There are six ways to help you deal with the simpledateformat class, which is not a thread safety problem

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【 High concurrency 】SimpleDateFormat Why classes are not thread safe ？

》, author ： ice   The river  .

First of all, let me ask you ： You used SimpleDateFormat Is class still safe ？ Why do you say SimpleDateFormat Classes are not thread-safe ？ Seek answers from this article with questions .

mention SimpleDateFormat class , I think I did Java The development of children's shoes will not feel strange . you 're right , It is Java The date time conversion class provided in . here , Why do you say SimpleDateFormat Class has thread safety problems ？ Some friends may ask questions ： We use it all the time in our production environment SimpleDateFormat Class to parse and format date and time type data , No problem all the time ！ My answer is ： you 're right , That's because your system can't reach SimpleDateFormat The amount of concurrency that went wrong with the class , So your system doesn't have much load ！

Next , Let's take a look at it together and make it SimpleDateFormat Why do classes have security problems , And how to solve it SimpleDateFormat Class security issues .

repeat SimpleDateFormat Thread safety of class

In order to reproduce SimpleDateFormat Thread safety of class , A relatively simple way is to use thread pool combination Java And the CountDownLatch Classes and Semaphore Class to reproduce thread safety issues .

of CountDownLatch Classes and Semaphore Class specific usage and underlying principles and source code analysis in 【 High concurrency topics 】 Later, I will analyze it in depth . here , You just need to know CountDownLatch Class enables a thread to wait for other threads to finish their own execution . and Semaphore Class can be understood as a counting semaphore , Must be released by the thread that gets it , Often used to limit the number of threads that access resources , For example, current limiting, etc .

Okay , Let's look at the reproduction first SimpleDateFormat Class thread safety issues code , As shown below .

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  test SimpleDateFormat Thread security problem
 */
public class SimpleDateFormatTest01 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 private static final int THREAD_COUNT = 20;
 //SimpleDateFormat object
 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(&quot;2020-01-01&quot;);
 } catch (ParseException e) {
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }catch (NumberFormatException e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

You can see , stay SimpleDateFormatTest01 Class , First, we define two constants , One is the total number of times the program is executed , One is the number of threads running at the same time . The program combines line pool and CountDownLatch Class and Semaphore Class to simulate high concurrency business scenarios . among , The only code for date conversion is the following line .

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

When the program catches an exception , Print relevant information , And exit the whole program . When the program runs correctly , prints " All threads successfully formatted date ".

The output information of the running program is as follows .

Exception in thread "pool-1-thread-4" Exception in thread "pool-1-thread-1" Exception in thread "pool-1-thread-2"  Threads ：pool-1-thread-7  Failed to format date
Threads ：pool-1-thread-9  Failed to format date
Threads ：pool-1-thread-10  Failed to format date
Exception in thread "pool-1-thread-3" Exception in thread "pool-1-thread-5" Exception in thread "pool-1-thread-6"  Threads ：pool-1-thread-15  Failed to format date
Threads ：pool-1-thread-21  Failed to format date
Exception in thread "pool-1-thread-23"  Threads ：pool-1-thread-16  Failed to format date
Threads ：pool-1-thread-11  Failed to format date
java.lang.ArrayIndexOutOfBoundsException
Threads ：pool-1-thread-27  Failed to format date
at java.lang.System.arraycopy(Native Method)
at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:597)
at java.lang.StringBuffer.append(StringBuffer.java:367)
at java.text.DigitList.getLong(DigitList.java:191) Threads ：pool-1-thread-25  Failed to format date
at java.text.DecimalFormat.parse(DecimalFormat.java:2084)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
Threads ：pool-1-thread-14  Failed to format date
at java.text.DateFormat.parse(DateFormat.java:364)
at io.binghe.concurrent.lab06.SimpleDateFormatTest01.lambda$main$0(SimpleDateFormatTest01.java:47)
Threads ：pool-1-thread-13  Failed to format date at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
Threads ：pool-1-thread-20  Failed to format date 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:2084)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
at io.binghe.concurrent.lab06.SimpleDateFormatTest01.lambda$main$0(SimpleDateFormatTest01.java:47)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
java.lang.NumberFormatException: For input string: ""
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:2084)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1869)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
at java.text.DateFormat.parse(DateFormat.java:364)
Process finished with exit code 1

explain , Use in high parallel mode SimpleDateFormat Class threw an exception while formatting the date ,SimpleDateFormat Classes are not thread-safe !!!

