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();
}
}
参考：
原文地址：https://www.cnblogs.com/yy3b2007com/p/11360895.html
时间： 2024-10-28 09:44:31