Integer中有个静态内部类IntegerCache，里面有个cache[],也就是Integer常量池，常量池的大小为一个字节（-128~127）。
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private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[];
static {
// 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;
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() {}
}
例如：Integer a = 10; 调用的是Integer.valueOf()方法，其实现如下：
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public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
完成了自动装箱操作。
本地测试代码如下：
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public class IntegerCacheTest {
public static void main(String[] args) {
//从缓存池中读取
Integer a1 = 1;
Integer b1 = 1;
System.out.println(a1 == b1);
//超过缓存区间
Integer a2 = 155;
Integer b2 = 155;
System.out.println(a2 == b2);
//没有走缓存
Integer a3 = new Integer(-1);
Integer b3 = new Integer(-1);
System.out.println(a3 == b3);
//自动拆箱
System.out.println("a3 == -1:" + (a3 == -1));
}
}
