@Slf4j
public class Test {
 
    private static ExecutorService service;
 
    static {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("thread-%d").build();
        service = new ThreadPoolExecutor(50, 100, 1, TimeUnit.SECONDS, new ArrayBlockingQueue<>(100), threadFactory);
    }
 
    private static final SimpleDateFormat SIMPLE_DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
 
    public static void main(String[] args) {
        String oldDate = SIMPLE_DATE_FORMAT.format(new Date());
 
        for (int i = 0; i < 100; i++) {
            service.execute(()->{
                Date parse = null;
                try {
                    parse = SIMPLE_DATE_FORMAT.parse(oldDate);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
                String newDate = SIMPLE_DATE_FORMAT.format(parse);
                //多次转换，查看是否相等
                log.info(StringUtils.equals(oldDate, newDate) + "");
            });
 
        }
    }
}

上述代码执行后出现大量空指针异常和false的结果。

证明SimpleDateFormat确实不是线程安全的。