Integer缓存机制

Intgeter的缓存机制

Integer缓存的源码如下,IntegerCache在类加载的时候，创建了256个缓存Integer对象，范围在-128~127.

private static class IntegerCache {
        static final int low = -128;
        static final int high;
        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
            VM.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() {}
    }

Integer的值-128到127之间时，会引用缓存中创建好的对象，不会创建新的对象所以num1 ==num2输出为true。

超过这个范围就会创建新的对象，num3 == num4 输出为false

public static void main(String argv[]){
        Integer num1 = 1;
        Integer num2 = 1;

        Integer num3 = 129;
        Integer num4 = 129;

        System.out.println(num1 == num2); //true
        System.out.println(num3 == num4); //false

    }