首页 > 代码库 > IntegerCache类

IntegerCache类

先看代码实例现象:

技术分享

问题:为什么都是比较数值,第一个为true,第二个确为false呢?

查找源码(java.lang.Integer),看到如下代码:

/**
 * 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;
    }

    private IntegerCache() {}
}

原来是因为Integer类型使用了缓存机制,即默认在JVM启动的时候设定了[-127~128]范围内的int包装类,这样在实际使用并在范围内的时候,直接从缓存中取实例,不用再new了。

这样做的好处是:提升JVM的性能。

坏处是:用==来比较Integer类型的时候,可能出现问题。

 

解决方案:

  • 设定JVM启动参数-XX:AutoBoxCacheMax=<size>,改编默认的缓存最大(不推荐)
  • 不要用==比较大小,用equals比较(推荐)

同样使用缓存的还有ShortCache, LongCache

IntegerCache类