前言
接着源码阅读:new Date之旅，补充几种关于获取当前时间的方式。
Date
在Java中，获取当前日期最简单的方法之一就是直接实例化位于Java包java.util的Date类。
Date date = new Date();

System.currentTimeMillis
获取标准时间可以通过System.currentTimeMillis()方法获取，此方法不受时区影响，得到的结果是时间戳格式的。
long time = System.currentTimeMillis();

源码：
	 /**
     * Returns the current time in milliseconds.  Note that
     * while the unit of time of the return value is a millisecond,
     * the granularity of the value depends on the underlying
     * operating system and may be larger.  For example, many
     * operating systems measure time in units of tens of
     * milliseconds.
     *
     * <p> See the description of the class <code>Date</code> for
     * a discussion of slight discrepancies that may arise between
     * "computer time" and coordinated universal time (UTC).
     *
     * @return  the difference, measured in milliseconds, between
     *          the current time and midnight, January 1, 1970 UTC.
     * @see     java.util.Date
     */
    public static native long currentTimeMillis();

Calendar
Calendar类，专门用于转换特定时刻和日历字段之间的日期和时间。
Calendar calendar = Calendar.getInstance(); 

源码：
/**
     * Gets a calendar using the default time zone and locale. The
     * <code>Calendar</code> returned is based on the current time
     * in the default time zone with the default
     * {@link Locale.Category#FORMAT FORMAT} locale.
     *
     * @return a Calendar.
     */
    public static Calendar getInstance()
    {
        return createCalendar(TimeZone.getDefault(), Locale.getDefault(Locale.Category.FORMAT));
    }

   	private static Calendar createCalendar(TimeZone zone,
                                           Locale aLocale)
    {
        CalendarProvider provider =
            LocaleProviderAdapter.getAdapter(CalendarProvider.class, aLocale)
                                 .getCalendarProvider();
        if (provider != null) {
            try {
                return provider.getInstance(zone, aLocale);
            } catch (IllegalArgumentException iae) {
                // fall back to the default instantiation
            }
        }

        Calendar cal = null;

        if (aLocale.hasExtensions()) {
            String caltype = aLocale.getUnicodeLocaleType("ca");
            if (caltype != null) {
                switch (caltype) {
                case "buddhist":
                cal = new BuddhistCalendar(zone, aLocale);
                    break;
                case "japanese":
                    cal = new JapaneseImperialCalendar(zone, aLocale);
                    break;
                case "gregory":
                    cal = new GregorianCalendar(zone, aLocale);
                    break;
                }
            }
        }
        if (cal == null) {
            // If no known calendar type is explicitly specified,
            // perform the traditional way to create a Calendar:
            // create a BuddhistCalendar for th_TH locale,
            // a JapaneseImperialCalendar for ja_JP_JP locale, or
            // a GregorianCalendar for any other locales.
            // NOTE: The language, country and variant strings are interned.
            if (aLocale.getLanguage() == "th" && aLocale.getCountry() == "TH") {
                cal = new BuddhistCalendar(zone, aLocale);
            } else if (aLocale.getVariant() == "JP" && aLocale.getLanguage() == "ja"
                       && aLocale.getCountry() == "JP") {
                cal = new JapaneseImperialCalendar(zone, aLocale);
            } else {
                cal = new GregorianCalendar(zone, aLocale);
            }
        }
        return cal;
    }

LocalDateTime
LocalDateTime，是Java中最常用的Date / Time类，代表前两个类的组合 – 即日期和时间的值。
LocalDateTime dateTime = LocalDateTime.now(); 

源码：
	 /**
     * Obtains the current date-time from the system clock in the default time-zone.
     * <p>
     * This will query the {@link Clock#systemDefaultZone() system clock} in the default
     * time-zone to obtain the current date-time.
     * <p>
     * Using this method will prevent the ability to use an alternate clock for testing
     * because the clock is hard-coded.
     *
     * @return the current date-time using the system clock and default time-zone, not null
     */
    public static LocalDateTime now() {
        return now(Clock.systemDefaultZone());
    }
  	/**
     * Obtains the current date-time from the specified clock.
     * <p>
     * This will query the specified clock to obtain the current date-time.
     * Using this method allows the use of an alternate clock for testing.
     * The alternate clock may be introduced using {@link Clock dependency injection}.
     *
     * @param clock  the clock to use, not null
     * @return the current date-time, not null
     */
    public static LocalDateTime now(Clock clock) {
        Objects.requireNonNull(clock, "clock");
        final Instant now = clock.instant();  // called once
        ZoneOffset offset = clock.getZone().getRules().getOffset(now);
        return ofEpochSecond(now.getEpochSecond(), now.getNano(), offset);
    }
 	 /**
     * Obtains an instance of {@code LocalDateTime} using seconds from the
     * epoch of 1970-01-01T00:00:00Z.
     * <p>
     * This allows the {@link ChronoField#INSTANT_SECONDS epoch-second} field
     * to be converted to a local date-time. This is primarily intended for
     * low-level conversions rather than general application usage.
     *
     * @param epochSecond  the number of seconds from the epoch of 1970-01-01T00:00:00Z
     * @param nanoOfSecond  the nanosecond within the second, from 0 to 999,999,999
     * @param offset  the zone offset, not null
     * @return the local date-time, not null
     * @throws DateTimeException if the result exceeds the supported range,
     *  or if the nano-of-second is invalid
     */
    public static LocalDateTime ofEpochSecond(long epochSecond, int nanoOfSecond, ZoneOffset offset) {
        Objects.requireNonNull(offset, "offset");
        NANO_OF_SECOND.checkValidValue(nanoOfSecond);
        long localSecond = epochSecond + offset.getTotalSeconds();  // overflow caught later
        long localEpochDay = Math.floorDiv(localSecond, SECONDS_PER_DAY);
        int secsOfDay = (int)Math.floorMod(localSecond, SECONDS_PER_DAY);
        LocalDate date = LocalDate.ofEpochDay(localEpochDay);
        LocalTime time = LocalTime.ofNanoOfDay(secsOfDay * NANOS_PER_SECOND + nanoOfSecond);
        return new LocalDateTime(date, time);
    }


