Simple Date Format类到底为啥不是线程安全的？

摘要：我们就一起看下在高并发下Simple Date Format类为何会出现安全问题，以及如何解决Simple Date Format类的安全问题。

本文分享自华为云社区《【高并发】SimpleDateFormat类到底为啥不是线程安全的？》，作者：冰 河。
首先问下大家：你使用的Simple Date Format类还安全吗？为什么说Simple Date Format 类不是线程安全的？带着问题从本文中寻求答案。
提起Simple Date Format 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Simple Date Format 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Simple Date Format 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Simple Date Format 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Simple Date Format 类为何会出现安全问题，以及如何解决 Simple Date Format 类的安全问题。
重现 Simple Date Format类的线程安全问题

为了重现Simple Date Format类的线程安全问题，一种比较简单的方式就是使用线程池结合Java并发包中的Count Down Latch类和Semaphore类来重现线程安全问题。
有关Count Down Latch 类和Semaphore类的具体用法和底层原理与源码解析在【高并发专题】后文会深度分析。这里，大家只需要知道 Count Down Latch 类可以使一个线程等待其他线程各自执行完毕后再执行。而Semaphore类可以理解为一个计数信号量，必须由获取它的线程释放，经常用来限制访问某些资源的线程数量，例如限流等。
好了，先来看下重现Simple Date Format 类的线程安全问题的代码，如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 测试SimpleDateFormat的线程不安全问题
12. */
13. public class SimpleDateFormatTest01 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. //SimpleDateFormat对象
19. private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
20. public static void main(String[] args) throws InterruptedException {
21. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
22. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
23. ExecutorService executorService = Executors.newCachedThreadPool();
24. for (int i = 0; i < EXECUTE_COUNT; i++){
25. executorService.execute(() -> {
26. try {
27. semaphore.acquire();
28. try {
29. simpleDateFormat.parse("2020-01-01");
30. } catch (ParseException e) {
31. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
32. e.printStackTrace();
33. System.exit(1);
34. }catch (NumberFormatException e){
35. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
36. e.printStackTrace();
37. System.exit(1);
38. }
39. semaphore.release();
40. } catch (InterruptedException e) {
41. System.out.println("信号量发生错误");
42. e.printStackTrace();
43. System.exit(1);
44. }
45. countDownLatch.countDown();
46. });
47. }
48. countDownLatch.await();
49. executorService.shutdown();
50. System.out.println("package io.binghe.concurrent.lab06;
51. import java.text.ParseException;
52. import java.text.SimpleDateFormat;
53. import java.util.concurrent.CountDownLatch;
54. import java.util.concurrent.ExecutorService;
55. import java.util.concurrent.Executors;
56. import java.util.concurrent.Semaphore;
57. /**
58. * @author binghe
59. * @version 1.0.0
60. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
61. */
62. public class SimpleDateFormatTest02 {
63. //执行总次数
64. private static final int EXECUTE_COUNT = 1000;
65. //同时运行的线程数量
66. private static final int THREAD_COUNT = 20;
67. public static void main(String[] args) throws InterruptedException {
68. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
69. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
70. ExecutorService executorService = Executors.newCachedThreadPool();
71. for (int i = 0; i < EXECUTE_COUNT; i++){
72. executorService.execute(() -> {
73. try {
74. semaphore.acquire();
75. try {
76. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
77. simpleDateFormat.parse("2020-01-01");
78. } catch (ParseException e) {
79. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
80. e.printStackTrace();
81. System.exit(1);
82. }catch (NumberFormatException e){
83. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
84. e.printStackTrace();
85. System.exit(1);
86. }
87. semaphore.release();
88. } catch (InterruptedException e) {
89. System.out.println("信号量发生错误");
90. e.printStackTrace();
91. System.exit(1);
92. }
93. countDownLatch.countDown();
94. });
95. }
96. countDownLatch.await();
97. executorService.shutdown();
98. System.out.println("synchronized (simpleDateFormat){
99. simpleDateFormat.parse("2020-01-01");
100. }");
101. }
102. }");
103. }
104. }

