java解析日期格式_日期/时间格式/解析，Java8样式

java 解析日期格式

自Java 几乎 开始以来&＃xff0c;Java开发人员就通过java.util.Date类&＃xff08;自JDK 1.0起&＃xff09;和java.util.Calendar类&＃xff08;自JDK 1.1起 &＃xff09;来处理日期和时间。 在这段时间内&＃xff0c;成千上万&＃xff08;甚至数百万&＃xff09;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&＃xff0c;不足为奇的是&＃xff0c;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 &＃xff08; Dates and Times &＃xff09;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&＃xff0c;先比较一下DateFormat / SimpleDateFormat和DateTimeFormatter的Javadoc描述。 下表包含可区分的信息&＃xff0c;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&＃xff0c;新的DateTimeFormatter是线程安全的且不可变的&＃xff0c;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 &＃61; "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 &＃61; 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 &＃61; 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 &＃61; "2014/09/03 13:59:50 MDT";

在Java 8之前&＃xff0c;用于日期和时间的标准Java方法是通过Date和Calendar类&＃xff0c;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 &＃61; new SimpleDateFormat(dateTimeFormatPattern);final String nowString &＃61; format.format(now);out.println("Date &＃39;" &＃43; now &＃43; "&＃39; formatted with SimpleDateFormat and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;: " &＃43; nowString);
}/*** Demonstrate parsing a java.util.Date from a String* via SimpleDateFormat.*/
public void demonstrateSimpleDateFormatParsing()
{final DateFormat format &＃61; new SimpleDateFormat(dateTimeFormatPattern);try{final Date parsedDate &＃61; format.parse(dateTimeString);out.println("&＃39;" &＃43; dateTimeString &＃43; "&＃39; is parsed with SimpleDateFormat as " &＃43; parsedDate);}// DateFormat.parse(String) throws a checked exceptioncatch (ParseException parseException){out.println("ERROR: Unable to parse date/time String &＃39;"&＃43; dateTimeString &＃43; "&＃39; with pattern &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;.");}
}

在Java 8中&＃xff0c;首选日期/时间类不再位于java.util包中&＃xff0c;并且首选日期/时间处理类现在位于java.time包中。 同样&＃xff0c;首选的日期/时间格式/解析类不再位于java.text包中&＃xff0c;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&＃xff08;无时间信息&＃xff09;&＃xff0c;仅对时间建模的类&＃xff08;无日期信息&＃xff09;&＃xff0c;对日期和时间信息进行建模的类&＃xff0c;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&＃xff08;例如&＃xff0c;是否支持日期或时间或时区信息&＃xff09;会影响可以应用的模式&＃xff0c;但格式化和解析这些方法的方法通常是相似的。 在本文中&＃xff0c;我将ZonedDateTime类用作示例。 之所以选择该选项&＃xff0c;是因为它包含日期&＃xff0c;时间和时区信息&＃xff0c;并允许我使用一种匹配模式&＃xff0c;该模式涉及所有三个特征&＃xff0c;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&＃xff0c;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&＃xff0c;以获取日期和/或时间信息&＃xff0c;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 &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final String nowString &＃61; formatter.format(now8);out.println(now8 &＃43; " formatted with DateTimeFormatter and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;: " &＃43; 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&＃xff08;CharSequence&＃xff0c;DateTimeFormatter&＃xff09;方法&＃xff0c;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&＃39;s parse(String, DateTimeFormatter) method.*/
public void demonstrateDateTimeFormatParsingTemporalStaticMethod()
{final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime &＃61; ZonedDateTime.parse(dateTimeString, formatter);out.println("&＃39;" &＃43; dateTimeString&＃43; "&＃39; is parsed with DateTimeFormatter and ZonedDateTime.parse as "&＃43; zonedDateTime);
}

从字符串解析ZonedDateTime第二种方法是通过DateTimeFormatter的parse&＃xff08;CharSequence&＃xff0c;TemporalQuery &＃xff09;方法。 在下面的代码清单中对此进行了说明&＃xff0c;该清单也提供了演示Java 8 方法引用的用法的机会&＃xff08;请参见ZonedDateTime::from &＃xff09;。

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&＃39;s* from(TemporalAccessor) used as a Java 8 method reference.*/
public void demonstrateDateTimeFormatParsingMethodReference()
{final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime &＃61; formatter.parse(dateTimeString, ZonedDateTime::from);out.println("&＃39;" &＃43; dateTimeString&＃43; "&＃39; is parsed with DateTimeFormatter and ZoneDateTime::from as "&＃43; 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&＃xff0c;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&＃xff0c;JDK 8的DateTimeFormatter可以通过DateTimeFormatter.toFormat&＃xff08;&＃xff09;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 &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final Format format &＃61; formatter.toFormat();final String nowString &＃61; format.format(now8);out.println("ZonedDateTime " &＃43; now &＃43; " formatted with DateTimeFormatter/Format (and "&＃43; format.getClass().getCanonicalName() &＃43; ") and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;: " &＃43; 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&＃xff0c; java.util.Date有方法从&＃xff08;即时&＃xff09;和toInstant&＃xff08;&＃xff09;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&＃xff0c;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&＃39;t have timezone informationfinal Instant instant &＃61; now.toInstant();final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern).withZone(ZoneId.systemDefault());final String formattedInstance &＃61; formatter.format(instant);out.println("Instant " &＃43; instant &＃43; " formatted with DateTimeFormatter and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39; to &＃39;" &＃43; formattedInstance &＃43; "&＃39;");final Instant instant2 &＃61;formatter.parse(formattedInstance, ZonedDateTime::from).toInstant();out.println(formattedInstance &＃43; " parsed back to " &＃43; instant2);
}

