优雅处理时间之LocalDateTime

矫情吗；* 2024-03-26 22:00 34阅读 0赞

### 前言 ###

**LocalDate**，**LocalTime**和**LocalDateTime**是**JDK1.8**新增日期处理类，能够以**更快更优雅且线程安全**的方式实现日期处理。**DateTimeFormatter**可以对基于**LocalDate**，**LocalTime**和**LocalDateTime**获取到的日期的格式进行自定义，且**线程安全**。本篇文章将对其常见用法进行介绍。

### 正文 ###

#### 一. 获取当前日期时间 ####

**LocalDate**，**LocalTime**和**LocalDateTime**提供了静态方法**now()** 用于获取当前日期时间。

LocalDate date = LocalDate.now();
    LocalTime time = LocalTime.now();
    LocalDateTime dateTime = LocalDateTime.now();
    System.out.println("date: " + date.toString());
    System.out.println("time: " + time.toString());
    System.out.println("date time: " + dateTime.toString());
    复制代码

![format_png][]

#### 二. 日期时间格式化 ####

使用**DateTimeFormatter**可以对获取到的日期时间按照预置的格式进行格式化。

1. 格式化LocalDate

DateTimeFormatter dateFormatter1 = DateTimeFormatter.ofPattern("yyyy-MM-dd");
    DateTimeFormatter dateFormatter2 = DateTimeFormatter.ofPattern("yyyyMMdd");
    DateTimeFormatter dateFormatter3 = DateTimeFormatter.ofPattern("yyyy.MM.dd");
    DateTimeFormatter dateFormatter4 = DateTimeFormatter.ofPattern("yyyy年MM月dd日");
    LocalDate date = LocalDate.now();
    System.out.println("date -> " + date.toString());
    String formatDate1 = dateFormatter1.format(date);
    String formatDate2 = dateFormatter2.format(date);
    String formatDate3 = dateFormatter3.format(date);
    String formatDate4 = dateFormatter4.format(date);
    System.out.println("yyyy-MM-dd -> " + formatDate1);
    System.out.println("yyyyMMdd -> " + formatDate2);
    System.out.println("yyyy.MM.dd -> " + formatDate3);
    System.out.println("yyyy年MM月dd日 -> " + formatDate4);
    复制代码

![format_png 1][]

2. 格式化LocalTime

DateTimeFormatter timeFormatter1 = DateTimeFormatter.ofPattern("HH:mm:ss");
    DateTimeFormatter timeFormatter2 = DateTimeFormatter.ofPattern("HHmmss");
    DateTimeFormatter timeFormatter3 = DateTimeFormatter.ofPattern("HH mm ss");
    DateTimeFormatter timeFormatter4 = DateTimeFormatter.ofPattern("HH时mm分ss秒");
    LocalTime time = LocalTime.now();
    System.out.println("time -> " + time.toString());
    String formatTime1 = timeFormatter1.format(time);
    String formatTime2 = timeFormatter2.format(time);
    String formatTime3 = timeFormatter3.format(time);
    String formatTime4 = timeFormatter4.format(time);
    System.out.println("HH:mm:ss -> " + formatTime1);
    System.out.println("HHmmss -> " + formatTime2);
    System.out.println("HH mm ss -> " + formatTime3);
    System.out.println("HH时mm分ss秒 -> " + formatTime4);
    复制代码

![format_png 2][]

3. 格式化LocalDateTime

DateTimeFormatter dateTimeFormatter1 = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
    DateTimeFormatter dateTimeFormatter2 = DateTimeFormatter.ofPattern("yyyyMMddHHmmss");
    DateTimeFormatter dateTimeFormatter3 = DateTimeFormatter.ofPattern("yyyy.MM.dd HH:mm:ss");
    DateTimeFormatter dateTimeFormatter4 = DateTimeFormatter.ofPattern("yyyy年MM月dd日HH时mm分ss秒");
    LocalDateTime dateTime = LocalDateTime.now();
    System.out.println("date time -> " + dateTime);
    String formatDateTime1 = dateTimeFormatter1.format(dateTime);
    String formatDateTime2 = dateTimeFormatter2.format(dateTime);
    String formatDateTime3 = dateTimeFormatter3.format(dateTime);
    String formatDateTime4 = dateTimeFormatter4.format(dateTime);
    System.out.println("yyyy-MM-dd HH:mm:ss -> " + formatDateTime1);
    System.out.println("yyyyMMddHHmmss -> " + formatDateTime2);
    System.out.println("yyyy.MM.dd HH:mm:ss -> " + formatDateTime3);
    System.out.println("yyyy年MM月dd日HH时mm分ss秒 -> " + formatDateTime4);
    复制代码

