Pengenalan kepada API Tarikh / Masa Java 8

Pengaturcaraan

1. Gambaran keseluruhan

Java 8 diperkenalkan API baru untuk Tarikh dan Masa untuk menangani kelemahan yang lebih tua java.util.Date dan java.util.Calendar .

Sebagai sebahagian daripada artikel ini, mari kita mulakan dengan masalah dalam API Tarikh dan Kalendar yang ada dan mari kita bincangkan bagaimana API Tarikh dan Waktu Java 8 menanganinya.

Kami juga akan melihat beberapa kelas teras projek Java 8 baru yang merupakan sebahagian daripada java.time pakej seperti LOCALDATE , localtime, LocalDateTime, ZonedDateTime, Tempoh, Tempoh dan API disokong mereka.

2. Masalah dengan API Tarikh / Masa yang Ada

  • Keselamatan Thread - Kelas Tarikh dan Kalendar tidak selamat di benang, menyebabkan pembangun dapat menangani masalah sakit kepala yang sukar untuk diselesaikan dan menulis kod tambahan untuk menangani keselamatan utas. Sebaliknya API Tarikh dan Waktu baru yang diperkenalkan di Java 8 tidak dapat diubah dan selamat diuraikan, sehingga menghilangkan kepeningan bersamaan dengan pemaju.
  • API Design dan Kemudahan Persefahaman - The Tarikh dan Kalendar API yang kurang direka dengan kaedah yang tidak mencukupi untuk melaksanakan operasi sehari-hari. API Tarikh / Masa baru berpusat pada ISO dan mengikuti model domain yang konsisten untuk tarikh, masa, tempoh dan tempoh. Terdapat pelbagai kaedah utiliti yang menyokong operasi biasa.
  • ZonedDate and Time - Pembangun terpaksa menulis logik tambahan untuk menangani logik zon waktu dengan API lama, sedangkan dengan API baru, pengendalian zon waktu dapat dilakukan dengan Local dan ZonedDate / Time API.

3. Menggunakan LocalDate , LocalTime dan LocalDateTime

Kelas yang paling kerap digunakan ialah LocalDate , LocalTime dan LocalDateTime . Seperti namanya, mereka mewakili Tarikh / Waktu tempatan dari konteks pemerhati.

Kelas-kelas ini digunakan terutamanya ketika zon waktu tidak perlu dinyatakan secara eksplisit dalam konteksnya. Sebagai sebahagian daripada bahagian ini, kami akan merangkumi API yang paling biasa digunakan.

3.1. Bekerja Dengan LocalDate

The LOCALDATE mewakili tarikh dalam format ISO (dd-mm-dd) tanpa masa .

Ia boleh digunakan untuk menyimpan tarikh seperti hari lahir dan hari gaji.

Contoh tarikh semasa dapat dibuat dari jam sistem seperti di bawah:

LocalDate localDate = LocalDate.now();

The LOCALDATE mewakili hari tertentu, bulan dan tahun boleh diperolehi menggunakan " daripada " kaedah atau dengan menggunakan " hurai " kaedah. Contohnya, coretan kod di bawah mewakili LocalDate untuk 20 Februari 2015:

LocalDate.of(2015, 02, 20); LocalDate.parse("2015-02-20");

The LOCALDATE menyediakan pelbagai kaedah utiliti untuk mendapatkan pelbagai maklumat. Mari kita lihat sebilangan kaedah API ini.

Coretan kod berikut mendapat tarikh tempatan semasa dan menambah satu hari:

LocalDate tomorrow = LocalDate.now().plusDays(1);

Contoh ini memperoleh tarikh semasa dan mengurangkan satu bulan. Perhatikan bagaimana ia menerima enum sebagai unit masa:

LocalDate previousMonthSameDay = LocalDate.now().minus(1, ChronoUnit.MONTHS);

Dalam dua contoh kod berikut, kami menguraikan tarikh "2016-06-12" dan masing-masing memperoleh hari dalam seminggu dan hari bulan tersebut. Perhatikan nilai pulangan, yang pertama adalah objek yang mewakili DayOfWeek sementara yang kedua dalam int mewakili nilai ordinal bulan:

DayOfWeek sunday = LocalDate.parse("2016-06-12").getDayOfWeek(); int twelve = LocalDate.parse("2016-06-12").getDayOfMonth();

Kita boleh menguji sama ada tarikh berlaku dalam tahun lompat. Dalam contoh ini, kami menguji jika tarikh semasa berlaku adalah tahun lompat:

boolean leapYear = LocalDate.now().isLeapYear();

Hubungan tarikh dengan tarikh yang lain dapat ditentukan berlaku sebelum atau selepas tarikh lain:

boolean notBefore = LocalDate.parse("2016-06-12") .isBefore(LocalDate.parse("2016-06-11")); boolean isAfter = LocalDate.parse("2016-06-12") .isAfter(LocalDate.parse("2016-06-11"));

