Java Integer的缓存机制与策略

[alshat]

先让我们看一段伪代码:

Integer i1 = 100;
Integer i2 = 100;
System.out.println("i1 == i2: " + (i1 == i2));
Integer i3 = 1000;
Integer i4 = 1000;
System.out.println("i3 == i4: " + (i3 == i4));

执行一下, 结果如下

i1 == i2: true
i3 == i4: false

和大多数人心里的结果是不一样的吧

我们先来反编译一下这段伪代码, 反编译结果如下:

Integer i1 = Integer.valueOf(100);
Integer i2 = Integer.valueOf(100);
System.out.println("i1 == i2: " + (i1 == i2));
Integer i3 = Integer.valueOf(1000);
Integer i4 = Integer.valueOf(1000);
System.out.println("i3 == i4: " + (i3 == i4));

很奇怪吧, 我们的代码 Integer i1 = 100; 被编译为了 Integer i1 = Integer.valueOf(100);

其实这就是Java的自动装箱(autoboxing), 有关于自动拆装箱请移步

我们来看下Integer的valueOf方法的源码:

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

我们看到, 当传入值在IntegerCache.low 和 IntegerCache.high 之间时, valueOf方法会直接返回IntegerCache.cache这个数组中的某一特定下标的值

那么我们就再找到IntegerCache.cache所在的这部分源码:

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() {}
    }

未完待续