为了完整起见&＃xff0c;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 &＃61; "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 &＃61; 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 &＃61; 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 &＃61; "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 &＃61; new SimpleDateFormat(dateTimeFormatPattern);final String nowString &＃61; format.format(now);out.println("Date &＃39;" &＃43; now &＃43; "&＃39; formatted with SimpleDateFormat and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;: " &＃43; nowString);}/*** Demonstrate parsing a java.util.Date from a String* via SimpleDateFormat.*/public void demonstrateSimpleDateFormatParsing(){final DateFormat format &＃61; new SimpleDateFormat(dateTimeFormatPattern);try{final Date parsedDate &＃61; format.parse(dateTimeString);out.println("&＃39;" &＃43; dateTimeString &＃43; "&＃39; is parsed with SimpleDateFormat as " &＃43; parsedDate);}// DateFormat.parse(String) throws a checked exceptioncatch (ParseException parseException){out.println("ERROR: Unable to parse date/time String &＃39;"&＃43; dateTimeString &＃43; "&＃39; with pattern &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;.");}}/*** Demonstrate presenting ZonedDateTime as a String matching* provided pattern via DateTimeFormatter and its* ofPattern(String) method.*/public void demonstrateDateTimeFormatFormatting(){final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final String nowString &＃61; formatter.format(now8);out.println(now8 &＃43; " formatted with DateTimeFormatter and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;: " &＃43; nowString);}/*** Demonstrate parsing ZonedDateTime from provided String* via ZonedDateTime&＃39;s parse(String, DateTimeFormatter) method.*/public void demonstrateDateTimeFormatParsingTemporalStaticMethod(){final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime &＃61; ZonedDateTime.parse(dateTimeString, formatter);out.println("&＃39;" &＃43; dateTimeString&＃43; "&＃39; is parsed with DateTimeFormatter and ZonedDateTime.parse as "&＃43; zonedDateTime);}/*** Demonstrate parsing ZonedDateTime from String* via DateTimeFormatter.parse(String, TemporaryQuery)* with the Temple Query in this case being ZonedDateTime&＃39;s* from(TemporalAccessor) used as a Java 8 method reference.*/public void demonstrateDateTimeFormatParsingMethodReference(){final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final ZonedDateTime zonedDateTime &＃61; formatter.parse(dateTimeString, ZonedDateTime::from);out.println("&＃39;" &＃43; dateTimeString&＃43; "&＃39; is parsed with DateTimeFormatter and ZoneDateTime::from as "&＃43; zonedDateTime);}/*** Demonstrate formatting ZonedDateTime via DateTimeFormatter,* but using implementation of Format.*/public void demonstrateDateTimeFormatAndFormatFormatting(){final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern);final Format format &＃61; formatter.toFormat();final String nowString &＃61; format.format(now8);out.println("ZonedDateTime " &＃43; now &＃43; " formatted with DateTimeFormatter/Format (and "&＃43; format.getClass().getCanonicalName() &＃43; ") and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39;: " &＃43; nowString);}/*** Demonstrate formatting and parsing an instance of Instant.*/public void demonstrateDateTimeFormatFormattingAndParsingInstant(){// Instant instances don&＃39;t have timezone informationfinal Instant instant &＃61; now.toInstant();final DateTimeFormatter formatter &＃61;DateTimeFormatter.ofPattern(dateTimeFormatPattern).withZone(ZoneId.systemDefault());final String formattedInstance &＃61; formatter.format(instant);out.println("Instant " &＃43; instant &＃43; " formatted with DateTimeFormatter and &＃39;"&＃43; dateTimeFormatPattern &＃43; "&＃39; to &＃39;" &＃43; formattedInstance &＃43; "&＃39;");final Instant instant2 &＃61;formatter.parse(formattedInstance, ZonedDateTime::from).toInstant();out.println(formattedInstance &＃43; " parsed back to " &＃43; instant2);}/*** Demonstrate java.text.SimpleDateFormat and* java.time.format.DateTimeFormatter.** &＃64;param arguments Command-line arguments; none anticipated.*/public static void main(final String[] arguments){final DateFormatDemo demo &＃61; 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&＃xff0c;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