SimpleDateFormat非线程安全

//成员变量calendar，多线程下共享
protected Calendar calendar;

private StringBuffer format(Date var1, StringBuffer var2, FieldDelegate var3) {
        this.calendar.setTime(var1);
        boolean var4 = this.useDateFormatSymbols();
        int var5 = 0;

        while(var5 < this.compiledPattern.length) {
            int var6 = this.compiledPattern[var5] >>> 8;
            int var7 = this.compiledPattern[var5++] & 255;
            if (var7 == 255) {
                var7 = this.compiledPattern[var5++] << 16;
                var7 |= this.compiledPattern[var5++];
            }

            switch(var6) {
            case 100:
                var2.append((char)var7);
                break;
            case 101:
                var2.append(this.compiledPattern, var5, var7);
                var5 += var7;
                break;
            default:
                this.subFormat(var6, var7, var3, var2, var4);
            }
        }

        return var2;
    }

解决方案：
阿里规范

【强制】SimpleDateFormat 是线程不安全的类，一般不要定义为static变量，如果定义为static，必须加锁，或者使用DateUtils工具类。 正例：注意线程安全，使用DateUtils。亦推荐如下处理：

public class DateUtils {

    private static ThreadLocal<DateFormat> threadLocal = new ThreadLocal<DateFormat>(){
        @Override
        protected DateFormat initialValue() {
            return new SimpleDateFormat("yyyy-MM-dd");
        }
    };

    public static Date parse(String formatPattern, String dateString) throws ParseException {
        return threadLocal.get().parse(dateString);
    }

    public static String format(String formatPattern, Date date) {
        return threadLocal.get().format(date);
    }
}

说明：如果是JDK8的应用，可以使用Instant代替Date，LocalDateTime代替Calendar，DateTimeFormatter代替SimpleDateFormat，官方给出的解释：simple beautiful strong immutable thread-safe。

public class DateUtils {

        public static final DateTimeFormatter DATE_TIME_FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd");

        public static LocalDate parse(String dateString){
            return LocalDate.parse(dateString, DATE_TIME_FORMATTER);
        }

        public static String format(LocalDate target) {
            return target.format(DATE_TIME_FORMATTER);
        }
    }