说到Java中的Integer包装类，大家或多或少的都听说过Integer缓存，下面我们先从Integer缓存相关的一道笔试题讲起。

先来看一段代码：

Integer a1 = Integer.valueOf(13);
Integer a2 = Integer.valueOf(13);
Integer a3 = Integer.valueOf(133);
Integer a4 = Integer.valueOf(133);
System.out.println(a1 == a2);   // 输出 true
System.out.println(a3 == a4);   // 输出 false

要想深入了解其中的道理，必须学会分析JDK里面的Integer类源码。

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

// 静态内部类实现[-128, 127]的缓存
private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];
    static {
        // high 值通过JVM进行设置，默认为127
       //  可以通过 vm 参数指定：-XX:AutoBoxCacheMax。
        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);
                // 最大缓存上限 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() {}
    }

stringSize() 函数判断int长度

    final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,
                                      99999999, 999999999, Integer.MAX_VALUE };
    // Requires positive x 参数必须为正数
    static int stringSize(int x) {
        for (int i=0; ; i++)
            if (x <= sizeTable[i])
                return i+1;
    }

parseInt 函数

public static int parseInt(String s, int radix)
                throws NumberFormatException
    {
        if (s == null) {
            throw new NumberFormatException("null");
        }
        if (radix < Character.MIN_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " less than Character.MIN_RADIX");
        }
        if (radix > Character.MAX_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " greater than Character.MAX_RADIX");
        }
        int result = 0;
        boolean negative = false;
        int i = 0, len = s.length();
        int limit = -Integer.MAX_VALUE;
        int multmin;
        int digit;
        if (len > 0) {
            char firstChar = s.charAt(0);
            if (firstChar < '0') { // Possible leading "+" or "-"
                if (firstChar == '-') {
                    negative = true;
                    limit = Integer.MIN_VALUE;
                } else if (firstChar != '+')
                    throw NumberFormatException.forInputString(s);
                if (len == 1) // Cannot have lone "+" or "-"
                    throw NumberFormatException.forInputString(s);
                i++;
            }
            multmin = limit / radix;
            while (i < len) {
                digit = Character.digit(s.charAt(i++),radix);
                if (digit < 0) {
                    throw NumberFormatException.forInputString(s);
                }
                if (result < multmin) {
                    throw NumberFormatException.forInputString(s);
                }
                result *= radix;
                if (result < limit + digit) {
                    throw NumberFormatException.forInputString(s);
                }
                result -= digit;
            }
        } else {
            throw NumberFormatException.forInputString(s);
        }
        return negative ? result : -result;
}

参考
【JDK】：java.lang.Integer源码解析