代码一

public class Test {

    public static void main(String[] args) {
        Integer i1 = 127;
        Integer i2 = 127;
        System.out.println(i1 == i2); // true
    }
}

代码二

public class Test {

    public static void main(String[] args) {
        Integer i1 = 128;
        Integer i2 = 128;
        System.out.println(i1 == i2); // false
    }
}

原因
Integer i1声明变量时，调用了下面的方法

public static Integer valueOf(int i) {
    assert IntegerCache.high >= 127;
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

而cache由一个静态内部类初始化，取值范围[-128, 127]代码如下

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) {
            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);
        }
        high = h;

        cache = new Integer[(high - low) + 1];
        int j = low;
        for(int k = 0; k < cache.length; k++)
            cache[k] = new Integer(j++);
    }

    private IntegerCache() {}
}