由自动装箱和拆箱引发我看Integer源码

package com.sitech.test;
/**
 * 自动装箱和拆箱 jdk1.6
 * @author liaowp
 *
 */
public class TestInteger {

    public static void main(String[] args) {
        Integer i1 = 80, i2 = 80, i3 = 999, i4 = 999;
        System.out.println(i1 == i2);//true
        System.out.println(i3 == i4);//false
    }
}

    /**
     * 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) {//是一个静态方法 IntegerCache是一个内部类
        assert IntegerCache.high >= 127;//断言  参考http://lavasoft.blog.51cto.com/62575/43735/
        if (i >= IntegerCache.low && i <= IntegerCache.high)//如果i大于对于IntegerCache.low()且i小于等于IntegerCache.high
            return IntegerCache.cache[i + (-IntegerCache.low)];//直接从缓存取出来
        return new Integer(i);//新创建一个Integer对象
    }

    /**
     * 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 -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 {//静态类哦
        static final int low = -128;//最小值是-128
        static final int high;//最高
        static final Integer cache[];//缓存数组  这三个都final，不可修改的

        static {//静态代码块   静态代码块会比改造方法先执行
            // high value may be configured by property
            int h = 127;//默认的
            String integerCacheHighPropValue =//定义一个String 
                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");//取得设置的值
            if (integerCacheHighPropValue != null) {//如果设置了就用设置的值
                int i = parseInt(integerCacheHighPropValue);//把String转换为int
                i = Math.max(i, 127);//获得i和127的更大的一个，其实是不能小与默认的
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - (-low));//如果取的小的那个，不能超过Integer的最大值+low
            }
            high = h;//最大值为127

            cache = new Integer[(high - low) + 1];//创建缓存数组大小
            int j = low;//最小值
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);//缓存初始化
        }

        private IntegerCache() {}//私有构造
    }