SimpleDateFormat 是 JDK 提供的一个日期格式化和解析类,但它是非线程安全的,原因如下。
parse方法
Date parsedDate;
try {
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;
}
其中,calb.establish(calendar)初始化了calendar日历数据
跟进查看
//1-----------------------
cal.clear();
// 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]);//2--------------
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);//2-----------
}
//3-------------------
return cal;
参考1- 2- 3-注释,线程不安全的原因,不能保证多核模式下,多线程执行是顺序执行的原子性
解决方法
1. 加锁
自建方法包裹代码
public class SimpleDateFormatTest {
static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public static synchronized Date parse(String s) throws ParseException {
return sdf.parse(s);
}
public static void main(String[] args) {
for (int i = 0; i <20 ; ++i) {
Thread thread = new Thread(() -> {
public void run() {
try {
System.out.println(parse("2018-07-17 10:52:16"));
} catch (ParseException e) {
e.printStackTrace();
}
}
};
thread.start()
}
}
}
串行执行,在大批量调用效率很低,不推荐
2.每次调用创建一个对象
public Date parse(String s) throws ParseException {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
return sdf.parse(s);
}
缺点,耗内存,GC压力增大,每次创建对象销毁,分配和销毁内存空间,整理内存,GC。不推荐
3.每个线程创建一个对象
public class SimpleDateFormatTest {
static ThreadLocal<SimpleDateFormat> local= new ThreadLocal<SimpleDateFormat>(){
@Override
protected SimpleDateFormat initialValue(){
return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
}
};
public static void main(String[] args) {
for (int i = 0; i < 5; ++i) {
Thread thread = new Thread(() -> {
public void run() {
try {
System.out.println(local.get().parse("2018-07-17 11:01:16"));
} catch (ParseException e) {
e.printStackTrace();
}finally {
//TODO 线程运行结束清除,避免内存泄露
local.remove();
}
}
});
thread.start();
}
}
}
每个线程一个对象,提高format对象利用率,推荐使用
但是ThreadLocal的存储是自定义map,key是线程相关弱引用,随线程结束而结束,value是我们创建的对象是强引用,需要每次结束后remove()显示调用,set()方法也可以