结果


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Java 当前时间的上一个月时间、上一年时间


获取当前时间
  /**
   * 获取当前时间
   * @return Timestamp
   * @author  zhangyao
   * date: 2018/8/22 11:36
   */
  public static Timestamp now() {
      return new Timestamp(Calendar.getInstance().getTimeInMillis());
  }


上个月、上一年;time可以用获取当前时间的值
    /**
   * 上一个月
   */
  private Timestamp lastMonthTime(Timestamp time){
      Calendar c = Calendar.getInstance();
      c.setTime(time);
      c.add(Calendar.MONTH, -1);
      return  new Timestamp(c.getTimeInMillis());
  }

  private Timestamp lastYearTime(Timestamp time){
      //过去一年
      Calendar c = Calendar.getInstance();
      c.setTime(time);
      c.add(Calendar.YEAR, -1);
      return  new Timestamp(c.getTimeInMillis());
  }

获取java当前时间增加一小时怎么写
首先新建Date对象，然后转化成Calendar对象，利用其中的方法得到一小时后的Calendar对象，再转化成Date。
		//设置创建时间
        Date creatTime = new Date();
        noteEatlyWarningInfo.setCreatTime(creatTime);
        //设置生效时间为一小时后
        Calendar cal = Calendar.getInstance();
        cal.setTime(creatTime);
        cal.add(Calendar.HOUR, 1);// 24小时制
        Date effectivetime = cal.getTime();
        noteEatlyWarningInfo.setEffectiveTime(effectivetime);

方法1:new SimpleDateFormat(“yyyyMMddHHmmssSSS”) .format(new Date() );
方法2:new SimpleDateFormat(“yyyy-MM-dd HH:mm:ss:SSS”) .format(new Date() );
一. 获取当前系统时间和日期并格式化输出:
import java.util.Date;
import java.text.SimpleDateFormat;
public class NowString {
public static void main(String[] args) {
SimpleDateFormat df = new SimpleDateFormat(“yyyy-MM-dd HH:mm:ss”);//设置日期格式
System.out.println(df.format(new Date()));// new Date()为获取当前系统时间
}
}
二. 在数据库里的日期只以年-月-日的方式输出，可以用下面两种方法：
1、用convert()转化函数：
String sqlst = “select convert(varchar(10),bookDate,126) as convertBookDate from roomBook where bookDate between ’2007-4-10′ and ’2007-4-25′”;
System.out.println(rs.getString(“convertBookDate”));
2、利用SimpleDateFormat类：
先要输入两个java包：
import java.util.Date;
import java.text.SimpleDateFormat;
然后：
定义日期格式：SimpleDateFormat sdf = new SimpleDateFormat(yy-MM-dd);
sql语句为：String sqlStr = “select bookDate from roomBook where bookDate between ’2007-4-10′ and ’2007-4-25′”;
输出：
System.out.println(df.format(rs.getDate(“bookDate”)));
************************************************************
java中获取当前日期和时间的方法
import java.util.Date;
import java.util.Calendar;
import java.text.SimpleDateFormat;
public class TestDate{
public static void main(String[] args){
Date now = new Date();
SimpleDateFormat dateFormat = new SimpleDateFormat(“yyyy/MM/dd HH:mm:ss”);//可以方便地修改日期格式
String hehe = dateFormat.format( now );
System.out.println(hehe);
Calendar c = Calendar.getInstance();//可以对每个时间域单独修改
int year = c.get(Calendar.YEAR);
int month = c.get(Calendar.MONTH);
int date = c.get(Calendar.DATE);
int hour = c.get(Calendar.HOUR_OF_DAY);
int minute = c.get(Calendar.MINUTE);
int second = c.get(Calendar.SECOND);
System.out.println(year + “/” + month + “/” + date + ” ” +hour + “:” +minute + “:” + second);
}
}
有时候要把String类型的时间转换为Date类型，通过以下的方式，就可以将你刚得到的时间字符串转换为Date类型了。
SimpleDateFormat sdf=new SimpleDateFormat(“yyyy-MM-dd”);
java.util.Date time=null;
try {
time= sdf.parse(sdf.format(new Date()));
} catch (ParseException e) {
e.printStackTrace();
}