java 解析日期格式

自Java 几乎 开始以来，Java开发人员就通过java.util.Date类（自JDK 1.0起）和java.util.Calendar类（自JDK 1.1起 ）来处理日期和时间。 在这段时间内，成千上万（甚至数百万）的Java开发人员已使用java.text.DateFormat和java.text.SimpleDateFormat格式化并解析了Java日期和时间。 鉴于多年来这样做的频率，不足为奇的是，有很多关于这些类的日期和时间的解析和格式设置的 在线 示例和教程 。 经典的Java教程在“ 格式设置”课程 （ Dates and Times ）中介绍了这些java.util和java.text类。 Java教程中新的Date Time路径涵盖了Java 8中有关日期和时间以及它们的格式和解析的新类。 这篇文章提供了一些实际的例子。

在通过示例演示Java 8样式的日期/时间解析/格式化之前，先比较一下DateFormat / SimpleDateFormat和DateTimeFormatter的Javadoc描述。 下表包含可区分的信息，这些信息可通过比较每个格式类的Javadoc来直接或间接收集。 从此表中可能最重要的观察结果是，新的DateTimeFormatter是线程安全的且不可变的，并且DateTimeFormatter提供了用于解析和格式化日期和时间的API的概述。

这篇文章的其余部分将使用示例演示如何使用java.time构造在Java 8中格式化和解析日期。 这些示例将使用以下字符串模式和实例。

/** Pattern to use for String representation of Dates/Times. */
private final String dateTimeFormatPattern = "yyyy/MM/dd HH:mm:ss z";/*** java.util.Date instance representing now that can* be formatted using SimpleDateFormat based on my* dateTimeFormatPattern field.*/
private final Date now = new Date();/*** java.time.ZonedDateTime instance representing now that can* be formatted using DateTimeFormatter based on my* dateTimeFormatPattern field.** Note that ZonedDateTime needed to be used in this example* instead of java.time.LocalDateTime or java.time.OffsetDateTime* because there is zone information in the format provided by* my dateTimeFormatPattern field and attempting to have* DateTimeFormatter.format(TemporalAccessor) instantiated* with a format pattern that includes time zone details* will lead to DateTimeException for instances of* TemporalAccessor that do not have time zone information* (such as LocalDateTime and OffsetDateTime).*/
private final ZonedDateTime now8 = ZonedDateTime.now();/*** String that can be used by both SimpleDateFormat and* DateTimeFormatter to parse respective date/time instances* from this String.*/
private final String dateTimeString = "2014/09/03 13:59:50 MDT";

在Java 8之前，用于日期和时间的标准Java方法是通过Date和Calendar类，而用于解析和格式化日期的标准方法是通过DateFormat和SimpleDateFormat 。 下一个代码清单演示了这些经典方法。

使用SimpleDateFormat格式化和解析Java日期

/*** Demonstrate presenting java.util.Date as String matching* provided pattern via use of SimpleDateFormat.*/
public void demonstrateSimpleDateFormatFormatting()
{final DateFormat format = new SimpleDateFormat(dateTimeFormatPattern);final String nowString = format.format(now);out.println("Date '" + now + "' formatted with SimpleDateFormat and '"+ dateTimeFormatPattern + "': " + nowString);
}/*** Demonstrate parsing a java.util.Date from a String* via SimpleDateFormat.*/
public void demonstrateSimpleDateFormatParsing()
{final DateFormat format = new SimpleDateFormat(dateTimeFormatPattern);try{final Date parsedDate = format.parse(dateTimeString);out.println("'" + dateTimeString + "' is parsed with SimpleDateFormat as " + parsedDate);}// DateFormat.parse(String) throws a checked exceptioncatch (ParseException parseException){out.println("ERROR: Unable to parse date/time String '"+ dateTimeString + "' with pattern '"+ dateTimeFormatPattern + "'.");}
}

