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Calendar.getInstance()是非常耗时的操作。需要明确的是，日历实际上不是单例。每次调用Calendar.getInstance()都会返回一个新对象。
查看源码：
public static Calendar getInstance() { Calendar cal = createCalendar(TimeZone.getDefaultRef(), Locale.getDefault(Locale.Category.FORMAT)); cal.sharedZone = true; return cal; }再看createCalendar方法
private static Calendar createCalendar(TimeZone zone, Locale aLocale) { Calendar cal = null; String caltype = aLocale.getUnicodeLocaleType("ca"); if (caltype == null) { // Calendar type is not specified. // If the specified locale is a Thai locale, // returns a BuddhistCalendar instance. if ("th".equals(aLocale.getLanguage()) && ("TH".equals(aLocale.getCountry()))) { cal = new BuddhistCalendar(zone, aLocale); } else { cal = new GregorianCalendar(zone, aLocale); } } else if (caltype.equals("japanese")) { cal = new JapaneseImperialCalendar(zone, aLocale); } else if (caltype.equals("buddhist")) { cal = new BuddhistCalendar(zone, aLocale); } else { // Unsupported calendar type. // Use Gregorian calendar as a fallback. cal = new GregorianCalendar(zone, aLocale); } return cal; }代码中可以清楚的看到是每次都创建的。
看看使用System.currentTimeMillis()和Calendar.getInstance()的性能差别
public class Test1 { public static void main(String[] args) { int runs = 10000; long start = System.nanoTime(); Calendar cal = null; for(int i=0;i<runs;i++) cal = Calendar.getInstance(); long time = System.nanoTime() - start; System.out.println("Calendar.getInstance() took on average "+time/runs+" ns. "+cal); long start2 = System.nanoTime(); long now = 0; for(int i=0;i<runs;i++) now = System.currentTimeMillis(); long time2 = System.nanoTime() - start2; System.out.println("System.currentTimeMillis() took on average "+time2/runs+" ns. "+now); }结果：
Calendar.getInstance() took on average 29348 ns. java.util.GregorianCalendar[time=1569646753165,.......]System.currentTimeMillis() took on average 89 ns. 1569646753167微信公众号:51码农网