestRunnable implements Runnable {

        @Override
        public void run() {
            while(true) {

                long dateTime = 0;
                String dateStr = "2016-05-09 08:21:02";

                try{
                    Date dt = sdf.parse(dateStr);
                    dateTime = dt.getTime();
                } catch(Exception e){
                    e.printStackTrace();
                }

                if(dateTime < 0) {
                    System.out.println(dateTime);
                }
            }
        }
    }

java.lang.NumberFormatException: For input string: ""
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Long.parseLong(Long.java:453)
	at java.lang.Long.parseLong(Long.java:483)
	at java.text.DigitList.getLong(DigitList.java:194)
	at java.text.DecimalFormat.parse(DecimalFormat.java:1316)
	at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2088)
	at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1455)
	at java.text.DateFormat.parse(DateFormat.java:355)
-125744830738000
-61916917138000
-61916917138000
	at com.tiza.ngp.rp.sanhui.util.Utils$TestRunnable.run(Utils.java:214)
	at java.lang.Thread.run(Thread.java:745)
java.lang.NumberFormatException: For input string: ""
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Long.parseLong(Long.java:453)

public abstract class DateFormat extends Format {

    /**
     * The {@link Calendar} instance used for calculating the date-time fields
     * and the instant of time. This field is used for both formatting and
     * parsing.
     *
     * <p>Subclasses should initialize this field to a {@link Calendar}
     * appropriate for the {@link Locale} associated with this
     * <code>DateFormat</code>.
     * @serial
     */
    protected Calendar calendar;

    ...
}