SimpleDateFormat学习使用

提起SimpleDateFormat类，想必做过Java开发的童鞋都不会感到陌生。没错，它就是Java中提供的日期时间的转化类。这里，
为什么说SimpleDateFormat类有线程安全问题呢？有些小伙伴可能会提出疑问：我们生产环境上一直在使用SimpleDateFormat
类来解析和格式化日期和时间类型的数据，一直都没有问题啊！我的回答是：没错，那是因为你们的系统达不到
SimpleDateFormat类出现问题的并发量，也就是说你们的系统没啥负载！
接下来，我们就一起看下在高并发下SimpleDateFormat类为何会出现安全问题，以及如何解决SimpleDateFormat类的安全问
题。

重现SimpleDateFormat线程安全问题

public class SimpleDateFormatTest01 {//执行总次数private static final int EXECUTE_COUNT = 1000;//同时运行的线程数量private static final int THREAD_COUNT = 20;//SimpleDateFormat对象private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");public static void main(String[] args) throws InterruptedException {final Semaphore semaphore = new Semaphore(THREAD_COUNT);final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);ExecutorService executorService = Executors.newCachedThreadPool();for (int i = 0; i < EXECUTE_COUNT; i++) {executorService.execute(() -> {try {semaphore.acquire();try {simpleDateFormat.parse("2020-01-01");} catch (ParseException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);} catch (NumberFormatException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);}semaphore.release();} catch (InterruptedException e) {System.out.println("信号量发生错误");e.printStackTrace();System.exit(1);}countDownLatch.countDown();});}countDownLatch.await();executorService.shutdown();System.out.println("所有线程格式化日期成功");}
}

运行结果

java.lang.NumberFormatException: For input string: ""
线程：pool-1-thread-1 格式化日期失败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:2051)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.xysd.concurrent.lab06.SimpleDateFormatTest01.lambda$main$0(SimpleDateFormatTest01.java:72)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)at java.lang.Thread.run(Thread.java:748)

原因

//SimpleDateFormat
public Date parse(String text, ParsePosition pos) {CalendarBuilder calb = new CalendarBuilder();Date parsedDate;try {//TODOparsedDate = calb.establish(calendar).getTime();if (ambiguousYear[0]) {if (parsedDate.before(defaultCenturyStart)) {parsedDate = calb.addYear(100).establish(calendar).getTime();}}}catch (IllegalArgumentException e) {pos.errorIndex = start;pos.index = oldStart;return null;}return parsedDate;}

//CalendarBuilder
//clear()和set()这两个方法执行不是原子操作所以造成线程不安全
Calendar establish(Calendar cal) {boolean weekDate = isSet(WEEK_YEAR)&& field[WEEK_YEAR] > field[YEAR];if (weekDate && !cal.isWeekDateSupported()) {// Use YEAR insteadif (!isSet(YEAR)) {set(YEAR, field[MAX_FIELD + WEEK_YEAR]);}weekDate = false;}//这里线clearcal.clear();for (int stamp = MINIMUM_USER_STAMP; stamp < nextStamp; stamp++) {for (int index = 0; index <= maxFieldIndex; index++) {if (field[index] == stamp) {//然后再setcal.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);}return cal;}

解决方案一方法局部变量

public class SimpleDateFormatTest01 {//执行总次数private static final int EXECUTE_COUNT = 1000;//同时运行的线程数量private static final int THREAD_COUNT = 20;public static void main(String[] args) throws InterruptedException {final Semaphore semaphore = new Semaphore(THREAD_COUNT);final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);ExecutorService executorService = Executors.newCachedThreadPool();for (int i = 0; i < EXECUTE_COUNT; i++) {executorService.execute(() -> {try {semaphore.acquire();try {SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");simpleDateFormat.parse("2020-01-01");} catch (ParseException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);} catch (NumberFormatException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);}semaphore.release();} catch (InterruptedException e) {System.out.println("信号量发生错误");e.printStackTrace();System.exit(1);}countDownLatch.countDown();});}countDownLatch.await();executorService.shutdown();System.out.println("所有线程格式化日期成功");}
}

