Java-JUC（十四）：SimpleDateFormat是线程不安全的
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SimpleDateFormat是Java提供的一个格式化和解析日期的工具类，日常开发中应该经常会用到，但是由于它是线程不安全的，多线程公用一个SimpleDateFormat实例对日期进行解析、格式化都会导致程序出错，接下来就讨论下它为何是线程不安全的，以及如何避免。
问题复现
编写测试代码如下：
private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); public static void main(String[] args) String[] waitingFormatTimeItems = "2019-08-06", "2019-08-07", "2019-08-08" ; for (int i = 0; i < waitingFormatTimeItems.length; i++) final int i2 = i; Thread thread = new Thread(new Runnable() @Override public void run() for (int j = 0; j < 100; j++) String str = waitingFormatTimeItems[i2]; String str2 = null; Date parserDate = null; try parserDate = sdf.parse(str); catch (ParseException e) e.printStackTrace(); str2 = sdf.format(parserDate); System.out.println("i: " + i2 + "\\tj: " + j + "\\tThreadName: " + Thread.currentThread().getName() + "\\t" + str + "\\t" + str2); if (!str.equals(str2)) throw new RuntimeException("date conversion failed after " + j + " iterations. Expected " + str + " but got " + str2); ); thread.start();
运行会抛出java.lang.RuntimeException，说明处理的结果时不正确的，从下边日志也看出来。
i: 2 j: 0 ThreadName: Thread-2 2019-08-08 2208-09-17 Exception in thread "Thread-2" Exception in thread "Thread-1" Exception in thread "Thread-0" i: 1 j: 0 ThreadName: Thread-1 2019-08-07 2208-09-17 i: 0 j: 0 ThreadName: Thread-0 2019-08-06 2208-09-17 java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 2019-08-08 but got 2208-09-17 at dx.test.ThreadLocalTest$2.run(ThreadLocalTest.java:36) at java.lang.Thread.run(Thread.java:748) java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 2019-08-07 but got 2208-09-17 at dx.test.ThreadLocalTest$2.run(ThreadLocalTest.java:36) at java.lang.Thread.run(Thread.java:748) java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 2019-08-06 but got 2208-09-17 at dx.test.ThreadLocalTest$2.run(ThreadLocalTest.java:36) at java.lang.Thread.run(Thread.java:748)
测试代码多运行几次，会发现抛出 java.lang.NumberFormatException 异常：
Exception in thread "Thread-1" Exception in thread "Thread-0" Exception in thread "Thread-2" 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:2056)
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.dx.test.ThreadLocalTest$2.run(ThreadLocalTest.java:29)
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:2056)
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.dx.test.ThreadLocalTest$2.run(ThreadLocalTest.java:29)
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:2056)
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.dx.test.ThreadLocalTest$2.run(ThreadLocalTest.java:29)
at java.lang.Thread.run(Thread.java:748)
问题分析
首先看下SimpleDateFormat的类图结构：
从类图和源代码从都可以发现，SimpleDateFormat内部依赖于Calendar对象，通过下边代码分析会发现：实际上SimpleDateFormat的线程不安全就是因为Calendar是线程不安全的。
Calendar内部存储的日期数据的变量field，time等都是不安全的，更重要的Calendar内部函数操作对变量操作是不具有原子性的操作。
SimpleDateFormat#parse方法：
@Override public Date parse(String text, ParsePosition pos) checkNegativeNumberExpression(); int start = pos.index; int oldStart = start; int textLength = text.length(); boolean[] ambiguousYear = false; //(1)解析日期字符串放入CalendarBuilder的实例calb中 CalendarBuilder calb = new CalendarBuilder(); for (int i = 0; i < compiledPattern.length; ) int tag = compiledPattern[i] >>> 8; int count = compiledPattern[i++] & 0xff; if (count == 255) count = compiledPattern[i++] << 16; count |= compiledPattern[i++]; switch (tag) case TAG_QUOTE_ASCII_CHAR: if (start >= textLength || text.charAt(start) != (char)count) pos.index = oldStart; pos.errorIndex = start; return null; start++; break; case TAG_QUOTE_CHARS: while (count-- > 0) if (start >= textLength || text.charAt(start) != compiledPattern[i++]) pos.index = oldStart; pos.errorIndex = start; return null; start++; break; default: // Peek the next pattern to determine if we need to obey the number of pattern letters for parsing. // It‘s required when parsing contiguous digit text (e.g., "20010704") with a pattern which has no delimiters between fields, like "yyyyMMdd". boolean obeyCount = false; // In Arabic, a minus sign for a negative number is put after the number. Even in another locale, a minus sign can be put after a number using DateFormat.setNumberFormat(). // If both the minus sign and the field-delimiter are ‘-‘, subParse() needs to determine whether a ‘-‘ after a number in the given text is a delimiter or is a minus sign for the preceding number. // We give subParse() a clue based on the information in compiledPattern. boolean useFollowingMinusSignAsDelimiter = false; if (i < compiledPattern.length) int nextTag = compiledPattern[i] >>> 8; if (!