Batasan tarikh boleh didapati dari tarikh tertentu. Dalam dua contoh berikut, kami mendapat LocalDateTime yang mewakili awal hari (2016-06-12T00: 00) pada tarikh yang ditentukan dan LocalDate yang masing-masing mewakili awal bulan (2016-06-01):

LocalDateTime beginningOfDay = LocalDate.parse("2016-06-12").atStartOfDay(); LocalDate firstDayOfMonth = LocalDate.parse("2016-06-12") .with(TemporalAdjusters.firstDayOfMonth());

Sekarang mari kita lihat bagaimana kita bekerja dengan waktu tempatan.

3.2. Bekerja Dengan LocalTime

The localtime mewakili masa tanpa tarikh .

Mirip dengan LocalDate , contoh LocalTime dapat dibuat dari jam sistem atau dengan menggunakan kaedah "parse" dan "of". Lihat dengan cepat beberapa API yang biasa digunakan di bawah.

Contoh LocalTime semasa dapat dibuat dari jam sistem seperti di bawah:

LocalTime now = LocalTime.now();

Dalam contoh kod di bawah , kami membuat LocalTime yang mewakili 06:30 pagi dengan menguraikan representasi rentetan:

LocalTime sixThirty = LocalTime.parse("06:30");

Kaedah Factory "of" dapat digunakan untuk membuat LocalTime . Contohnya kod di bawah ini membuat LocalTime yang mewakili 06:30 pagi menggunakan kaedah kilang:

LocalTime sixThirty = LocalTime.of(6, 30);

Contoh di bawah ini membuat LocalTime dengan menguraikan rentetan dan menambahkan satu jam padanya dengan menggunakan API "plus". Hasilnya adalah LocalTime yang menunjukkan pukul 07:30 pagi:

LocalTime sevenThirty = LocalTime.parse("06:30").plus(1, ChronoUnit.HOURS);

Terdapat pelbagai kaedah mendapatkan yang boleh digunakan untuk mendapatkan unit waktu tertentu seperti jam, min dan saat seperti di bawah:

int six = LocalTime.parse("06:30").getHour();

We can also check if a specific time is before or after another specific time. The below code sample compares two LocalTime for which the result would be true:

boolean isbefore = LocalTime.parse("06:30").isBefore(LocalTime.parse("07:30"));

The max, min and noon time of a day can be obtained by constants in LocalTime class. This is very useful when performing database queries to find records within a given span of time. For example, the below code represents 23:59:59.99:

LocalTime maxTime = LocalTime.MAX

Now let's dive into LocalDateTime.

3.3. Working With LocalDateTime

The LocalDateTime is used to represent a combination of date and time.

This is the most commonly used class when we need a combination of date and time. The class offers a variety of APIs and we will look at some of the most commonly used ones.

An instance of LocalDateTime can be obtained from the system clock similar to LocalDate and LocalTime:

LocalDateTime.now();

The below code samples explain how to create an instance using the factory “of” and “parse” methods. The result would be a LocalDateTime instance representing 20 February 2015, 06:30 AM:

LocalDateTime.of(2015, Month.FEBRUARY, 20, 06, 30);
LocalDateTime.parse("2015-02-20T06:30:00");

There are utility APIs to support addition and subtraction of specific units of time like days, months, year and minutes are available. The below code samples demonstrates the usage of “plus” and “minus” methods. These APIs behave exactly like their counterparts in LocalDate and LocalTime:

localDateTime.plusDays(1);
localDateTime.minusHours(2);

Getter methods are available to extract specific units similar to the date and time classes. Given the above instance of LocalDateTime, the below code sample will return the month February:

localDateTime.getMonth();

4. Using ZonedDateTime API

Java 8 provides ZonedDateTime when we need to deal with time zone specific date and time. The ZoneId is an identifier used to represent different zones. There are about 40 different time zones and the ZoneId are used to represent them as follows.

In this code snippet we create a Zone for Paris:

ZoneId zoneId = ZoneId.of("Europe/Paris");

A set of all zone ids can be obtained as below:

Set allZoneIds = ZoneId.getAvailableZoneIds();

The LocalDateTime can be converted to a specific zone:

ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, zoneId);

The ZonedDateTime provides parse method to get time zone specific date time:

ZonedDateTime.parse("2015-05-03T10:15:30+01:00[Europe/Paris]");

Another way to work with time zone is by using OffsetDateTime. The OffsetDateTime is an immutable representation of a date-time with an offset. This class stores all date and time fields, to a precision of nanoseconds, as well as the offset from UTC/Greenwich.

