Java Review - SimpleDateFormat线程不安全原因的源码分析及解决办法
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文章目录
概述
SimpleDateFormat是Java提供的一个格式化和解析日期的工具类，在日常开发中经常会用到，但是由于它是线程不安全的，所以多线程共用一个SimpleDateFormat实例对日期进行解析或者格式化会导致程序出错。
这里来揭示它为何是线程不安全的，以及如何避免该问题。
复现问题
import java.text.ParseException;
import java.text.SimpleDateFormat;
/**
* @author 小工匠
* @version 1.0
* @description: TODO
* @date 2021/11/21 14:56
* @mark: show me the code , change the world
*/
public class SimpleDateFormatTest
// 1 创建单例实例
private static SimpleDateFormat sdf = new SimpleDateFormat("YYYY-MM-dd HH:mm:ss");
public static void main(String[] args)
// 2 开启多个线程，并且欧东
for (int i = 0; i < 10; i++)
Thread thread = new Thread(() ->
try
// 3 使用单例日期解析文本
System.out.println(sdf.parse("2021-11-19 15:15:00"));
catch (ParseException e)
e.printStackTrace();
);
thread.start();
代码（1）创建了SimpleDateFormat的一个实例
代码（2）创建10个线程，每个线程都共用同一个sdf对象对文本日期进行解析。
多运行几次代码就会抛出java.lang.NumberFormatException异常，增加线程的个数有利于复现该问题
Exception in thread "Thread-0" Exception in thread "Thread-2" Exception in thread "Thread-1" Exception in thread "Thread-6" Exception in thread "Thread-4" Exception in thread "Thread-8" Exception in thread "Thread-9" Exception in thread "Thread-5" Exception in thread "Thread-7" 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.artisan.bfzm.chapter11.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:26)
at java.lang.Thread.run(Thread.java:748)
java.lang.ArrayIndexOutOfBoundsException: 20
at java.text.DigitList.append(DigitList.java:151)
at java.text.DecimalFormat.subparse(DecimalFormat.java:2278)
at java.text.DecimalFormat.parse(DecimalFormat.java:2036)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.artisan.bfzm.chapter11.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:26)
at java.lang.Thread.run(Thread.java:748)
java.lang.NumberFormatException: multiple points
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890)
at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
at java.lang.Double.parseDouble(Double.java:538)
at java.text.DigitList.getDouble(DigitList.java:169)
at java.text.DecimalFormat.parse(DecimalFormat.java:2089)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.artisan.bfzm.chapter11.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:26)
at java.lang.Thread.run(Thread.java:748)
java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Long.parseLong(Long.java:601)
at java.lang.Long.parseLong(Long.java:631)
at java.text.DigitList.getLong(DigitList.java:195)
at java.text.DecimalFormat.parse(DecimalFormat.java:2084)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.artisan.bfzm.chapter11.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:26)
at java.lang.Thread.run(Thread.java:748)
java.lang.NumberFormatException: empty String
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
at java.lang.Double.parseDouble(Double.java:538)
at java.text.DigitList.getDouble(DigitList.java:169)
at java.text.DecimalFormat.parse(DecimalFormat.java:2089)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.artisan.bfzm.chapter11.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:26)
at java.lang.Thread.run(Thread.java:748)
java.lang.NumberFormatException: empty String
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
at java.lang.Double.parseDouble(Double.java:538)
at java.text.DigitList.getDouble(DigitList.java:169)
at java.text.DecimalFormat.parse(DecimalFormat.java:2089)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.artisan.bfzm.chapter11.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:26)
at java.lang.Thread.run(Thread.java:748)
java.lang.ArrayIndexOutOfBoundsException: 19
at java.text.DigitList.append(DigitList.java:151)
at java.text.DecimalFormat.subparse(DecimalFormat.java:2278)
at java.text.DecimalFormat.parse(DecimalFormat.java:2036)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at com.artisan.bfzm.chapter11.SimpleDateFormatTest.lambda$main$0(SimpleDateFormatTest.java:26)
at java.lang.Thread.run(Thread.java:748)
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)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at
.....
源码分析
为了便于分析，首先来看SimpleDateFormat的类图结构
可以看到，每个SimpleDateFormat实例里面都有一个Calendar对象，后面我们就会知道，SimpleDateFormat之所以是线程不安全的，就是因为Calendar是线程不安全的。
Calendar之所以是线程不安全的，是因为其中存放日期数据的变量都是线程不安全的，比如fields、time等。
下面从代码层面来看下parse方法做了什么事情。
public Date parse(String source) throws ParseException
ParsePosition pos = new ParsePosition(0);
Date result = parse(source, pos);
if (pos.index == 0)
throw new ParseException("Unparseable date: \\"" + source + "\\"" ,
pos.errorIndex);
return result;
@Override
public Date parse(String text, ParsePosition pos)
.......
// 1 解析日期字符串，将解析好的数据放入CalendarBuilder对象中
CalendarBuilder calb = new CalendarBuilder();
.......
.......
Date parsedDate;
try
// 2 使用calb中解析好的日期数据设置calendar
parsedDate = calb.establish(calendar).getTime();
catch (IllegalArgumentException e)
.......
.......
return null;
.......
.......
return parsedDate;
-
代码（1）的主要作用是解析日期字符串并把解析好的数据放入 CalendarBuilder的实例calb中。CalendarBuilder是一个建造者模式，用来存放后面需要的数据。
-
代码（2）使用calb中解析好的日期数据设置calendar。
calb.establish的代码如下
Calendar establish(Calendar cal)
.....
// 3 重置日期对象cal的属性值
cal.clear();
// 4 使用calb中的属性设置cal
// Set the fields from the min stamp to the max stamp so that
// the field resolution works in the Calendar.
for (int stamp = MINIMUM_USER_STAMP; stamp < nextStamp; stamp++)
for (int index = 0; index <= maxFieldIndex; index++)
if (field[index] == stamp)
cal.set(index, field[MAX_FIELD + index]);
break;
if (weekDate)
int weekOfYear = isSet(WEEK_OF_YEAR) ? field[MAX_FIELD + WEEK_OF_YEAR] : 1;
int dayOfWeek = isSet(DAY_OF_WEEK) ?
field[MAX_FIELD + DAY_OF_WEEK] : cal.getFirstDayOfWeek();
if (!isValidDayOfWeek(dayOfWeek) && cal.isLenient())
if (dayOfWeek >= 8)
dayOfWeek--;
weekOfYear += dayOfWeek / 7;
dayOfWeek = (dayOfWeek % 7) + 1;
else
while (dayOfWeek <= 0)
dayOfWeek += 7;
weekOfYear--;
dayOfWeek = toCalendarDayOfWeek(dayOfWeek);
cal.setWeekDate(field[MAX_FIELD + WEEK_YEAR], weekOfYear, dayOfWeek);
// 5 返回cal
return cal;
- 代码（3）重置Calendar对象里面的属性值，如下所示。
public final void clear()
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