Next , Let's take a look at ,SimpleDateFormat Why classes are not thread safe .

SimpleDateFormat Why classes are not thread safe ？

that , Next , Let's see what really causes SimpleDateFormat The root cause of class thread insecurity .

By looking at SimpleDateFormat Class source code , We learned that ：SimpleDateFormat It is inherited from DateFormat class ,DateFormat Class maintains a global Calendar Variable , As shown below .

/**
 * The {@link Calendar} instance used for calculating the date-time fields
 * and the instant of time. This field is used for both formatting and
 * parsing.
 *
 * <p>Subclasses should initialize this field to a {@link Calendar}
 * appropriate for the {@link Locale} associated with this
 * <code>DateFormat</code>.
 * @serial
 */
protected Calendar calendar; Copy

As can be seen from the notes , This Calendar Object is used for both formatting and parsing date and time . Next , Let's see parse() The method is close to the last part .

@Override
public Date parse(String text, ParsePosition pos){
 ################ Omit here N Line code ##################
 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;
 }
 return parsedDate;
}

so , The final return value is by calling CalendarBuilder.establish（） Method of obtaining , And the parameters of this method are exactly the ones mentioned above Calendar object .

Next , Let's see CalendarBuilder.establish（） Method , As shown below .

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;
 }
 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]);
 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;
}

stay CalendarBuilder.establish（） The method has been called cal.clear（） And cal.set（）, That's to say, clean up first cal Value set in object , And then reset the new value . because Calendar There is no thread safety mechanism inside , And neither of these operations is atomic , So when multiple threads operate one at the same time SimpleDateFormat It will cause cal The value of is chaotic . Similarly , 

format（） The same is true of methods .

therefore , SimpleDateFormat Class is not thread safe because ：

DateFormat Class Calendar Objects are shared by multiple threads , and Calendar Object itself does not support thread safety .

that , I learned that SimpleDateFormat Classes are not thread-safe , And cause SimpleDateFormat Classes are not thread safe , So how to solve this problem ？ Next , Let's discuss how to solve SimpleDateFormat Class thread safety in high concurrency scenarios .

solve SimpleDateFormat Thread safety of class

solve SimpleDateFormat There are many ways to solve the thread safety problem of class in high concurrency scenario , here , Just list a few common ways for reference ,

You can also give more solutions in the comments area .

1. Local variable method

The easiest way to do that is to put SimpleDateFormat Class objects are defined as local variables , The code shown below , take SimpleDateFormat Class objects are defined in parse（String） On top of the method , Problem solvable .

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  Local variable method to solve SimpleDateFormat Thread safety of class
 */
public class SimpleDateFormatTest02 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 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(&quot;yyyy-MM-dd&quot;);
 simpleDateFormat.parse(&quot;2020-01-01&quot;);
 } catch (ParseException e) {
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }catch (NumberFormatException e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

Now run the modified program , The output is as follows .

All threads successfully formatted date

As for why using local variables in high concurrency scenarios can solve the problem of thread security , Will be in 【JVM project 】 Of JVM In depth analysis of memory mode related content , I don't want to introduce it too much .

Of course , This approach creates a lot of SimpleDateFormat Class object , Affects program performance , therefore ,

This method is not recommended in the actual production environment .