The OffSetDateTime instance can be created as below using ZoneOffset. Here we create a LocalDateTime representing 6:30 am on 20th February 2015:

LocalDateTime localDateTime = LocalDateTime.of(2015, Month.FEBRUARY, 20, 06, 30);

Then we add two hours to the time by creating a ZoneOffset and setting for the localDateTime instance:

ZoneOffset offset = ZoneOffset.of("+02:00"); OffsetDateTime offSetByTwo = OffsetDateTime .of(localDateTime, offset);

We now have a localDateTime of 2015-02-20 06:30 +02:00. Now let's move on to how to modify date and time values using the Period and Duration classes.

5. Using Period and Duration

The Period class represents a quantity of time in terms of years, months and days and the Duration class represents a quantity of time in terms of seconds and nano seconds.

5.1. Working With Period

The Period class is widely used to modify values of given a date or to obtain the difference between two dates:

LocalDate initialDate = LocalDate.parse("2007-05-10");

The Date can be manipulated using Period as shown in the following code snippet:

LocalDate finalDate = initialDate.plus(Period.ofDays(5));

The Period class has various getter methods such as getYears, getMonths and getDays to get values from a Period object. The below code example returns an int value of 5 as we try to get difference in terms of days:

int five = Period.between(initialDate, finalDate).getDays();

The Period between two dates can be obtained in a specific unit such as days or month or years, using ChronoUnit.between:

long five = ChronoUnit.DAYS.between(initialDate, finalDate);

This code example returns five days. Let's continue by taking a look at the Duration class.

5.2. Working With Duration

Similar to Period, the Duration class is use to deal with Time. In the following code we create a LocalTime of 6:30 am and then add a duration of 30 seconds to make a LocalTime of 06:30:30am:

LocalTime initialTime = LocalTime.of(6, 30, 0); LocalTime finalTime = initialTime.plus(Duration.ofSeconds(30));

The Duration between two instants can be obtained either as a Duration or as a specific unit. In the first code snippet we use the between() method of the Duration class to find the time difference between finalTime and initialTime and return the difference in seconds:

long thirty = Duration.between(initialTime, finalTime).getSeconds();

In the second example we use the between() method of the ChronoUnit class to perform the same operation:

long thirty = ChronoUnit.SECONDS.between(initialTime, finalTime);

Now we will look at how to convert existing Date and Calendar to new Date/Time.

6. Compatibility with Date and Calendar

Java 8 has added the toInstant() method which helps to convert existing Date and Calendar instance to new Date Time API as in the following code snippet:

LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault()); LocalDateTime.ofInstant(calendar.toInstant(), ZoneId.systemDefault());

The LocalDateTime can be constructed from epoch seconds as below. The result of the below code would be a LocalDateTime representing 2016-06-13T11:34:50:

LocalDateTime.ofEpochSecond(1465817690, 0, ZoneOffset.UTC);

Now let's move on to Date and Time formatting.

7. Date and Time Formatting

Java 8 provides APIs for the easy formatting of Date and Time:

LocalDateTime localDateTime = LocalDateTime.of(2015, Month.JANUARY, 25, 6, 30);

The below code passes an ISO date format to format the local date. The result would be 2015-01-25 :

String localDateString = localDateTime.format(DateTimeFormatter.ISO_DATE);

The DateTimeFormatter provides various standard formatting options. Custom patterns can be provided to format method as well, like below, which would return a LocalDate as 2015/01/25:

localDateTime.format(DateTimeFormatter.ofPattern("yyyy/MM/dd"));

We can pass in formatting style either as SHORT, LONG or MEDIUM as part of the formatting option. The below code sample would give an output representing LocalDateTime in 25-Jan-2015, 06:30:00:

localDateTime .format(DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM)) .withLocale(Locale.UK);

Let us take a look at alternatives available to Java 8 Core Date/Time APIs.

8. Backport and Alternate Options

8.1. Using Project Threeten

For organization that are on the path of moving to Java 8 from Java 7 or Java 6 and want to use date and time API, project threeten provides the backport capability. Developers can use classes available in this project to achieve the same functionality as that of new Java 8 Date and Time API and once they move to Java 8, the packages can be switched. Artifact for the project threeten can be found in the maven central repository:

 org.threeten threetenbp 1.3.1

8.2. Joda-Time Library

Another alternative for Java 8 Date and Time library is Joda-Time library. In fact Java 8 Date Time APIs has been led jointly by the author of Joda-Time library (Stephen Colebourne) and Oracle. This library provides pretty much all capabilities that is supported in Java 8 Date Time project. The Artifact can be found in the maven central by including the below pom dependency in your project:

 joda-time joda-time 2.9.4

9. Conclusion

Java 8 provides a rich set of APIs with consistent API design for easier development.

The code samples for the above article can be found in the Java 8 Date/Time git repository.