在Java 8中，首选日期/时间类不再位于java.util包中，并且首选日期/时间处理类现在位于java.time包中。 同样，首选的日期/时间格式/解析类不再位于java.text包中，而是来自java.time.format包。

java.time包提供了许多用于对日期和/或时间进行建模的类。 这些包括仅对日期建模的类（无时间信息），仅对时间建模的类（无日期信息），对日期和时间信息进行建模的类，使用时区信息的类以及不包含时区信息的类。 尽管类的特性（例如，是否支持日期或时间或时区信息）会影响可以应用的模式，但格式化和解析这些方法的方法通常是相似的。 在本文中，我将ZonedDateTime类用作示例。 之所以选择该选项，是因为它包含日期，时间和时区信息，并允许我使用一种匹配模式，该模式涉及所有三个特征，例如Date或Calendar实例。 这使得比较新旧方法变得更加容易。

DateTimeFormatter类提供了ofPattern方法，以基于所提供的日期/时间模式String提供DateTimeFormatter的实例。 然后可以在该DateTimeFormatter实例上调用一种格式方法，以获取日期和/或时间信息，格式为与提供的模式匹配的String。 下一个代码清单说明了基于提供的模式从ZonedDateTime格式化String这种方法。

将ZonedDateTime格式化为字符串

/*** Demonstrate presenting ZonedDateTime as a String matching* provided pattern via DateTimeFormatter and its* ofPattern(String) method.*/
public void demonstrateDateTimeFormatFormatting()
{final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final String nowString = formatter.format(now8);out.println(now8 + " formatted with DateTimeFormatter and '"+ dateTimeFormatPattern + "': " + nowString);
}

基于模式从字符串中解析日期/时间类很容易完成。 有两种方法可以实现。 一种方法是将DateTimeFormatter的实例传递给静态ZonedDateTime.parse（CharSequence，DateTimeFormatter）方法，该方法返回从提供的字符序列派生并基于提供的模式的ZonedDateTime实例。 下一个代码清单对此进行了说明。

使用静态ZonedDateTime.parse方法从字符串解析ZonedDateTime

/*** Demonstrate parsing ZonedDateTime from provided String* via ZonedDateTime's parse(String, DateTimeFormatter) method.*/
public void demonstrateDateTimeFormatParsingTemporalStaticMethod()
{final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime = ZonedDateTime.parse(dateTimeString, formatter);out.println("'" + dateTimeString+ "' is parsed with DateTimeFormatter and ZonedDateTime.parse as "+ zonedDateTime);
}

从字符串解析ZonedDateTime第二种方法是通过DateTimeFormatter的parse（CharSequence，TemporalQuery <T>）方法。 在下面的代码清单中对此进行了说明，该清单也提供了演示Java 8 方法引用的用法的机会（请参见ZonedDateTime::from ）。

使用DateTimeFormatter.parse方法从字符串解析ZonedDateTime

/*** Demonstrate parsing ZonedDateTime from String* via DateTimeFormatter.parse(String, TemporaryQuery)* with the Temple Query in this case being ZonedDateTime's* from(TemporalAccessor) used as a Java 8 method reference.*/
public void demonstrateDateTimeFormatParsingMethodReference()
{final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime = formatter.parse(dateTimeString, ZonedDateTime::from);out.println("'" + dateTimeString+ "' is parsed with DateTimeFormatter and ZoneDateTime::from as "+ zonedDateTime);
}

很少有项目可以作为可以从Java 8开始的未开发项目。因此，将JDK 8之前的日期/时间类与JDK 8中引入的新日期/时间类连接在一起的类将很有帮助。例如，JDK 8的DateTimeFormatter可以通过DateTimeFormatter.toFormat（）方法提供JDK 8之前的抽象Format类的降序实例。 下一个代码清单中对此进行了演示。

