java坑之SimpleDateFormat线程安全问题

我们经常会使用simpleDateFormat进行日期的格式转换，便于我们在页面进行数据输出时，符合业务需要的时间格式进行展示。

不过SimpleDateFormat本身是具有线程安全问题，当我们加固SimpleDateFormat作为静态变量时，就会导致异常。

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

public static void main(String[] args) {
    ThreadPoolExecutor threadPoolExecutor =
            new ThreadPoolExecutor(10, 100, 1, TimeUnit.MINUTES,
                    new LinkedBlockingDeque<>(1000));

    while(true) {
        threadPoolExecutor.execute(() -> {
            String format = sdf.format(new Date());
            try {
                Date parseDate = sdf.parse(format);
                String format2 = sdf.format(parseDate);
                System.out.println(format.equals(format2));
            } catch (ParseException e) {
                e.printStackTrace();
            }
        });

    }
}

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:601)
	at java.lang.Long.parseLong(Long.java:631)
	at java.text.DigitList.getLong(DigitList.java:195)
	at java.text.DecimalFormat.parse(DecimalFormat.java:2084)
	at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
	at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
	at java.text.DateFormat.parse(DateFormat.java:364)
	at com.huawei.it.api.controller.WhiteController.lambda$main$1(WhiteController.java:112)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
Exception in thread "pool-1-thread-7" Exception in thread "pool-1-thread-3" Exception in thread "pool-1-thread-1" java.lang.NumberFormatException: multiple points
	at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890)
	at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
	at java.lang.Double.parseDouble(Double.java:538)
	at java.text.DigitList.getDouble(DigitList.java:169)
	at java.text.DecimalFormat.parse(DecimalFormat.java:2089)

原因分析：

1.SimpleDateFormat中存在一个成员变量Calendar

2.parse()函数和format()函数均使用该变量进行保存

3.SimpleDateFormat为静态，则成员变量也静态共享

4.Calendar不存在线程安全机制，线程并发时，SimpleDateFormat存在线程安全问题