![format_png 3][]

#### 三. 日期时间字符串转LocalDateTime ####

使用**DateTimeFormatter**也可以读取预置格式的日期时间字符串。

DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
    TemporalAccessor parsedDateTime = dateTimeFormatter.parse("2021-07-01 18:00:00");
    LocalDateTime dateTime = LocalDateTime.from(parsedDateTime);
    复制代码

#### 四. 日期时间之间的转换 ####

**LocalDate**，**LocalTime**和**LocalDateTime**之间可以方便的进行转换。

LocalDateTime dateTime = LocalDateTime.now();
    LocalDate date = dateTime.toLocalDate();
    LocalTime time = dateTime.toLocalTime();
    LocalDateTime gettedDateTime = LocalDateTime.of(date, time);
    System.out.println("date time: " + dateTime.toString());
    System.out.println("date: " + date.toString());
    System.out.println("time: " + time.toString());
    System.out.println("getted date time: " + gettedDateTime.toString());
    复制代码

![format_png 4][]

**LocalDate**，**LocalTime**和**LocalDateTime**提供了静态方法**of()** 来构造自身，以**LocalDateTime**为例，方法签名如下。

public static LocalDateTime of(int year, Month month, int dayOfMonth, int hour, int minute)
    public static LocalDateTime of(int year, Month month, int dayOfMonth, int hour, int minute, int second)
    public static LocalDateTime of(int year, Month month, int dayOfMonth, int hour, int minute, int second, int nanoOfSecond)
    public static LocalDateTime of(int year, int month, int dayOfMonth, int hour, int minute)
    public static LocalDateTime of(int year, int month, int dayOfMonth, int hour, int minute, int second)
    public static LocalDateTime of(int year, int month, int dayOfMonth, int hour, int minute, int second, int nanoOfSecond)
    public static LocalDateTime of(LocalDate date, LocalTime time)
    复制代码

#### 五. 获取某天在年（月，周）的第几天 ####

**LocalDate**和**LocalDateTime**可以方便的获取某天在年（月，周）的第几天。

LocalDate date = LocalDate.now();
    LocalDateTime dateTime = LocalDateTime.now();
    
    int dateDayOfYear = date.getDayOfYear();
    int dateTimeDayOfYear = dateTime.getDayOfYear();
    System.out.println("date day of year: " + dateDayOfYear);
    System.out.println("date time day of year: " + dateTimeDayOfYear);
    
    int dateDayOfMonth = date.getDayOfMonth();
    int dateTimeDayOfMonth = dateTime.getDayOfMonth();
    System.out.println("date day of month: " + dateDayOfMonth);
    System.out.println("date time day of month: " + dateTimeDayOfMonth);
    
    DayOfWeek dateDayOfWeek = date.getDayOfWeek();
    DayOfWeek dateTimeDayOfWeek = dateTime.getDayOfWeek();
    System.out.println("date day of week: " + dateDayOfWeek.getValue());
    System.out.println("date time day of week: " + dateTimeDayOfWeek.getValue());
    复制代码

![format_png 5][]

#### 六. 获取某月一共多少天 ####

**LocalDate**和**LocalDateTime**可以获取某月一共有多少天，获取方式如下。

LocalDate date = LocalDate.now();
    LocalDate lastDayOfMonth = date.with(TemporalAdjusters.lastDayOfMonth());
    System.out.println("days of month: " + lastDayOfMonth.getDayOfMonth());
    
    LocalDateTime dateTime = LocalDateTime.now();
    LocalDateTime lastDateTimeOfMonth = dateTime.with(TemporalAdjusters.lastDayOfMonth());
    System.out.println("days of month: " + lastDateTimeOfMonth.getDayOfMonth());
    复制代码

![format_png 6][]

#### 七. 日期时间加减 ####

**LocalDate**，**LocalTime**和**LocalDateTime**可以实现对日期时间的加减，以**LocalDate**为例进行说明。

LocalDate date = LocalDate.of(2021, 1, 1);
    LocalDate fiveDaysAgo = date.minus(5, ChronoUnit.DAYS);
    LocalDate fiveDaysLater = date.plus(5, ChronoUnit.DAYS);
    System.out.println("today: " + date.toString());
    System.out.println("five days ago: " + fiveDaysAgo.toString());
    System.out.println("five days later: " + fiveDaysLater.toString());
    复制代码