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可以看到，在SimpleDateFormatTest01类中，首先定义了两个常量，一个是程序执行的总次数，一个是同时运行的线程数量。程序中结合线程池和Count Down Latch类与Semaphore类来模拟高并发的业务场景。其中，有关日期转化的代码只有如下一行。

1. simpleDateFormat.parse("2020-01-01");

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当程序捕获到异常时，打印相关的信息，并退出整个程序的运行。当程序正确运行后，会打印“package io.binghe.concurrent.lab06;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
/**
* @author binghe
* @version 1.0.0
* @description 局部变量法解决SimpleDateFormat类的线程安全问题
*/
public class SimpleDateFormatTest02 {
//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("synchronized (simpleDateFormat){
simpleDateFormat.parse("2020-01-01");
}");
}
}”。
运行程序输出的结果信息如下所示。

1. Exception in thread "pool-1-thread-4" Exception in thread "pool-1-thread-1" Exception in thread "pool-1-thread-2" 线程：pool-1-thread-7 格式化日期失败
2. 线程：pool-1-thread-9 格式化日期失败
3. 线程：pool-1-thread-10 格式化日期失败
4. Exception in thread "pool-1-thread-3" Exception in thread "pool-1-thread-5" Exception in thread "pool-1-thread-6" 线程：pool-1-thread-15 格式化日期失败
5. 线程：pool-1-thread-21 格式化日期失败
6. Exception in thread "pool-1-thread-23" 线程：pool-1-thread-16 格式化日期失败
7. 线程：pool-1-thread-11 格式化日期失败
8. java.lang.ArrayIndexOutOfBoundsException
9. 线程：pool-1-thread-27 格式化日期失败
10. at java.lang.System.arraycopy(Native Method)
11. at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:597)
12. at java.lang.StringBuffer.append(StringBuffer.java:367)
13. at java.text.DigitList.getLong(DigitList.java:191)线程：pool-1-thread-25 格式化日期失败
14. at java.text.DecimalFormat.parse(DecimalFormat.java:2084)
15. at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
16. at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
17. 线程：pool-1-thread-14 格式化日期失败
18. at java.text.DateFormat.parse(DateFormat.java:364)
19. at io.binghe.concurrent.lab06.SimpleDateFormatTest01.lambda$main$0(SimpleDateFormatTest01.java:47)
20. 线程：pool-1-thread-13 格式化日期失败at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
21. at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
22. at java.lang.Thread.run(Thread.java:748)
23. java.lang.NumberFormatException: For input string: ""
24. at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
25. 线程：pool-1-thread-20 格式化日期失败at java.lang.Long.parseLong(Long.java:601)
26. at java.lang.Long.parseLong(Long.java:631)
27. at java.text.DigitList.getLong(DigitList.java:195)
28. at java.text.DecimalFormat.parse(DecimalFormat.java:2084)
29. at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
30. at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
31. at java.text.DateFormat.parse(DateFormat.java:364)
32. at io.binghe.concurrent.lab06.SimpleDateFormatTest01.lambda$main$0(SimpleDateFormatTest01.java:47)
33. at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
34. at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
35. at java.lang.Thread.run(Thread.java:748)
36. java.lang.NumberFormatException: For input string: ""
37. at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
38. at java.lang.Long.parseLong(Long.java:601)
39. at java.lang.Long.parseLong(Long.java:631)
40. at java.text.DigitList.getLong(DigitList.java:195)
41. at java.text.DecimalFormat.parse(DecimalFormat.java:2084)
42. at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
43. at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
44. at java.text.DateFormat.parse(DateFormat.java:364)
45. Process finished with exit code 1

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说明，在高并发下使用Simple Date Format 类格式化日期时抛出了异常，Simple Date Forma t类不是线程安全的！！！
接下来，我们就看下，Simple Date Format 类为何不是线程安全的。

Simple Date Format 类为何不是线程安全的？

那么，接下来，我们就一起来看看真正引起Simple Date Format类线程不安全的根本原因。
通过查看Simple Date Format类的源码，我们得知：Simple Date Format是继承自Date Format类，Date Format类中维护了一个全局的Calendar变量，如下所示。

