学步园

其实这些东西比较简单，只是需要讲一下小整数的缓冲池（python的底层也是有的，这是相互抄袭的结果？？），先看看代码

private static class IntegerCache {
        static final int high;
        static final Integer cache[];
        //静态块中完成整数缓冲池的初始化
        //为什么需要使用静态块？不使用静态块的话，high必须直接赋值，否则无法赋值
        //使用了静态块，high可以动态的赋值，甚至根据JVM的启动参数设置high的值
        static {
            final int low = -128;

            // high value may be configured by property
            int h = 127;
            //获得high的值
            if (integerCacheHighPropValue != null) {
                // Use Long.decode here to avoid invoking methods that
                // require Integer's autoboxing cache to be initialized
                int i = Long.decode(integerCacheHighPropValue).intValue();
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - -low);
            }
            high = h;
            //初始化整数缓冲池
            cache = new Integer[(high - low) + 1];
            int j = low;
            for(int k = 0; k < cache.length; k++)
                cache[k] = new Integer(j++);
        }
        //禁止构造
        private IntegerCache() {}
    }

其中的integerCacheHighPropValue由JVM的启动参数决定。-XX:AutoBoxCacheMax=<size>

/**
     * 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. During VM initialization the
     * getAndRemoveCacheProperties method may be used to get and remove any system
     * properites that configure the cache size. At this time, the size of the
     * cache may be controlled by the vm option -XX:AutoBoxCacheMax=<size>.
     */

    // value of java.lang.Integer.IntegerCache.high property (obtained during VM init)
    private static String integerCacheHighPropValue;
    //从虚拟机的启动参数中获得integerCacheHighPropValue
    static void getAndRemoveCacheProperties() {
        if (!sun.misc.VM.isBooted()) {
            Properties props = System.getProperties();
            integerCacheHighPropValue =
                (String)props.remove("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null)
                System.setProperties(props);  // remove from system props
        }
    }