![format_png 7][]

#### 八. 日期时间比较 ####

**LocalDate**，**LocalTime**和**LocalDateTime**可以实现日期时间之间的比较，以**LocalDate**为例进行说明。

LocalDate date = LocalDate.of(2021, 1, 1);
    LocalDate fiveDaysAgo = date.minus(5, ChronoUnit.DAYS);
    LocalDate fiveDaysLater = date.plus(5, ChronoUnit.DAYS);
    
    System.out.println("compare result bwtween date and fiveDaysAgo: " + date.compareTo(fiveDaysAgo));
    System.out.println("compare result bwtween date and fiveDaysLater: " + date.compareTo(fiveDaysLater));
    System.out.println("compare result bwtween date and date: " + date.compareTo(date));
    复制代码

![format_png 8][]

比较的结果为两个日期时间的差值，通过判断比较结果的正负，可以判断日期时间的先后。

#### 九. 时间戳处理 ####

以**LocalDateTime**为例，演示如何进行**LocalDateTime**和时间戳的相互转换（**注意时区的处理**）。

1. 时间戳转换为LocalDateTime

示例代码如下。

public class LocalDateTimeLearnTest {
    
        @Test
        public void 时间戳转换为格式化时间() {
            DateTimeFormatter dateTimeFormatter = DateTimeFormatter
                    .ofPattern("yyyy-MM-dd'T'HH:mm:ss");
            
            // 2022-10-01T10:00:00
            long timeStamp = 1664589600000L;
    
            LocalDateTime localDateTime_8 = LocalDateTime.ofEpochSecond(
                    timeStamp / 1000, 0, ZoneOffset.ofHours(8));
            LocalDateTime localDateTime_0 = LocalDateTime.ofEpochSecond(
                    timeStamp / 1000, 0, ZoneOffset.ofHours(0));
            System.out.println(dateTimeFormatter.format(localDateTime_8));
            System.out.println(dateTimeFormatter.format(localDateTime_0));
        }
    
    }
    复制代码

运行结果如下。

![format_png 9][]

2. LocalDateTime转换时间戳

示例代码如下。

public class LocalDateTimeLearnTest {
    
        @Test
        public void 格式化时间转换为时间戳() {
            DateTimeFormatter dateTimeFormatter = DateTimeFormatter
                    .ofPattern("yyyy-MM-dd'T'HH:mm:ss");
    
            // 得到格式化时间LocalDateTime
            String dateTimeString = "2022-10-01T10:00:00";
            TemporalAccessor temporalAccessor = dateTimeFormatter.parse(dateTimeString);
            LocalDateTime dateTime = LocalDateTime.from(temporalAccessor);
            
            // 计算时间戳
            long timeStamp_8 = dateTime.toInstant(ZoneOffset.ofHours(8)).toEpochMilli();
            long timeStamp_0 = dateTime.toInstant(ZoneOffset.ofHours(0)).toEpochMilli();
            System.out.println("东八区时间戳：" + timeStamp_8);
            System.out.println("零时区时间戳：" + timeStamp_0);
    
            // 计算东八区与零时区相差小时数
            long diff = (timeStamp_0 - timeStamp_8) / 1000 / 60 / 60;
            System.out.println("东八区与零时区相差小时数：" + diff);
        }
    
    }
    复制代码

运行结果如下。

![format_png 10][]

#### 十. 为什么线程安全 ####

首先说明一下为什么**SimpleDateFormat**线程不安全。已知**SimpleDateFormat**是用于格式化和分析**Date**的类，当**SimpleDateFormat**对象被声明为静态变量时，**SimpleDateFormat**对象的使用是线程不安全的。以**SimpleDateFormat**的**format()** 方法为例，看一下其实现。