1. /**
2.   * The {@link Calendar} instance used for calculating the date-time fields
3.   * and the instant of time. This field is used for both formatting and
4.   * parsing.
5.   *
6.   * <p>Subclasses should initialize this field to a {@link Calendar}
7.   * appropriate for the {@link Locale} associated with this
8.   * DateFormat.
9.   * @serial
10.   */
11. protected Calendar calendar;

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从注释可以看出，这个Calendar对象既用于格式化也用于解析日期时间。接下来，我们再查看parse()方法接近最后的部分。

1. @Override
2. public Date parse(String text, ParsePosition pos){
3.     ################此处省略N行代码##################
4. Date parsedDate;
5. try {
6. parsedDate = calb.establish(calendar).getTime();
7. // If the year value is ambiguous,
8. // then the two-digit year == the default start year
9. if (ambiguousYear[0]) {
10. if (parsedDate.before(defaultCenturyStart)) {
11. parsedDate = calb.addYear(100).establish(calendar).getTime();
12. }
13. }
14. }
15. // An IllegalArgumentException will be thrown by Calendar.getTime()
16. // if any fields are out of range, e.g., MONTH == 17.
17. catch (IllegalArgumentException e) {
18. pos.errorIndex = start;
19. pos.index = oldStart;
20. return null;
21. }
22. return parsedDate;
23. }

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可见，最后的返回值是通过调用CalendarBuilder.establish()方法获得的，而这个方法的参数正好就是前面的Calendar对象。
接下来，我们再来看看CalendarBuilder.establish()方法，如下所示。

1. Calendar establish(Calendar cal) {
2. boolean weekDate = isSet(WEEK_YEAR)
3. && field[WEEK_YEAR] > field[YEAR];
4. if (weekDate && !cal.isWeekDateSupported()) {
5. // Use YEAR instead
6. if (!isSet(YEAR)) {
7. set(YEAR, field[MAX_FIELD + WEEK_YEAR]);
8. }
9. weekDate = false;
10. }
11. cal.clear();
12. // Set the fields from the min stamp to the max stamp so that
13. // the field resolution works in the Calendar.
14. for (int stamp = MINIMUM_USER_STAMP; stamp < nextStamp; stamp++) {
15. for (int index = 0; index <= maxFieldIndex; index++) {
16. if (field[index] == stamp) {
17. cal.set(index, field[MAX_FIELD + index]);
18. break;
19. }
20. }
21. }
22. if (weekDate) {
23. int weekOfYear = isSet(WEEK_OF_YEAR) ? field[MAX_FIELD + WEEK_OF_YEAR] : 1;
24. int dayOfWeek = isSet(DAY_OF_WEEK) ?
25. field[MAX_FIELD + DAY_OF_WEEK] : cal.getFirstDayOfWeek();
26. if (!isValidDayOfWeek(dayOfWeek) && cal.isLenient()) {
27. if (dayOfWeek >= 8) {
28. dayOfWeek--;
29. weekOfYear += dayOfWeek / 7;
30. dayOfWeek = (dayOfWeek % 7) + 1;
31. } else {
32. while (dayOfWeek <= 0) {
33. dayOfWeek += 7;
34. weekOfYear--;
35. }
36. }
37. dayOfWeek = toCalendarDayOfWeek(dayOfWeek);
38. }
39. cal.setWeekDate(field[MAX_FIELD + WEEK_YEAR], weekOfYear, dayOfWeek);
40. }
41. return cal;
42. }

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此时运行修改后的程序，输出结果如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }

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至于在高并发场景下使用局部变量为何能解决线程的安全问题，会在【JVM专题】的JVM内存模式相关内容中深入剖析，这里不做过多的介绍了。
当然，这种方式在高并发下会创建大量的SimpleDateFormat类对象，影响程序的性能，所以，这种方式在实际生产环境不太被推荐。
2.synchronized锁方式

将SimpleDateFormat类对象定义成全局静态变量，此时所有线程共享SimpleDateFormat类对象，此时在调用格式化时间的方法时，对SimpleDateFormat对象进行同步即可，代码如下所示。