(nextTag == TAG_QUOTE_ASCII_CHAR || nextTag == TAG_QUOTE_CHARS)) obeyCount = true; if (hasFollowingMinusSign && (nextTag == TAG_QUOTE_ASCII_CHAR || nextTag == TAG_QUOTE_CHARS)) int c; if (nextTag == TAG_QUOTE_ASCII_CHAR) c = compiledPattern[i] & 0xff; else c = compiledPattern[i+1]; if (c == minusSign) useFollowingMinusSignAsDelimiter = true; start = subParse(text, start, tag, count, obeyCount, ambiguousYear, pos, useFollowingMinusSignAsDelimiter, calb); if (start < 0) pos.index = oldStart; return null; // At this point the fields of Calendar have been set. Calendar // will fill in default values for missing fields when the time // is computed. pos.index = start; Date parsedDate; try //（2）使用calb中解析好的日期数据设置calendar parsedDate = calb.establish(calendar).getTime(); // If the year value is ambiguous, // then the two-digit year == the default start year if (ambiguousYear[0]) if (parsedDate.before(defaultCenturyStart)) parsedDate = calb.addYear(100).establish(calendar).getTime(); // An IllegalArgumentException will be thrown by Calendar.getTime() // if any fields are out of range, e.g., MONTH == 17. catch (IllegalArgumentException e) pos.errorIndex = start; pos.index = oldStart; return null; return parsedDate;
CalendarBuilder#establish方法：
Calendar establish(Calendar cal) boolean weekDate = isSet(WEEK_YEAR) && field[WEEK_YEAR] > field[YEAR]; if (weekDate && !cal.isWeekDateSupported()) // Use YEAR instead if (!isSet(YEAR)) set(YEAR, field[MAX_FIELD + WEEK_YEAR]); weekDate = false; //（3）重置日期对象cal的属性值 cal.clear(); //(4) 使用calb中中属性设置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)返回设置好的cal对象 return cal;
Calendar#clear()方法：
代码（3）重置Calendar对象里面的属性值，如下代码：
public final void clear() for (int i = 0; i < fields.length; ) stamp[i] = fields[i] = 0; // UNSET == 0 isSet[i++] = false; areAllFieldsSet = areFieldsSet = false; isTimeSet = false;
代码（4）使用calb中解析好的日期数据设置cal对象
代码（5） 返回设置好的cal对象
代码（3）、（4）、（5）这几步骤一起操作不具有原子性，当A线程操作了（3）、（4），当将要执行（5）返回结果之前，如果B线程执行（3）会导致线程A的结果错误。
那么多线程下如何保证SimpleDateFormat的安全性呢？
1）每个线程使用时，都new一个SimpleDateFormat的实例，这保证每个线程都用各自的Calendar实例。
public static void main(String[] args) String[] waitingFormatTimeItems = "2019-08-06", "2019-08-07", "2019-08-08" ; for (int i = 0; i < waitingFormatTimeItems.length; i++) final int i2 = i; Thread thread = new Thread(new Runnable() @Override public void run() SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); for (int j = 0; j < 100; j++) String str = waitingFormatTimeItems[i2]; String str2 = null; Date parserDate = null;try parserDate = sdf.parse(str); catch (ParseException e) e.printStackTrace(); str2 = sdf.format(parserDate); System.out.println("i: " + i2 + "\\tj: " + j + "\\tThreadName: " + Thread.currentThread().getName() + "\\t" + str + "\\t" + str2); if (!str.equals(str2)) throw new RuntimeException("date conversion failed after " + j + " iterations. Expected " + str + " but got " + str2); ); thread.start();
这种方式缺点：每个线程都 new 一个对象，并且使用后由于没有其它引用，都需要被回收，开销比较大。
2）经过分析最终导致SimpleDateFormat的线程不安全原因是步骤（3）、（4）、（5）不是一个原子性操作，那么就可以对其进行同步，让（3）、（4）、（5）成为原子操作，可以使用ReetentLock。Synchronized等进行同步。
private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); public static void main(String[] args) String[] waitingFormatTimeItems = "2019-08-06", "2019-08-07", "2019-08-08" ; for (int i = 0; i < waitingFormatTimeItems.length; i++) final int i2 = i; Thread thread = new Thread(new Runnable() @Override public void run() for (int j = 0; j < 100; j++) String str = waitingFormatTimeItems[i2]; String str2 = null; Date parserDate = null; synchronized (sdf) try parserDate = sdf.parse(str); catch (ParseException e) e.printStackTrace(); str2 = sdf.format(parserDate); System.out.println("i: " + i2 + "\\tj: " + j + "\\tThreadName: " + Thread.currentThread().getName() + "\\t" + str + "\\t" + str2); if (!str.equals(str2)) throw new RuntimeException("date conversion failed after " + j + " iterations. Expected " + str + " but got " + str2); ); thread.start();
使用了同步锁，意味着多线程下会竞争锁，在高并发情况下会导致系统响应性能下降。
3）使用ThreadLocal，这样每个线程只需要使用一个SimpleDateFormat实例，在多线程下比第一种节省了对象的销毁开销，并且不需要对多线程进行同步，代码如下：
当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其他线程所对应的副本。
ThreadLocal包含定义了一个ThreadLocalMap，ThreadLocalMap的key为弱引用的线程（ThreadLocal<?>），要保存的线程局部变量的值为value（Object）.
private static ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal<SimpleDateFormat>() @Override protected SimpleDateFormat initialValue() return new SimpleDateFormat("yyyy-MM-dd"); ; ; public static void main(String[] args) String[] waitingFormatTimeItems = "2019-08-06", "2019-08-07", "2019-08-08" ; for (int i = 0; i < waitingFormatTimeItems.length; i++) final int i2 = i; Thread thread = new Thread(new Runnable() @Override public void run() SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); for (int j = 0; j < 100; j++) String str = waitingFormatTimeItems[i2]; String str2 = null; Date parserDate = null; try parserDate = threadLocal.get().parse(str); catch (ParseException e) e.printStackTrace(); str2 = threadLocal.get().format(parserDate); System.out.println("i: " + i2 + "\\tj: " + j + "\\tThreadName: " + Thread.currentThread().getName() + "\\t" + str + "\\t" + str2); if (!str.equals(str2)) throw new RuntimeException("date conversion failed after " + j + " iterations. Expected " + str + " but got " + str2); ); thread.start();
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