從源碼看java中Integer的緩存問題

在開始詳細的說明問題之前，我們先看一段代碼

public static void compare1(){
        Integer i1 = 127, i2 = 127, i3 = 128, i4 = 128;
        System.out.println(i1  == i2);
        System.out.println(i1.equals(i2));
        System.out.println(i3  == i4);
        System.out.println(i3.equals(i4));
    }

這段代碼輸出的結果是什么呢？

答案是：

是不是感到奇怪呢？為什么127的時候==是true，128的時候就變成了false？其實要回答這個問題不難。

Integer在賦值的時候會發生自動裝箱操作，調用Integer的valueOf方法，那么我們看一下java的源碼（1.8）：

/**
     * Returns an {@code Integer} instance representing the specified
     * {@code int} value.  If a new {@code Integer} instance is not
     * required, this method should generally be used in preference to
     * the constructor {@link #Integer(int)}, as this method is likely
     * to yield significantly better space and time performance by
     * caching frequently requested values.
     *
     * This method will always cache values in the range -128 to 127,
     * inclusive, and may cache other values outside of this range.
     *
     * @param  i an {@code int} value.
     * @return an {@code Integer} instance representing {@code i}.
     * @since  1.5
     */
    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

這里根據源碼可以看出，在傳入的i值在IntegerCache.low和IntegerCache.high之間的時候，會返回IntegerCache.cache數組里面的數，不在這個范圍才會new一個Integer對象。接下來我們看一下IntegerCache數組的初始化情況：

/**
     * 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() {}
    }

我們看到IntegerCache的low定義為-128，high默認定義為127.但是high是可以配置的，如果沒有配置才是127.我們不去看配置的情況，因為java默認是沒有配置的。看一下cache數組，長度為high-low+1，從-128開始到127，存在cache數組內。

從上面的代碼中可以看出，java在申請一個大於-128小於127的數時，其實是從cache中直接取出來用的，如果不在這個范圍則是new了一個Integer對象。

對於==，他比較的是地址。對於int來說比較的是值。

對於equals，比較的是內容（要看equals的具體實現）。看一下Integer里面的實現：

/**
     * Compares this object to the specified object.  The result is
     * {@code true} if and only if the argument is not
     * {@code null} and is an {@code Integer} object that
     * contains the same {@code int} value as this object.
     *
     * @param   obj   the object to compare with.
     * @return  {@code true} if the objects are the same;
     *          {@code false} otherwise.
     */
    public boolean equals(Object obj) {
        if (obj instanceof Integer) {
            return value == ((Integer)obj).intValue();
        }
        return false;
    }

它比較的確實是值的大小。

因此i1==i2和i1.equals（i2）都是true

i3==i4為false

i3.equals（i4）為true。