Integer缓冲池[-128,127]

Integer 是 int的封装类。有时候不得不用引用类型。比如集合里面的泛型。
如果在项目中你使用了Integer那么你可能就会陷入一个巨大的坑中。

        Integer i=1290;
        Integer ii=1290;
        System.out.println(i==ii);//1  false
        System.out.println(i.equals(ii));//2 true
        Integer a=127;
        Integer b=127;
        System.out.println(a==b);//3 true

1为什么是fasle呢？这是因为‘’==‘’用在引用类型中判断的是地址。所以会是 false
3为什么等于true了呢？不理解怎么办？来一起看下源码。

先使用jd-jui看上面代码被虚拟机翻译成了是什么

     Integer localInteger1 = Integer.valueOf(1290);
    Integer localInteger2 = Integer.valueOf(1290);
    System.out.println(localInteger1 == localInteger2);
    System.out.println(localInteger1.equals(localInteger2));
    Integer localInteger3 = Integer.valueOf(127);
    Integer localInteger4 = Integer.valueOf(127);
    System.out.println(localInteger3 == localInteger4);

通过反编译可以看出 Integer i=1290; 这样简写的方式会被翻译成Integer.valueOf(1290)运行；
那就看下Integer.valueOf()这个方法的源码

/**
     * 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);
    }

大概意思就是说，为了提升性能这个方法会缓存-128到127的值。继续深入IntegerCache类


    /**
     * 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;
        }
        …………

这是Integer的内部类，定义了Integer缓冲池最小值和最大值[-128,127]，并且可以看出最大值可以根据本地环境自定义设置，默认127。
通过valueOf(int i) 这个方法我们就找到了答案。如果i>=-128&&i<=127就在缓冲池里取值，否则就是新创建一个对象。

再看一下 2 为什么为true,那我们就看下 Integer 的 equals方法的实现

     public boolean equals(Object obj) {
        if (obj instanceof Integer) {
            return value == ((Integer)obj).intValue();
        }
        return false;
    }

相信大家都能看懂吧哈哈

Integer缓冲池[-128,127]

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