Introducción a la API de fecha / hora de Java 8

Programación

1. Información general

Java 8 introdujo nuevas API para fecha y hora para abordar las deficiencias de los antiguos java.util.Date y java.util.Calendar .

Como parte de este artículo, comencemos con los problemas en las API de fecha y calendario existentes y analicemos cómo las nuevas API de fecha y hora de Java 8 los abordan.

También vamos a ver algunas de las clases principales del nuevo proyecto Java 8 que forman parte de la java.time paquete como LocalDate , LocalTime, LocalDateTime, ZonedDateTime, Período, Duración y sus APIs compatibles.

2. Problemas con las API de fecha / hora existentes

  • Seguridad de subprocesos : las clases Fecha y Calendario no son seguras para subprocesos, lo que deja a los desarrolladores para lidiar con el dolor de cabeza de problemas de concurrencia difíciles de depurar y escribir código adicional para manejar la seguridad de subprocesos. Por el contrario, las nuevas API de fecha y hora introducidas en Java 8 son inmutables y seguras para subprocesos, lo que elimina el dolor de cabeza de la concurrencia de los desarrolladores.
  • Diseño de API y facilidad de comprensión : las API de fecha y calendario están mal diseñadas y tienen métodos inadecuados para realizar las operaciones diarias. Las nuevas API de fecha / hora se centran en ISO y siguen modelos de dominio coherentes para la fecha, la hora, la duración y los períodos. Existe una amplia variedad de métodos de utilidad que admiten las operaciones más comunes.
  • ZonedDate and Time : los desarrolladores tenían que escribir lógica adicional para manejar la lógica de la zona horaria con las API antiguas, mientras que con las nuevas API, el manejo de la zona horaria se puede realizar con las API locales y ZonedDate / Time .

3. Uso de LocalDate , LocalTime y LocalDateTime

Las clases más utilizadas son LocalDate , LocalTime y LocalDateTime . Como indican sus nombres, representan la fecha / hora local del contexto del observador.

Estas clases se utilizan principalmente cuando no se requiere que la zona horaria se especifique explícitamente en el contexto. Como parte de esta sección, cubriremos las API más utilizadas.

3.1. Trabajar con LocalDate

El LocalDate representa una fecha en formato ISO (aaaa-MM-dd) sin tiempo .

Se puede utilizar para almacenar fechas como cumpleaños y días de pago.

Se puede crear una instancia de la fecha actual a partir del reloj del sistema como se muestra a continuación:

LocalDate localDate = LocalDate.now();

El LocalDate que representa un día, mes y año específicos se puede obtener usando el método " of " o usando el método " parse ". Por ejemplo, los siguientes fragmentos de código representan LocalDate del 20 de febrero de 2015:

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

El LocalDate proporciona varios métodos de utilidad para obtener una variedad de información. Echemos un vistazo rápido a algunos de estos métodos de API.

El siguiente fragmento de código obtiene la fecha local actual y agrega un día:

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

Este ejemplo obtiene la fecha actual y resta un mes. Observe cómo acepta una enumeración como unidad de tiempo:

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

En los siguientes dos ejemplos de código, analizamos la fecha “2016-06-12” y obtenemos el día de la semana y el día del mes respectivamente. Tenga en cuenta los valores de retorno, el primero es un objeto que representa el DayOfWeek mientras que el segundo en un int que representa el valor ordinal del mes:

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

Podemos probar si una fecha ocurre en un año bisiesto. En este ejemplo, probamos si la fecha actual es un año bisiesto:

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

Se puede determinar que la relación de una fecha con otra ocurre antes o después de otra fecha:

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"));

Los límites de fecha se pueden obtener a partir de una fecha determinada. En los siguientes dos ejemplos obtenemos LocalDateTime que representa el comienzo del día (2016-06-12T00: 00) de la fecha dada y LocalDate que representa el comienzo del mes (2016-06-01) respectivamente:

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

Ahora echemos un vistazo a cómo trabajamos con la hora local.

3.2. Trabajar con LocalTime

El LocalTime representa el tiempo sin una fecha .

De manera similar a LocalDate, se puede crear una instancia de LocalTime a partir del reloj del sistema o utilizando el método "parse" y "of". Eche un vistazo rápido a algunas de las API de uso común a continuación.

An instance of current LocalTime can be created from the system clock as below:

LocalTime now = LocalTime.now();

In the below code sample, we create a LocalTime representing 06:30 AM by parsing a string representation:

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

The Factory method “of” can be used to create a LocalTime. For example the below code creates LocalTime representing 06:30 AM using the factory method:

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

The below example creates a LocalTime by parsing a string and adds an hour to it by using the “plus” API. The result would be LocalTime representing 07:30 AM:

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

Various getter methods are available which can be used to get specific units of time like hour, min and secs like below:

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.