2. Synchronous lock mode

take SimpleDateFormat Class objects are defined as global static variables , At this point, all threads share SimpleDateFormat Class object , In this case, when calling the method of formatting time , Yes SimpleDateFormat Objects can be synchronized , The code is as follows .

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  adopt Synchronized Lock resolution SimpleDateFormat Thread safety of class
 */
public class SimpleDateFormatTest03 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 private static final int THREAD_COUNT = 20;
 //SimpleDateFormat object
 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(&quot;2020-01-01&quot;);
 }
 } catch (ParseException e) {
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }catch (NumberFormatException e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

here , The key code to solve the problem is as follows .

synchronized (simpleDateFormat){
simpleDateFormat.parse("2020-01-01");
}

Run the program , The output is as follows .

All threads successfully formatted date

It should be noted that , Although this way can solve SimpleDateFormat Thread safety of class , But because during the execution of the program , by SimpleDateFormat Class object with synchronization lock , Only one thread can execute at the same time parse（String） Method . here , Will affect the performance of the program , In a production environment that requires high concurrency ,

This method is not recommended .

3. Lock mode

Lock The way of locking is related to synchronized The principle of lock implementation is the same , It's all passed at the high level JVM To ensure the thread safety of the program . adopt Lock The code to solve the problem in lock mode is as follows .

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  adopt Lock Lock resolution SimpleDateFormat Thread safety of class
 */
public class SimpleDateFormatTest04 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 private static final int THREAD_COUNT = 20;
 //SimpleDateFormat object
 private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");
 //Lock object
 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(&quot;2020-01-01&quot;);
 } catch (ParseException e) {
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }catch (NumberFormatException e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }finally {
 lock.unlock();
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

From the code , First , Defined a Lock A global static variable of type as a handle to lock and release . And then in simpleDateFormat.parse（String） The code passed before lock.lock（） Lock . One thing to note here is that ： To prevent the lock from being released due to the program throwing an exception , Be sure to release the lock finally Block of code , As shown below .

finally {
lock.unlock();
} Copy

Run the program , The output is as follows .

All threads successfully formatted date

This way will also affect the performance of high concurrency scenarios ,

Not recommended for high concurrency production environments .

4.ThreadLocal The way

Use ThreadLocal Store what each thread has SimpleDateFormat Copy of object , It can effectively avoid the thread safety problems caused by multithreading , Use ThreadLocal The code to solve the thread safety problem is as follows .

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  adopt ThreadLocal solve SimpleDateFormat Thread safety of class
 */
public class SimpleDateFormatTest05 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 private static final int THREAD_COUNT = 20;
 private static ThreadLocal<DateFormat> threadLocal = new ThreadLocal<DateFormat>(){
 @Override
 protected DateFormat initialValue() {
 return new SimpleDateFormat(&quot;yyyy-MM-dd&quot;);
 }
 };
 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(&quot;2020-01-01&quot;);
 } catch (ParseException e) {
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }catch (NumberFormatException e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

From the code , Will each thread use SimpleDateFormat A copy is kept in ThreadLocal in , Each thread does not interfere with each other when it is used , So as to solve the problem of thread safety .

Run the program , The output is as follows .

All threads successfully formatted date

This way is more efficient ,

It is recommended to use in the production environment of high concurrency business scenario .

in addition , Use ThreadLocal It can also be written in the following form of code , The effect is the same .

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  adopt ThreadLocal solve SimpleDateFormat Thread safety of class
 */
public class SimpleDateFormatTest06 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 private static final int THREAD_COUNT = 20;
 private static ThreadLocal<DateFormat> threadLocal = new ThreadLocal<DateFormat>();
 private static DateFormat getDateFormat(){
 DateFormat dateFormat = threadLocal.get();
 if(dateFormat == null){
 dateFormat = new SimpleDateFormat(&quot;yyyy-MM-dd&quot;);
 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(&quot;2020-01-01&quot;);
 } catch (ParseException e) {
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }catch (NumberFormatException e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

5.DateTimeFormatter The way

DateTimeFormatter yes Java8 New date and time provided API Class in ,DateTimeFormatter Class is thread safe , It can be used directly in high concurrency scenarios DateTimeFormatter Class to handle date formatting . The code is as follows .