从JDK 8的DateTimeFormatter访问JDK 8之前的格式

/*** Demonstrate formatting ZonedDateTime via DateTimeFormatter,* but using implementation of Format.*/
public void demonstrateDateTimeFormatAndFormatFormatting()
{final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final Format format = formatter.toFormat();final String nowString = format.format(now8);out.println("ZonedDateTime " + now + " formatted with DateTimeFormatter/Format (and "+ format.getClass().getCanonicalName() + ") and '"+ dateTimeFormatPattern + "': " + nowString);
}

该即时与两个前JDK 8工作时类是特别重要的Date和Calendar结合类与JDK 8之所以引入新的日期和时间类Instant是如此重要的是， java.util.Date有方法从（即时）和toInstant（）分别从Instant获取Date和从Date获取Instant 。 因为Instant在将Java 8之前的日期/时间处理迁移到Java 8基线非常重要，所以下一个代码清单演示了Instant实例的格式和解析。

格式化和解析Instant实例

/*** Demonstrate formatting and parsing an instance of Instant.*/
public void demonstrateDateTimeFormatFormattingAndParsingInstant()
{// Instant instances don't have timezone informationfinal Instant instant = now.toInstant();final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern).withZone(ZoneId.systemDefault());final String formattedInstance = formatter.format(instant);out.println("Instant " + instant + " formatted with DateTimeFormatter and '"+ dateTimeFormatPattern + "' to '" + formattedInstance + "'");final Instant instant2 =formatter.parse(formattedInstance, ZonedDateTime::from).toInstant();out.println(formattedInstance + " parsed back to " + instant2);
}

