一、测试

测试代码如下： 

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);                }            }        }    }

输出结果：

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)

结果要么抛异常，要么结果为错误的负值。

二、简单分析

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.     *     * 
Subclasses should initialize this field to a {@link Calendar}     * appropriate for the {@link Locale} associated with this     * DateFormat.     * @serial     */    protected Calendar calendar;    ...}

DateFormat类使用了Calendar对象来维护parse和format过程中的日期时间值，当多线程同时使用同一个DateFormat对象，也就是多线程同时使用同一个Calendar对象来维护parse或format过程的日期时间值，必定会发生错乱。

引用Java api文档：

Synchronization

Date formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

三、解决方案

使用中要么为每个线程创建一个DateFormat实例，要么对其外部加锁。