SimpleDateFormat非线程安全

 1 //成员变量calendar，多线程下共享
 2 protected Calendar calendar;
 3 
 4 private StringBuffer format(Date var1, StringBuffer var2, FieldDelegate var3) {
 5         this.calendar.setTime(var1);
 6         boolean var4 = this.useDateFormatSymbols();
 7         int var5 = 0;
 8 
 9         while(var5 < this.compiledPattern.length) {
10             int var6 = this.compiledPattern[var5] >>> 8;
11             int var7 = this.compiledPattern[var5++] & 255;
12             if (var7 == 255) {
13                 var7 = this.compiledPattern[var5++] << 16;
14                 var7 |= this.compiledPattern[var5++];
15             }
16 
17             switch(var6) {
18             case 100:
19                 var2.append((char)var7);
20                 break;
21             case 101:
22                 var2.append(this.compiledPattern, var5, var7);
23                 var5 += var7;
24                 break;
25             default:
26                 this.subFormat(var6, var7, var3, var2, var4);
27             }
28         }
29 
30         return var2;
31     }

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);
        }
    }