为了完整起见，所有上述示例均来自下一个代码清单中显示的示例类。

DateFormatDemo.java

package dustin.examples.numberformatdemo;import static java.lang.System.out;import java.text.DateFormat;
import java.text.Format;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.Instant;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Date;/*** Demonstrates formatting dates as strings and parsing strings* into dates and times using pre-Java 8 (java.text.SimpleDateFormat)* and Java 8 (java.time.format.DateTimeFormatter) mechanisms.*/
public class DateFormatDemo
{/** Pattern to use for String representation of Dates/Times. */private final String dateTimeFormatPattern = "yyyy/MM/dd HH:mm:ss z";/*** java.util.Date instance representing now that can* be formatted using SimpleDateFormat based on my* dateTimeFormatPattern field.*/private final Date now = new Date();/*** java.time.ZonedDateTime instance representing now that can* be formatted using DateTimeFormatter based on my* dateTimeFormatPattern field.** Note that ZonedDateTime needed to be used in this example* instead of java.time.LocalDateTime or java.time.OffsetDateTime* because there is zone information in the format provided by* my dateTimeFormatPattern field and attempting to have* DateTimeFormatter.format(TemporalAccessor) instantiated* with a format pattern that includes time zone details* will lead to DateTimeException for instances of* TemporalAccessor that do not have time zone information* (such as LocalDateTime and OffsetDateTime).*/private final ZonedDateTime now8 = ZonedDateTime.now();/*** String that can be used by both SimpleDateFormat and* DateTimeFormatter to parse respective date/time instances* from this String.*/private final String dateTimeString = "2014/09/03 13:59:50 MDT";/*** Demonstrate presenting java.util.Date as String matching* provided pattern via use of SimpleDateFormat.*/public void demonstrateSimpleDateFormatFormatting(){final DateFormat format = new SimpleDateFormat(dateTimeFormatPattern);final String nowString = format.format(now);out.println("Date '" + now + "' formatted with SimpleDateFormat and '"+ dateTimeFormatPattern + "': " + nowString);}/*** Demonstrate parsing a java.util.Date from a String* via SimpleDateFormat.*/public void demonstrateSimpleDateFormatParsing(){final DateFormat format = new SimpleDateFormat(dateTimeFormatPattern);try{final Date parsedDate = format.parse(dateTimeString);out.println("'" + dateTimeString + "' is parsed with SimpleDateFormat as " + parsedDate);}// DateFormat.parse(String) throws a checked exceptioncatch (ParseException parseException){out.println("ERROR: Unable to parse date/time String '"+ dateTimeString + "' with pattern '"+ dateTimeFormatPattern + "'.");}}/*** Demonstrate presenting ZonedDateTime as a String matching* provided pattern via DateTimeFormatter and its* ofPattern(String) method.*/public void demonstrateDateTimeFormatFormatting(){final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final String nowString = formatter.format(now8);out.println(now8 + " formatted with DateTimeFormatter and '"+ dateTimeFormatPattern + "': " + nowString);}/*** Demonstrate parsing ZonedDateTime from provided String* via ZonedDateTime's parse(String, DateTimeFormatter) method.*/public void demonstrateDateTimeFormatParsingTemporalStaticMethod(){final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime = ZonedDateTime.parse(dateTimeString, formatter);out.println("'" + dateTimeString+ "' is parsed with DateTimeFormatter and ZonedDateTime.parse as "+ zonedDateTime);}/*** Demonstrate parsing ZonedDateTime from String* via DateTimeFormatter.parse(String, TemporaryQuery)* with the Temple Query in this case being ZonedDateTime's* from(TemporalAccessor) used as a Java 8 method reference.*/public void demonstrateDateTimeFormatParsingMethodReference(){final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime = formatter.parse(dateTimeString, ZonedDateTime::from);out.println("'" + dateTimeString+ "' is parsed with DateTimeFormatter and ZoneDateTime::from as "+ zonedDateTime);}/*** Demonstrate formatting ZonedDateTime via DateTimeFormatter,* but using implementation of Format.*/public void demonstrateDateTimeFormatAndFormatFormatting(){final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern);final Format format = formatter.toFormat();final String nowString = format.format(now8);out.println("ZonedDateTime " + now + " formatted with DateTimeFormatter/Format (and "+ format.getClass().getCanonicalName() + ") and '"+ dateTimeFormatPattern + "': " + nowString);}/*** Demonstrate formatting and parsing an instance of Instant.*/public void demonstrateDateTimeFormatFormattingAndParsingInstant(){// Instant instances don't have timezone informationfinal Instant instant = now.toInstant();final DateTimeFormatter formatter =DateTimeFormatter.ofPattern(dateTimeFormatPattern).withZone(ZoneId.systemDefault());final String formattedInstance = formatter.format(instant);out.println("Instant " + instant + " formatted with DateTimeFormatter and '"+ dateTimeFormatPattern + "' to '" + formattedInstance + "'");final Instant instant2 =formatter.parse(formattedInstance, ZonedDateTime::from).toInstant();out.println(formattedInstance + " parsed back to " + instant2);}/*** Demonstrate java.text.SimpleDateFormat and* java.time.format.DateTimeFormatter.** @param arguments Command-line arguments; none anticipated.*/public static void main(final String[] arguments){final DateFormatDemo demo = new DateFormatDemo();out.print("\n1: ");demo.demonstrateSimpleDateFormatFormatting();out.print("\n2: ");demo.demonstrateSimpleDateFormatParsing();out.print("\n3: ");demo.demonstrateDateTimeFormatFormatting();out.print("\n4: ");demo.demonstrateDateTimeFormatParsingTemporalStaticMethod();out.print("\n5: ");demo.demonstrateDateTimeFormatParsingMethodReference();out.print("\n6: ");demo.demonstrateDateTimeFormatAndFormatFormatting();out.print("\n7: ");demo.demonstrateDateTimeFormatFormattingAndParsingInstant();}
}

下一个屏幕快照中显示了运行上述演示的输出。

结论

与JDK 8之前的版本相比，JDK 8日期/时间类以及相关的格式和解析类更易于使用。 这篇文章试图演示如何应用这些新类并利用它们的某些优点。

翻译自: https://www.javacodegeeks.com/2014/09/datetime-formattingparsing-java-8-style.html

java 解析日期格式