今天看到一道'经典'面试题:
Integer a = 100;
Integer b = 100;
System.out.println(a==b);
Integer a2 = 200;
Integer b2 = 200;
System.out.println(a2==b2);
答案运行一下就能很容易的得出:true,false.
这个题很'经典',之前也看过讲解,大体上说由于jdk的优化,存在一个缓存机制导致,但是一直没有自己去看过源码,今天正好又遇到了,就仔细的看了一下源码.
以Integer为例,JDK中的关键代码:
/**
* 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
* sun.misc.VM class.
*/ 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() {}
}
从方法上的注释可以知道,JLS(Java语言规范)中定义了当处于-127-127之间的值时需要采用缓存机制,Oracle官网的原文如下:
If the value p being boxed is true, false, a byte, or a char in the range \u0000 to \u007f, or an int or short number between -128 and 127 (inclusive),
then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.
原文链接: https://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.7
当第一次调用这个缓存类的时候会初始化这个缓存, 从代码中很清楚的看到其实它就是直接初始化了一个数组,然后一口气将上下限之间对应的包装类都创建出来.且这个上限值
还可通过JVM参数XX:AutoBoxCacheMax=<size>来设置.可以想象出有了这个缓存类后,每次调用的时候只要在这个区间内的都可以不用再创建包装类,直接从缓存中取出来即可,
这个用法有点像String的常量池.装箱的源码如下:
/**
* 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)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
再回过头来看一下那道面试题也就很清楚原因了,一个是使用到了这个缓存,另一个由于超出默认范围,所以直接调用的构造方法创建的新对象.
明白了这个缓存机制后,其实很多的时候我们担心的性能问题就根本不存在了,因为大多数情况下这个装箱都是走的缓存.但同时也要注意,当我们错误的使用==来判断2个包装类的
值是否相等时,由于这个缓存机制的存在,可能导致错误极难被发现,或是随机产生错误判断.