**format()** 方法最终会调用到如下实现。

private StringBuffer format(Date date, StringBuffer toAppendTo,
                                FieldDelegate delegate) {
        // 调用了线程不安全的Calendar对象
        calendar.setTime(date);
    
        boolean useDateFormatSymbols = useDateFormatSymbols();
    
        for (int i = 0; i < compiledPattern.length; ) {
            int tag = compiledPattern[i] >>> 8;
            int count = compiledPattern[i++] & 0xff;
            if (count == 255) {
                count = compiledPattern[i++] << 16;
                count |= compiledPattern[i++];
            }
    
            switch (tag) {
            case TAG_QUOTE_ASCII_CHAR:
                toAppendTo.append((char)count);
                break;
    
            case TAG_QUOTE_CHARS:
                toAppendTo.append(compiledPattern, i, count);
                i += count;
                break;
    
            default:
                subFormat(tag, count, delegate, toAppendTo, useDateFormatSymbols);
                break;
            }
        }
        return toAppendTo;
    }
    复制代码

**calendar**是**SimpleDateFormat**从父类**DateFormat**继承的**Calendar**对象，如果将**SimpleDateFormat**对象声明为静态变量，则会存在多个线程同时使用**SimpleDateFormat**对象，因此多个线程同时调用**SimpleDateFormat**的**format()** 方法时，会导致多个线程同时调用**Calendar**的**setTime()** 方法，线程之间会产生相互影响，线程不安全。下面以一个简单例子演示一下。

public class PrintDate {
    
        private static SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
    
        public void printDate(Date date) {
            System.out.println(Thread.currentThread().getName() + " -> " + sdf.format(date));
        }
    
    }
    复制代码

测试方法如下所示。

public class PrintDateTest {
    
        private PrintDate printDate;
    
        @Before
        public void setUp() {
            printDate = new PrintDate();
        }
    
        @Test
        public void givenMultiThreads_whenPrintDateBySimpleDateFormat_thenThreadUnsafe() {
            ThreadPoolExecutor threadPool = new ThreadPoolExecutor(
                    2, 10, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<>(15));
            
            threadPool.execute(() -> {
                Date date = new Date(121, Calendar.JANUARY, 1);
                Thread.currentThread().setName(date.toString());
                for (int i = 0; i < 50; i++) {
                    printDate.printDate(date);
                }
            });
            threadPool.execute(() -> {
                Date date = new Date(121, Calendar.JANUARY, 2);
                Thread.currentThread().setName(date.toString());
                for (int i = 0; i < 50; i++) {
                    printDate.printDate(date);
                }
            });
    
            LockSupport.parkNanos(1000 * 1000 * 1000L);
        }
    
    }
    复制代码

启用两个线程循环50次调用**PrintDate**的**printDate()** 方法，结果如下所示。

![format_png 11][]

发现存在传入日期和打印日期不一致的情况。表明了当**SimpleDateFormat**对象被声明为静态变量时，**SimpleDateFormat**对象的使用是线程不安全的

那么**LocalDate**，**LocalTime**和**LocalDateTime**为什么线程安全呢。下面看一下它们的字段签名。

**LocalDate**字段签名。

public final class LocalDate
            implements Temporal, TemporalAdjuster, ChronoLocalDate, Serializable {
    
        // 省略
    
        private final int year;
    
        private final short month;
    
        private final short day;
    
        // 省略
    
    }
    复制代码

**LocalTime**字段签名。

public final class LocalTime
            implements Temporal, TemporalAdjuster, Comparable<LocalTime>, Serializable {
    
        // 省略
    
        private final byte hour;
    
        private final byte minute;
    
        private final byte second;
    
        private final int nano;
    
        // 省略
    
    }
    复制代码

**LocalDateTime**字段签名。

public final class LocalDateTime
            implements Temporal, TemporalAdjuster, ChronoLocalDateTime<LocalDate>, Serializable {
    
        // 省略
    
        private final LocalDate date;
    
        private final LocalTime time;
    
        // 省略
    
    }
    复制代码

**LocalDate**，**LocalTime**和**LocalDateTime**的表示日期时间的字段签名为**private final**，表示它们一经指定日期时间，便不能再被改变。其次，它们对日期时间的更改，都会返回一个新的对象。**LocalDate**，**LocalTime**和**LocalDateTime**的线程安全的实现和**String**的线程安全的实现是一样的，**String**类中存储字符的数组的签名也是**private final**，以及对字符串的操作会返回一个新的字符串对象。

### 总结 ###

**LocalDate**，**LocalTime**和**LocalDateTime**的使用比**Date**，**SimpleDateFormat**和**Calendar**的使用更方便和安全。善用**LocalDate**，**LocalTime**和**LocalDateTime**，可以使得对日期时间的处理更加优雅。

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