Introduction

Hello. It's Kecho. ** Do you guys use SimpleDateFormat and stream? ** ** It's a niche. I love stream. This time we will verify that SimpleDateFormat is thread unsafe.

Preparation

First, prepare a Date type object.

SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd hh:mm:ss");

Calendar day1 = Calendar.getInstance();
day1.set(2020, 0, 31, 0, 0, 0);
Calendar day2 = Calendar.getInstance();
day2.set(1995, 3, 6, 0, 0, 0);

Let's put it in the list.

for (int i = 0; i < 10; i++) {
	//Fill the list with 20 Date objects
	list.add(day1.getTime());
	list.add(day2.getTime());
}

First, process without executing in parallel.

list.stream().map(date -> sdf.format(date)).forEach(System.out::println);
//Convert each element to String in turn and output

The whole picture is below.

SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd hh:mm:ss");

Calendar day1 = Calendar.getInstance();
day1.set(2020, 0, 31, 0, 0, 0);
Calendar day2 = Calendar.getInstance();
day2.set(1995, 3, 6, 0, 0, 0);
List<Date> list = new ArrayList<>(); //Creating a list

for (int i = 0; i < 10; i++) {
	//Fill the list with Date objects
	list.add(day1.getTime());
	list.add(day2.getTime());
}

list.stream().map(date -> sdf.format(date)).forEach(System.out::println);

Run in single thread

2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00

There seems to be no problem.

Run in multiple threads

stream will do parallel processing just by adding parallel () to the intermediate operation.

list.stream().map(date -> sdf.format(date)).forEach(System.out::println);

Let's do it!

2020/01/31 12:00:00
1995/01/31 12:00:00 //what's this
1995/04/06 12:00:00
2020/01/31 12:00:00
2020/01/31 12:00:00
2020/01/31 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/06 12:00:00
2020/01/31 12:00:00
1995/04/31 12:00:00 //what's this

Some unexpected dates were output. I was able to confirm that it is thread unsafe.

Countermeasures

You may create a SimpleDateFormat object in each thread. Basically, it is better to use the Date And Time API implemented from Java 8.

Implementation using DateTimeFormatter

DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy/MM/dd hh:mm:ss");

LocalDateTime today = LocalDateTime.now();
LocalDateTime birthDay = LocalDateTime.of(1995, 4, 6, 12, 0, 0);

List<LocalDateTime> list = new ArrayList<>();

for (int i = 0; i < 10; i++) {
	//Fill the list with Date objects
	list.add(today);
	list.add(birthDay);
}

list.stream().parallel().map(date -> dtf.format(date)).forEach(System.out::println);

2020/01/31 09:33:20
1995/04/06 12:00:00
1995/04/06 12:00:00
2020/01/31 09:33:20
1995/04/06 12:00:00
2020/01/31 09:33:20
2020/01/31 09:33:20
2020/01/31 09:33:20
2020/01/31 09:33:20
2020/01/31 09:33:20
2020/01/31 09:33:20
1995/04/06 12:00:00
1995/04/06 12:00:00
1995/04/06 12:00:00
1995/04/06 12:00:00
1995/04/06 12:00:00
2020/01/31 09:33:20
2020/01/31 09:33:20
1995/04/06 12:00:00
1995/04/06 12:00:00

Sounds okay.

Summary

Use Date And Time API for Java 8 and above

Use stream to verify that SimpleDateFormat is thread unsafe