浅聊Integer的缓存

有一道常见的面试题，如下

public static void main(String[] args) {
    Integer a = 128;
    Integer b = 128;
    Integer c = 100;
    Integer d = 100;
    System.out.println(a == b); //false
    System.out.println(c == d); //true
}

为什么会导致这样的结果呢？那就是Integer缓存在作怪！通过反编译.class文件发现编译后代码变成了这样

public class IntegerTest {
  public static void main(String[] args) {
    Integer a = Integer.valueOf(128);
    Integer b = Integer.valueOf(128);
    Integer c = Integer.valueOf(100);
    Integer d = Integer.valueOf(100);
    System.out.println((a == b));
    System.out.println((c == d));
  }
}

综上可知，对于Integer类型的变量赋值字面量，其实是调用了Integer.valueOf()方法，因此秘密就在这个方法中

/**
     * 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);
    }

可以看到，如果赋予的值在IntegerCache.low和IntegerCache.high闭区间之内，直接从缓存中取出一个Integer缓存对象返回出去，否则的话就直接new一个新的对象。那么这个闭区间范围是多大呢？

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() {}
    }

通过IntegerCache声明类可以看出，下限IntegerCache.low是固定值-128，上限默认+127，通过设置java.lang.Integer.IntegerCache.high这个虚拟机参数可以调整这个值，而且这个值不能比+127小，例如，通过该虚拟机参数设置IntegerCache上限为200，那么示例代码均返回true

其实不光Integer，其他一些包装类比如Long、Byte等，都有类似的缓存机制存在。
以上，完毕！

标签：缓存,浅聊,int,high,static,IntegerCache,128,Integer
来源： https://blog.csdn.net/carbuser_xl/article/details/117306896