integer 快取及其聯想到的
阿新 • • 發佈:2019-07-14
首先,我們都知道integer
在-128至127(包含),是走快取的,該快取設計目的是:節省記憶體,提高效能。
猜想,Long
是否也存在類似快取設計?
public static void main(String[] args) { Integer integer1 = 3; Integer integer2 = 3; System.out.printf("integer1 == integer2:[%s]\n", integer1 == integer2); Integer integer3 = 300; Integer integer4 = 300; System.out.printf("integer3 == integer4結果:[%s]\n", integer3 == integer4); System.out.println("--------換行----------"); Long long1 = 3L; Long long2 = 3L; System.out.printf("long1 == long2:結果:[%s]\n", long1 == long2); Long long3 = 300L; Long long4 = 300L; System.out.printf("long3 == long4:結果:[%s]\n", long3 == long4); }
返回值如下:
integer1 == integer2:[true]
integer3 == integer4結果:[false]
--------換行----------
long1 == long2:結果:[true]
long3 == long4:結果:[false]
剖析
剖析integer
java.lang.integer
類中有個private static class IntegerCache
靜態內部類。其javadoc如下:
/** * Cache to support the object identity semantics of autoboxing for values between * -128 and 127 (inclusive) as required by JLS. * * The cache is initialized on first usage. The size of the cache * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option. * During VM initialization, java.lang.Integer.IntegerCache.high property * may be set and saved in the private system properties in the * sun.misc.VM class. */
其最大值(high)可以通過-XX:AutoBoxCacheMax=<size>
屬性來指定,但程式碼中有判斷,確保其不可小於127
剖析Long
java.lang.Long
類中有個private static class LongCache
靜態內部類。其程式碼如下:
private static class LongCache { private LongCache(){} static final Long cache[] = new Long[-(-128) + 127 + 1]; static { for(int i = 0; i < cache.length; i++) cache[i] = new Long(i - 128); } }
參考連結
下面這段文字,來自:https://github.com/hollischuang/toBeTopJavaer/blob/master/basics/java-basic/integer-cache.md
這種快取行為不僅適用於Integer物件。我們針對所有的整數型別的類都有類似的快取機制。
ByteCache用於快取Byte物件
ShortCache用於快取Short物件
LongCache用於快取Long物件
CharacterCache用於快取Character物件
Byte, Short, Long有固定範圍: -128 到 127。對於Character, 範圍是 0 到 127。除了Integer以外,這個範圍都不能