1. package io.binghe.concurrent.lab06;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author binghe * @version 1.0.0 * @description 通过Synchronized锁解决SimpleDateFormat类的线程安全问题 */public class SimpleDateFormatTest03 { //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 { 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("package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }"); }}

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此时，解决问题的关键代码如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 通过Synchronized锁解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest03 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. //SimpleDateFormat对象
19. private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
20. public static void main(String[] args) throws InterruptedException {
21. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
22. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
23. ExecutorService executorService = Executors.newCachedThreadPool();
24. for (int i = 0; i < EXECUTE_COUNT; i++){
25. executorService.execute(() -> {
26. try {
27. semaphore.acquire();
28. try {
29. synchronized (simpleDateFormat){
30. simpleDateFormat.parse("2020-01-01");
31. }
32. } catch (ParseException e) {
33. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
34. e.printStackTrace();
35. System.exit(1);
36. }catch (NumberFormatException e){
37. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
38. e.printStackTrace();
39. System.exit(1);
40. }
41. semaphore.release();
42. } catch (InterruptedException e) {
43. System.out.println("信号量发生错误");
44. e.printStackTrace();
45. System.exit(1);
46. }
47. countDownLatch.countDown();
48. });
49. }
50. countDownLatch.await();
51. executorService.shutdown();
52. System.out.println("synchronized (simpleDateFormat){
53. simpleDateFormat.parse("2020-01-01");
54. }");
55. }
56. }

复制代码

运行程序，输出结果如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }

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需要注意的是，虽然这种方式能够解决SimpleDateFormat类的线程安全问题，但是由于在程序的执行过程中，为SimpleDateFormat类对象加上了synchronized锁，导致同一时刻只能有一个线程执行parse(String)方法。此时，会影响程序的执行性能，在要求高并发的生产环境下，此种方式也是不太推荐使用的。
3.Lock锁方式

Lock锁方式与synchronized锁方式实现原理相同，都是在高并发下通过JVM的锁机制来保证程序的线程安全。通过Lock锁方式解决问题的代码如下所示。

1. package io.binghe.concurrent.lab06;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;/** * @author binghe * @version 1.0.0 * @description 通过Lock锁解决SimpleDateFormat类的线程安全问题 */public class SimpleDateFormatTest04 { //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"); //Lock对象 private static Lock lock = new ReentrantLock(); 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("package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }"); }}

复制代码

通过代码可以得知，首先，定义了一个Lock类型的全局静态变量作为加锁和释放锁的句柄。然后在simpleDateFormat.parse(String)代码之前通过lock.lock()加锁。这里需要注意的一点是：为防止程序抛出异常而导致锁不能被释放，一定要将释放锁的操作放到finally代码块中，如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. import java.util.concurrent.locks.Lock;
9. import java.util.concurrent.locks.ReentrantLock;
10. /**
11. * @author binghe
12. * @version 1.0.0
13. * @description 通过Lock锁解决SimpleDateFormat类的线程安全问题
14. */
15. public class SimpleDateFormatTest04 {
16. //执行总次数
17. private static final int EXECUTE_COUNT = 1000;
18. //同时运行的线程数量
19. private static final int THREAD_COUNT = 20;
20. //SimpleDateFormat对象
21. private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
22. //Lock对象
23. private static Lock lock = new ReentrantLock();
24. public static void main(String[] args) throws InterruptedException {
25. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
26. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
27. ExecutorService executorService = Executors.newCachedThreadPool();
28. for (int i = 0; i < EXECUTE_COUNT; i++){
29. executorService.execute(() -> {
30. try {
31. semaphore.acquire();
32. try {
33. lock.lock();
34. simpleDateFormat.parse("2020-01-01");
35. } catch (ParseException e) {
36. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
37. e.printStackTrace();
38. System.exit(1);
39. }catch (NumberFormatException e){
40. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
41. e.printStackTrace();
42. System.exit(1);
43. }finally {
44. lock.unlock();
45. }
46. semaphore.release();
47. } catch (InterruptedException e) {
48. System.out.println("信号量发生错误");
49. e.printStackTrace();
50. System.exit(1);
51. }
52. countDownLatch.countDown();
53. });
54. }
55. countDownLatch.await();
56. executorService.shutdown();
57. System.out.println("finally {
58. lock.unlock();
59. }");
60. }
61. }

