【Java】Integer变量相等(==)比较问题

题目

这是关于一段令人疑惑的Java代码:

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class TestIntegerCache {

public static void main(String[] args){
Integer i3 = 100;
Integer i4 = 100;
System.out.println(i3 == i4);

Integer i5 = 1000;
Integer i6 = 1000;
System.out.println(i5 == i6);
}

}

这么简单,执行结果是什么?

true
false

一个是true,一个是false!
这是为什么呢?为什么和大多数人心里想的不一样!

分析

根据Java编译机制,.java文件在编译以后会生成.class文件给JVM加载执行,于是找到.class文件,反编译看了一下,发现编译器在编译我们的代码时,很调皮(聪明的)的在我们声明的变量加上了valueOf方法 ,代码变成了如下:

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class TestIntegerCache {

public static void main(String[] args){
Integer i3 = Integer.valueOf(100);
Integer i4 = Integer.valueOf(100);
System.out.println(i3 == i4);

Integer i5 = Integer.valueOf(1000);
Integer i6 = Integer.valueOf(1000);
System.out.println(i5 == i6);
}
}

valueOf() 方法对它做了什么,我们看看源代码:

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/**
* 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
*/
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high) //我们看到这里有个缓存,在缓存区间就返回缓存里的
return IntegerCache.cache[i + (-IntegerCache.low)]; //缓存数组相应的对象
return new Integer(i); //不在缓存数组区间就new一个对象
}

我们发现,Integer的作者在写这个类时,为了避免重复创建对象,对Integer值做了缓存,如果这个值在缓存范围内,直接返回缓存好的对象,否则new一个新的对象返回,那究竟这个缓存到底缓存了哪些内容呢?看一下IntegerCache这个类:

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/**
* 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.
*/

private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];

static {
//检查虚拟机里是否有缓存区间配置项,如果有就赋成该值,没有就默认[-128, 127]
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null) {
try {
int i = parseInt(integerCacheHighPropValue);
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE
h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
} catch( NumberFormatException nfe) {
// If the property cannot be parsed into an int, ignore it.
}
}
high = h;

//创建缓存数组,并初始化(缓存值)
cache = new Integer[(high - low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++)
cache[k] = new Integer(j++);

// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}

private IntegerCache() {}
}

这是一个内部静态类,该类只能在Integer这个类的内部访问,这个类在初始化的时候,会去加载JVM的配置,如果有值,就用配置的值初始化缓存数组,否则就缓存-128127之间的值。
再来看看我们之前的代码:

看完这个,是不是明白了呢

参考资料

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