Java 基元的装箱

引文

运行如下代码：

public class Main {
    public static void main(String[] args) {
        Integer a = 100;
        Integer b = 100;
        System.out.println(a == b);

        Integer c = 200;
        Integer d = 200;
        System.out.println(c == d);
    }
}

运行结果：

true
false

为什么第二个比较的结果为 false ？

装箱过程

Integer c = 200 会调用 Integer c = Integer.valueOf(200) （自动装箱），而从 Integer.valueOf() 的源码可以看到，其并非简单的 new Integer(200) ：

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

可见 Integer 维护了一个 static IntegerCache 缓存池，当初始化的值位于缓存池范围内时，就直接返回缓存池内的对象；否则执行 new Integer() 。所以对于同一个缓存池范围内的值，返回的对象总是同一个；而范围外的值则不同。

    /**
     * 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
     * jdk.internal.misc.VM class.
     *
     * WARNING: The cache is archived with CDS and reloaded from the shared
     * archive at runtime. The archived cache (Integer[]) and Integer objects
     * reside in the closed archive heap regions. Care should be taken when
     * changing the implementation and the cache array should not be assigned
     * with new Integer object(s) after initialization.
     */    
    private static final class IntegerCache {
        static final int low = -128;
        static final int high;

        @Stable
        static final Integer[] cache;
        static Integer[] archivedCache;

        static {
            // high value may be configured by property
            int h = 127;
            String integerCacheHighPropValue =
                VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
            if (integerCacheHighPropValue != null) {
                try {
                    h = Math.max(parseInt(integerCacheHighPropValue), 127);
                    // Maximum array size is Integer.MAX_VALUE
                    h = Math.min(h, Integer.MAX_VALUE - (-low) -1);
                } catch( NumberFormatException nfe) {
                    // If the property cannot be parsed into an int, ignore it.
                }
            }
            high = h;

            // Load IntegerCache.archivedCache from archive, if possible
            CDS.initializeFromArchive(IntegerCache.class);
            int size = (high - low) + 1;

            // Use the archived cache if it exists and is large enough
            if (archivedCache == null || size > archivedCache.length) {
                Integer[] c = new Integer[size];
                int j = low;
                for(int i = 0; i < c.length; i++) {
                    c[i] = new Integer(j++);
                }
                archivedCache = c;
            }
            cache = archivedCache;
            // range [-128, 127] must be interned (JLS7 5.1.7)
            assert IntegerCache.high >= 127;
        }

        private IntegerCache() {}
    }

可见通常情况下，缓存池的范围为 [-128,127]，其中下限固定，上限可由JVM参数 java.lang.Integer.IntegerCache.high 指定，但不能低于 127。

所以，当初始化 Integer 为 200 时，返回的并不是同一个对象，比较的结果为 false 。