复制代码

运行程序，输出结果如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }

复制代码

此种方式同样会影响高并发场景下的性能，不太建议在高并发的生产环境使用。
4.ThreadLocal方式

使用ThreadLocal存储每个线程拥有的SimpleDateFormat对象的副本，能够有效的避免多线程造成的线程安全问题，使用ThreadLocal解决线程安全问题的代码如下所示。

1. package io.binghe.concurrent.lab06;import java.text.DateFormat;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author binghe * @version 1.0.0 * @description 通过ThreadLocal解决SimpleDateFormat类的线程安全问题 */public class SimpleDateFormatTest05 { //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 ThreadLocal threadLocal = new ThreadLocal(){ @Override protected DateFormat initialValue() { return 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 { threadLocal.get().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("package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }"); }}

复制代码

通过代码可以得知，将每个线程使用的SimpleDateFormat副本保存在ThreadLocal中，各个线程在使用时互不干扰，从而解决了线程安全问题。
运行程序，输出结果如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }

复制代码

此种方式运行效率比较高，推荐在高并发业务场景的生产环境使用。
另外，使用ThreadLocal也可以写成如下形式的代码，效果是一样的。

1. package io.binghe.concurrent.lab06;import java.text.DateFormat;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author binghe * @version 1.0.0 * @description 通过ThreadLocal解决SimpleDateFormat类的线程安全问题 */public class SimpleDateFormatTest06 { //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 ThreadLocal threadLocal = new ThreadLocal(); private static DateFormat getDateFormat(){ DateFormat 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("package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }"); }}

复制代码

5.DateTimeFormatter方式

DateTimeFormatter是Java8提供的新的日期时间API中的类，DateTimeFormatter类是线程安全的，可以在高并发场景下直接使用DateTimeFormatter类来处理日期的格式化操作。代码如下所示。

1. package io.binghe.concurrent.lab06;import java.time.LocalDate;import java.time.format.DateTimeFormatter;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author binghe * @version 1.0.0 * @description 通过DateTimeFormatter类解决线程安全问题 */public class SimpleDateFormatTest07 { //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("package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }"); }}

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可以看到，DateTimeFormatter类是线程安全的，可以在高并发场景下直接使用DateTimeFormatter类来处理日期的格式化操作。
运行程序，输出结果如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }

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使用DateTimeFormatter类来处理日期的格式化操作运行效率比较高，推荐在高并发业务场景的生产环境使用。
6.joda-time方式

joda-time是第三方处理日期时间格式化的类库，是线程安全的。如果使用joda-time来处理日期和时间的格式化，则需要引入第三方类库。这里，以Maven为例，如下所示引入joda-time库。

1. import org.joda.time.DateTime;
2. import org.joda.time.format.DateTimeFormat;
3. import org.joda.time.format.DateTimeFormatter;

