package com.javapapers.java;public class JavaIntegerCache {

    public static void main(String... strings) {

        Integer integer1 = 3; //整数3自动装箱

        Integer integer2 = 3; //整数3自动装箱

        if (integer1 == integer2) //两个自动装箱后对象比较

            System.out.println("integer1 == integer2");

        else

            System.out.println("integer1 != integer2");

        Integer integer3 = 300;//整数300自动装箱

        Integer integer4 = 300;//整数300自动装箱

        if (integer3 == integer4)//两个自动装箱后对象比较

            System.out.println("integer3 == integer4");

        else

            System.out.println("integer3 != integer4");

    }}

integer1 == integer2

integer3 != integer4

/**

     * 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) //先判断整数值为是否处于IntegerCache.low～IntegerCache.high之间的缓存值

            return IntegerCache.cache[i + (-IntegerCache.low)]; //是的话，则从缓存数组中取对应的包装类对象之间返回

        return new Integer(i);//否，则创建一个新的包装类对象返回

    }

/**

     * 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=} 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; //默认可缓存的最小值是 -128

        static final int high; //可缓存的最大值

        static final Integer cache[]; //缓存数组

        static {

            // high value may be configured by property 可缓存最大值可通过jvm参数指定

            int h = 127; //默认可缓存最大值为127

            String integerCacheHighPropValue =

                sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high"); //从JVM参数中获取可缓存最大值配置

            if (integerCacheHighPropValue != null) {

                try {

                    int i = parseInt(integerCacheHighPropValue);

                    i = Math.max(i, 127);//将配置值与127之间取较大值，即：可缓存值最小为127

                    // Maximum array size is Integer.MAX_VALUE

                    h = Math.min(i, Integer.MAX_VALUE - (-low) -1); //可缓存值不能超过整型大最大值

                } catch( NumberFormatException nfe) {

                    // If the property cannot be parsed into an int, ignore it.

                }

            }

            high = h;

            cache = new Integer[(high - low) + 1];//创建缓存数组

            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;//断言：可缓存最大值必须大于等于127

        }

        private IntegerCache() {}

    }

如果一个变量p的值是：

-128至127之间的整数

true 和 false的布尔值

‘\u0000’至 ‘\u007f’之间的字符

之间时，将p自动装箱成a和b两个对象，可以直接使用a==b判断a和b的值是否相等。