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  adopt DateTimeFormatter Class to solve the problem of thread safety
 */
public class SimpleDateFormatTest07 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 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(&quot;2020-01-01&quot;, formatter);
 }catch (Exception e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

You can see ,DateTimeFormatter Class is thread safe , It can be used directly in high concurrency scenarios DateTimeFormatter Class to handle date formatting .

Run the program , The output is as follows .

All threads successfully formatted date

Use DateTimeFormatter Class to handle the date formatting operation is more efficient ,

It is recommended to use in the production environment of high concurrency business scenario

.

6.joda-time The way

joda-time It's a class library for the third party to handle date and time formatting , It's thread safe . If you use joda-time To handle date and time formatting , We need to introduce a third-party class library . here , With Maven For example , Introduce as follows joda-time library .

<dependency>
<groupId>joda-time</groupId>
<artifactId>joda-time</artifactId>
<version>2.9.9</version>
</dependency>

introduce joda-time Behind the library , The program code is as follows .

package io.binghe.concurrent.lab06;
import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.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  adopt DateTimeFormatter Class to solve the problem of thread safety
 */
public class SimpleDateFormatTest08 {
 // The total number of execution
 private static final int EXECUTE_COUNT = 1000;
 // Number of threads running at the same time
 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(&quot;2020-01-01&quot;, dateTimeFormatter).toDate();
 }catch (Exception e){
 System.out.println(&quot; Threads ：&quot; + Thread.currentThread().getName() + &quot;  Failed to format date &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 semaphore.release();
 } catch (InterruptedException e) {
 System.out.println(&quot; Semaphore error &quot;);
 e.printStackTrace();
 System.exit(1);
 }
 countDownLatch.countDown();
 });
 }
 countDownLatch.await();
 executorService.shutdown();
 System.out.println(&quot; All threads successfully formatted date &quot;);
 }
}

here , It should be noted that ：DateTime Class is org.joda.time Class under package ,DateTimeFormat Classes and DateTimeFormatter Classes are org.joda.time.format Class under package , As shown below .

import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

Run the program , The output is as follows .

All threads successfully formatted date

Use joda-time Library to deal with the date format operation is more efficient ,

It is recommended to use in the production environment of high concurrency business scenario .

solve SimpleDateFormat Class thread safety issues of the program summary

To sum up ： In solving SimpleDateFormat Class thread safety problems in several solutions , The local variable method is due to the fact that each time the thread executes the format time , Will create SimpleDateFormat Class object , This leads to the creation of a large number of SimpleDateFormat object , Waste of running space and server performance , because JVM Creating and destroying objects is performance intensive . therefore ,

It is not recommended to use in production environment with high concurrency requirements

.

synchronized Lock mode and Lock Locking is essentially the same way of dealing with problems , By locking , Only one thread can perform the operation of formatting date and time at the same time . This way, though, has reduced SimpleDateFormat Object creation , But because of the synchronization lock , Performance degradation , therefore ,

It is not recommended to use in production environment with high concurrency requirements .

ThreadLocal By saving each thread's SimpleDateFormat Copy of class object , Make each thread at run time , Each uses its own bound SimpleDateFormat object , Mutual interference , The execution performance is relatively high ,

Recommended for high concurrency production environments .

DateTimeFormatter yes Java 8 Class that handles date and time provided in ,DateTimeFormatter Class itself is thread safe , Pressure measurement ,DateTimeFormatter Class to handle date and time performance is not bad . therefore ,

It is recommended to use in production environment under high concurrency scenario .

joda-time It's a class library for the third party to process dates and times , Thread safety , Performance has been tested by high concurrency ,

It is recommended to use in production environment under high concurrency scenario

.

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