valueOf()方法

1. 判断传入的i是否在cache中的low和high之间，如果是的话，则直接从cache中获取并赋值
2. 否则在堆中创建新的Integer对象并返回

public static Integer valueOf(int i) {
    if (i >= IntegerCache.low && i <= IntegerCache.high) {
        return IntegerCache.cache[i + (-IntegerCache.low)];
    }
    return new Integer(i);
}

IntegerCache类及静态代码块

加载类并做JVM初始化的时候执行创建缓存的操作，步骤如下：

1. low值固定为-128；
2. 从JVM参数中取：String integerCacheHighPropValue = sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high")
3. 如果获取到的integerCacheHighPropValue值不为空，
   • 则取该值和127中的最大值为i
   • 取i和Integer.MAX_VALUE - (-low) -1中的最小值，作为high
4. 创建缓存数组，将new 出来的对象放进数组

/**
 * 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() {}
}

VM参数，可以在命令启动时传入，或在IDEA中设置，如下图：

-XX:AutoBoxCacheMax=200

参考文章: 聊聊Java Integer缓存池IntegerCache