Java整数常量池

首先来看一段示例代码，我们会发现这样的现象 127 = 127,128 != 128。

public class Test {
    public static void main(String[] args) {
        Integer a = 127;
        Integer b = 127;
        Integer c = 128;
        Integer d = 128;
        System.out.println(a == b);//true
        System.out.print(c == d);//false

        Integer e = 1;
        int f = 1;
        System.out.println(e == f); //Integer和int进行比较时，Integer会自动拆箱为int，然后比较两个值是否相等。
    }
}

为什么会出现这样的现象，我们来看以下Integer类的源码：

/**
 * Cache to support the object identity semantics of autoboxing for values between
 * -128 and 127 (inclusive) as required by JLS(Java Language Specification).
 *
 * 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() {}
}

以上这段代码是Integer类的一个内部类IntegerCache，它缓存了-128 ~ 127之间的整数值，缓存的最小值为low表示的值，缓存的最大值为high表示的值。
在创建这个范围内的整数对象时，可以在常量池中直接引用，而无需重新创建，因此 a == b 返回true。超出此范围的值，会通过new新的对象，因此 c == d返回false。