Java自动装箱中的缓存原理

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.