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企业网服务器如何搭建_企业app下载_一键生成网站_陕西疫情最新消息

时间:2025/7/9 13:11:33来源:https://blog.csdn.net/qq_31860647/article/details/142268101 浏览次数: 0次
企业网服务器如何搭建_企业app下载_一键生成网站_陕西疫情最新消息

开始介绍之前,希望大家能想一想下面这串代码运行出来的结果是什么?

public class IntegerCacheTest {public static void main(String[] args) {fun1();fun2();fun3();fun4();}public static void fun1(){Integer num1 = -129;Integer num2 = -129;System.out.println(num1 == num2);}public static void fun2(){Integer num1 = -128;Integer num2 = -128;System.out.println(num1 == num2);}public static void fun3(){Integer num1 = 127;Integer num2 = 127;System.out.println(num1 == num2);}public static void fun4(){Integer num1 = 128;Integer num2 = 128;System.out.println(num1 == num2);}
}

-129和-129的Integer对象,相不相等?使用基本数据类型int定义的值肯定相等,但是Integer是引用数据类型,而且-129不在缓存区里,所以应该是false。

-128在不在缓存区呢?把它和127,128一块测试,效果如下:

false
true
true
false

从结果上看,说明128也并不在Integer的缓存区里。所以Integer的缓存范围是-128到127之间,并且包含两个边界值,一共256个数(别忘了有个0)。

下面对Integer的相关源码解析一下:
上面的代码Integer在创建对象时会使用到valueOf方法,下面来看一下这个方法:

    /*** Returns an {@code Integer} instance representing the specified* {@code int} value.  If a new {@code Integer} instance is not* required, this method should generally be used in preference to* the constructor {@link #Integer(int)}, as this method is likely* to yield significantly better space and time performance by* caching frequently requested values.** This method will always cache values in the range -128 to 127,* inclusive, and may cache other values outside of this range.** @param  i an {@code int} value.* @return an {@code Integer} instance representing {@code i}.* @since  1.5*/@IntrinsicCandidatepublic static Integer valueOf(int i) {if (i >= IntegerCache.low && i <= IntegerCache.high)return IntegerCache.cache[i + (-IntegerCache.low)];return new Integer(i);}

我们可以发现当i在满足IntegerCache.low和i <= IntegerCache.high的时候直接返回一个IntegerCache里缓存的数据,而不满足的条件则是直接new一个新的Integer。

下面查看一下IntegerCache的源码:

    /*** 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* jdk.internal.misc.VM class.** WARNING: The cache is archived with CDS and reloaded from the shared* archive at runtime. The archived cache (Integer[]) and Integer objects* reside in the closed archive heap regions. Care should be taken when* changing the implementation and the cache array should not be assigned* with new Integer object(s) after initialization.*/private static class IntegerCache {static final int low = -128;static final int high;static final Integer[] cache;static Integer[] archivedCache;static {// high value may be configured by propertyint h = 127;String integerCacheHighPropValue =VM.getSavedProperty("java.lang.Integer.IntegerCache.high");if (integerCacheHighPropValue != null) {try {h = Math.max(parseInt(integerCacheHighPropValue), 127);// Maximum array size is Integer.MAX_VALUEh = Math.min(h, Integer.MAX_VALUE - (-low) -1);} catch( NumberFormatException nfe) {// If the property cannot be parsed into an int, ignore it.}}high = h;// Load IntegerCache.archivedCache from archive, if possibleCDS.initializeFromArchive(IntegerCache.class);int size = (high - low) + 1;// Use the archived cache if it exists and is large enoughif (archivedCache == null || size > archivedCache.length) {Integer[] c = new Integer[size];int j = low;for(int i = 0; i < c.length; i++) {c[i] = new Integer(j++);}archivedCache = c;}cache = archivedCache;// range [-128, 127] must be interned (JLS7 5.1.7)assert IntegerCache.high >= 127;}private IntegerCache() {}}

可以看到low是-128,high虽然并没有在定义的时候赋值,但是执行静态代码块的时候把127赋值给了high,这就是缓存的数据范围。通过代码分析,可以看到**static final Integer[] cache;**保存了从-128到127的数据,所以我们创建Integer对象时如果在这个区间,其实使用的都是缓存对象,所以使用==判断是true,反之为false,这边使用的是设计模式中的享元模式。

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