解决方案二使用synchronized锁

public class SimpleDateFormatTest01 {//执行总次数private static final int EXECUTE_COUNT = 1000;//同时运行的线程数量private static final int THREAD_COUNT = 20;private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");public static void main(String[] args) throws InterruptedException {final Semaphore semaphore = new Semaphore(THREAD_COUNT);final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);ExecutorService executorService = Executors.newCachedThreadPool();for (int i = 0; i < EXECUTE_COUNT; i++) {executorService.execute(() -> {try {semaphore.acquire();try {synchronized(simpleDateFormat){simpleDateFormat.parse("2020-01-01");}} catch (ParseException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);} catch (NumberFormatException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);}semaphore.release();} catch (InterruptedException e) {System.out.println("信号量发生错误");e.printStackTrace();System.exit(1);}countDownLatch.countDown();});}countDownLatch.await();executorService.shutdown();System.out.println("所有线程格式化日期成功");}
}

解决方案三 Lock锁

public class SimpleDateFormatTest01 {//执行总次数private static final int EXECUTE_COUNT = 1000;//同时运行的线程数量private static final int THREAD_COUNT = 20;private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");private static Lock lock = new ReentrackLock();public static void main(String[] args) throws InterruptedException {final Semaphore semaphore = new Semaphore(THREAD_COUNT);final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);ExecutorService executorService = Executors.newCachedThreadPool();for (int i = 0; i < EXECUTE_COUNT; i++) {executorService.execute(() -> {try {semaphore.acquire();try {lock.lock();simpleDateFormat.parse("2020-01-01");} catch (ParseException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);} catch (NumberFormatException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);}finally{lock.unlock();}semaphore.release();} catch (InterruptedException e) {System.out.println("信号量发生错误");e.printStackTrace();System.exit(1);}countDownLatch.countDown();});}countDownLatch.await();executorService.shutdown();System.out.println("所有线程格式化日期成功");}
}

解决方案四 ThreadLocal

public class SimpleDateFormatTest01 {//执行总次数private static final int EXECUTE_COUNT = 1000;//同时运行的线程数量private static final int THREAD_COUNT = 20;//SimpleDateFormat对象private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");//本地线程private static ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal<SimpleDateFormat>() {@Overrideprotected SimpleDateFormat initialValue() {return new SimpleDateFormat("yyyy-MM-dd");}};private static DateFormat getDateFormat() {SimpleDateFormat dateFormat = threadLocal.get();if (dateFormat == null) {dateFormat = new SimpleDateFormat("yyyy-MM-dd");threadLocal.set(dateFormat);}return dateFormat;}public static void main(String[] args) throws InterruptedException {final Semaphore semaphore = new Semaphore(THREAD_COUNT);final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);ExecutorService executorService = Executors.newCachedThreadPool();for (int i = 0; i < EXECUTE_COUNT; i++) {executorService.execute(() -> {try {semaphore.acquire();try {getDateFormat().parse("2020-01-01");} catch (ParseException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);} catch (NumberFormatException e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);}semaphore.release();} catch (InterruptedException e) {System.out.println("信号量发生错误");e.printStackTrace();System.exit(1);}countDownLatch.countDown();});}countDownLatch.await();executorService.shutdown();System.out.println("所有线程格式化日期成功");}}

解决方案五 DateTimeFormatter

public class SimpleDateFormatTest01 {//执行总次数private static final int EXECUTE_COUNT = 1000;//同时运行的线程数量private static final int THREAD_COUNT = 20;private static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");public static void main(String[] args) throws InterruptedException {final Semaphore semaphore = new Semaphore(THREAD_COUNT);final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);ExecutorService executorService = Executors.newCachedThreadPool();for (int i = 0; i < EXECUTE_COUNT; i++) {executorService.execute(() -> {try {semaphore.acquire();try {LocalDate.parse("2020-01-01", formatter);} catch (Exception e) {System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");e.printStackTrace();System.exit(1);}semaphore.release();} catch (InterruptedException e) {System.out.println("信号量发生错误");e.printStackTrace();System.exit(1);}countDownLatch.countDown();});}countDownLatch.await();executorService.shutdown();System.out.println("所有线程格式化日期成功");}
}