复制代码

引入joda-time库后，实现的程序代码如下所示。

1. package io.binghe.concurrent.lab06;package io.binghe.concurrent.lab06;
2. import org.joda.time.DateTime;
3. import org.joda.time.format.DateTimeFormat;
4. import org.joda.time.format.DateTimeFormatter;
5. import java.util.concurrent.CountDownLatch;
6. import java.util.concurrent.ExecutorService;
7. import java.util.concurrent.Executors;
8. import java.util.concurrent.Semaphore;
9. /**
10. * @author binghe
11. * @version 1.0.0
12. * @description 通过DateTimeFormatter类解决线程安全问题
13. */
14. public class SimpleDateFormatTest08 {
15. //执行总次数
16. private static final int EXECUTE_COUNT = 1000;
17. //同时运行的线程数量
18. private static final int THREAD_COUNT = 20;
19. private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyy-MM-dd");
20. public static void main(String[] args) throws InterruptedException {
21. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
22. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
23. ExecutorService executorService = Executors.newCachedThreadPool();
24. for (int i = 0; i < EXECUTE_COUNT; i++){
25. executorService.execute(() -> {
26. try {
27. semaphore.acquire();
28. try {
29. DateTime.parse("2020-01-01", dateTimeFormatter).toDate();
30. }catch (Exception e){
31. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
32. e.printStackTrace();
33. System.exit(1);
34. }
35. semaphore.release();
36. } catch (InterruptedException e) {
37. System.out.println("信号量发生错误");
38. e.printStackTrace();
39. System.exit(1);
40. }
41. countDownLatch.countDown();
42. });
43. }
44. countDownLatch.await();
45. executorService.shutdown();
46. System.out.println("import org.joda.time.DateTime;
47. import org.joda.time.format.DateTimeFormat;
48. import org.joda.time.format.DateTimeFormatter;");
49. }
50. }import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;/** * @author binghe * @version 1.0.0 * @description 通过DateTimeFormatter类解决线程安全问题 */public class SimpleDateFormatTest08 { //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 dateTimeFormatter = DateTimeFormat.forPattern("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 { DateTime.parse("2020-01-01", dateTimeFormatter).toDate(); }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("package io.binghe.concurrent.lab06;
51. import java.text.ParseException;
52. import java.text.SimpleDateFormat;
53. import java.util.concurrent.CountDownLatch;
54. import java.util.concurrent.ExecutorService;
55. import java.util.concurrent.Executors;
56. import java.util.concurrent.Semaphore;
57. /**
58. * @author binghe
59. * @version 1.0.0
60. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
61. */
62. public class SimpleDateFormatTest02 {
63. //执行总次数
64. private static final int EXECUTE_COUNT = 1000;
65. //同时运行的线程数量
66. private static final int THREAD_COUNT = 20;
67. public static void main(String[] args) throws InterruptedException {
68. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
69. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
70. ExecutorService executorService = Executors.newCachedThreadPool();
71. for (int i = 0; i < EXECUTE_COUNT; i++){
72. executorService.execute(() -> {
73. try {
74. semaphore.acquire();
75. try {
76. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
77. simpleDateFormat.parse("2020-01-01");
78. } catch (ParseException e) {
79. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
80. e.printStackTrace();
81. System.exit(1);
82. }catch (NumberFormatException e){
83. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
84. e.printStackTrace();
85. System.exit(1);
86. }
87. semaphore.release();
88. } catch (InterruptedException e) {
89. System.out.println("信号量发生错误");
90. e.printStackTrace();
91. System.exit(1);
92. }
93. countDownLatch.countDown();
94. });
95. }
96. countDownLatch.await();
97. executorService.shutdown();
98. System.out.println("synchronized (simpleDateFormat){
99. simpleDateFormat.parse("2020-01-01");
100. }");
101. }
102. }"); }}

复制代码

这里，需要注意的是：DateTime类是org.joda.time包下的类，DateTimeFormat类和DateTimeFormatter类都是org.joda.time.format包下的类，如下所示。

1. package io.binghe.concurrent.lab06;
2. import org.joda.time.DateTime;
3. import org.joda.time.format.DateTimeFormat;
4. import org.joda.time.format.DateTimeFormatter;
5. import java.util.concurrent.CountDownLatch;
6. import java.util.concurrent.ExecutorService;
7. import java.util.concurrent.Executors;
8. import java.util.concurrent.Semaphore;
9. /**
10. * @author binghe
11. * @version 1.0.0
12. * @description 通过DateTimeFormatter类解决线程安全问题
13. */
14. public class SimpleDateFormatTest08 {
15. //执行总次数
16. private static final int EXECUTE_COUNT = 1000;
17. //同时运行的线程数量
18. private static final int THREAD_COUNT = 20;
19. private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyy-MM-dd");
20. public static void main(String[] args) throws InterruptedException {
21. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
22. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
23. ExecutorService executorService = Executors.newCachedThreadPool();
24. for (int i = 0; i < EXECUTE_COUNT; i++){
25. executorService.execute(() -> {
26. try {
27. semaphore.acquire();
28. try {
29. DateTime.parse("2020-01-01", dateTimeFormatter).toDate();
30. }catch (Exception e){
31. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
32. e.printStackTrace();
33. System.exit(1);
34. }
35. semaphore.release();
36. } catch (InterruptedException e) {
37. System.out.println("信号量发生错误");
38. e.printStackTrace();
39. System.exit(1);
40. }
41. countDownLatch.countDown();
42. });
43. }
44. countDownLatch.await();
45. executorService.shutdown();
46. System.out.println("import org.joda.time.DateTime;
47. import org.joda.time.format.DateTimeFormat;
48. import org.joda.time.format.DateTimeFormatter;");
49. }
50. }

