Date、System、Calendar獲取當前時間毫秒
Date、System、Calendar三個類都可以獲取getTime方法，若要考慮效能，應選哪一個才比較合適呢？
如下程式碼模擬所示：
public class Test35
{
public static void main(String[] args)
{
long time = 0L;
// 執行函式SCTM
time = SCTM();
// 獲取時間差並列印
System.out.println("System.currentTimeMillis cost [" + time + "] ms");
// 執行函式DGT
time = DGT();
// 獲取時間差並列印
System.out.println("new Date().getTime cost [" + time + "] ms");
// 執行函式G
time = GGIGTG();
// 獲取時間差並列印
System.out.println("Calendar.getInstance().getTime().getTime cost [" + time + "] ms");
}
/**
* new Date物件放入堆中即消耗記憶體，在通過System類引用靜態方法獲取 構造方法:this(System.currentTimeMillis())
*
*
*/
private static long DGT()
{
// 開啟空間1000000
List<Long> list = new ArrayList<>(10000000);
long startTime = System.currentTimeMillis();
for (int i = 0; i < 10000000; i++ )
{
long data = new Date().getTime();
list.add(data);
}
return (System.currentTimeMillis() - startTime);
}
/**
* 直接通過類引用靜態方法獲取
*
* @return
*/
private static long SCTM()
{
// 開啟空間1000000
List<Long> list = new ArrayList<>(10000000);
long startTime = System.currentTimeMillis();
for (int i = 0; i < 10000000; i++ )
{
long data = System.currentTimeMillis();
list.add(data);
}
return (System.currentTimeMillis() - startTime);
}
/**
* Calendar獲取getInstance靜態方法，但它呼叫createCalendar方法耗盡了大量時間去判斷邏輯程式碼，如createCalendar原始碼所示
* createCalendar(TimeZone.getDefault(), Locale.getDefault(Locale.Category.FORMAT));
*
* @return
*/
private static long GGIGTG()
{
// 開啟空間1000000
List<Long> list = new ArrayList<>(10000000);
long startTime = System.currentTimeMillis();
for (int i = 0; i < 10000000; i++ )
{
long data = Calendar.getInstance().getTime().getTime();
list.add(data);
}
return (System.currentTimeMillis() - startTime);
}
/**
* createCalendar(TimeZone.getDefault(), Locale.getDefault(Locale.Category.FORMAT));原始碼
*
* @see JDKcode createCalendar(zone,aLocale))
*/
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;
}
}
執行結果：
System.currentTimeMillis cost [2266] ms
new Date().getTime cost [3051] ms
Calendar.getInstance().getTime().getTime cost [6449] ms
注：方法耗時描述如javadoc註釋所示，由結果可知耗時最多是Calendar，其次是Date，耗時最少