复制代码

运行程序，输出结果如下所示。

1. package io.binghe.concurrent.lab06;
2. import java.text.ParseException;
3. import java.text.SimpleDateFormat;
4. import java.util.concurrent.CountDownLatch;
5. import java.util.concurrent.ExecutorService;
6. import java.util.concurrent.Executors;
7. import java.util.concurrent.Semaphore;
8. /**
9. * @author binghe
10. * @version 1.0.0
11. * @description 局部变量法解决SimpleDateFormat类的线程安全问题
12. */
13. public class SimpleDateFormatTest02 {
14. //执行总次数
15. private static final int EXECUTE_COUNT = 1000;
16. //同时运行的线程数量
17. private static final int THREAD_COUNT = 20;
18. public static void main(String[] args) throws InterruptedException {
19. final Semaphore semaphore = new Semaphore(THREAD_COUNT);
20. final CountDownLatch countDownLatch = new CountDownLatch(EXECUTE_COUNT);
21. ExecutorService executorService = Executors.newCachedThreadPool();
22. for (int i = 0; i < EXECUTE_COUNT; i++){
23. executorService.execute(() -> {
24. try {
25. semaphore.acquire();
26. try {
27. SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
28. simpleDateFormat.parse("2020-01-01");
29. } catch (ParseException e) {
30. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
31. e.printStackTrace();
32. System.exit(1);
33. }catch (NumberFormatException e){
34. System.out.println("线程：" + Thread.currentThread().getName() + " 格式化日期失败");
35. e.printStackTrace();
36. System.exit(1);
37. }
38. semaphore.release();
39. } catch (InterruptedException e) {
40. System.out.println("信号量发生错误");
41. e.printStackTrace();
42. System.exit(1);
43. }
44. countDownLatch.countDown();
45. });
46. }
47. countDownLatch.await();
48. executorService.shutdown();
49. System.out.println("synchronized (simpleDateFormat){
50. simpleDateFormat.parse("2020-01-01");
51. }");
52. }
53. }

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使用joda-time库来处理日期的格式化操作运行效率比较高，推荐在高并发业务场景的生产环境使用。
解决SimpleDateFormat类的线程安全问题的方案总结

综上所示：在解决解决SimpleDateFormat类的线程安全问题的几种方案中，局部变量法由于线程每次执行格式化时间时，都会创建SimpleDateFormat类的对象，这会导致创建大量的SimpleDateFormat对象，浪费运行空间和消耗服务器的性能，因为JVM创建和销毁对象是要耗费性能的。所以，不推荐在高并发要求的生产环境使用。
synchronized锁方式和Lock锁方式在处理问题的本质上是一致的，通过加锁的方式，使同一时刻只能有一个线程执行格式化日期和时间的操作。这种方式虽然减少了SimpleDateFormat对象的创建，但是由于同步锁的存在，导致性能下降，所以，不推荐在高并发要求的生产环境使用。
ThreadLocal通过保存各个线程的SimpleDateFormat类对象的副本，使每个线程在运行时，各自使用自身绑定的SimpleDateFormat对象，互不干扰，执行性能比较高，推荐在高并发的生产环境使用。
DateTimeFormatter是Java 8中提供的处理日期和时间的类，DateTimeFormatter类本身就是线程安全的，经压测，DateTimeFormatter类处理日期和时间的性能效果还不错（后文单独写一篇关于高并发下性能压测的文章）。所以，推荐在高并发场景下的生产环境使用。
joda-time是第三方处理日期和时间的类库，线程安全，性能经过高并发的考验，推荐在高并发场景下的生产环境使用。
 
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