JDK源码分析

Integer

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

private static class IntegerCache {
        //最小值
        static final int low = -128;
        //最大值
        static final int high;
        static final Integer cache[];

        static {
            // high value may be configured by property
            //最大值
            int h = 127;
            String integerCacheHighPropValue =
               //可以在jvm中设置最大值，可设置的范围是（127-Integer.MAX_VALUE之间 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.
                }
            }
            //覆盖最大值
            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() {}
    }

• Integer的缓存默认范围是：-128~127
• Integer可设置最大值：127~Integer.MAX_VALUE

Long

public static Long valueOf(long l) {
        final int offset = 128;
        if (l >= -128 && l <= 127) { // will cache
            return LongCache.cache[(int)l + offset];
        }
        return new Long(l);
    }
    
private static class LongCache {
        private LongCache(){}
        //缓存范围是：-128~127；不可在jvm中改变缓存范围的最大值
        static final Long cache[] = new Long[-(-128) + 127 + 1];

        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Long(i - 128);
        }
    }

• Long的缓存范围是：-128~127

Character

public static Character valueOf(char c) {
        if (c <= 127) { // must cache
            return CharacterCache.cache[(int)c];
        }
        return new Character(c);
    }
private static class CharacterCache {
        private CharacterCache(){}
    
        static final Character cache[] = new Character[127 + 1];
        //缓存范围是0~127
        static {
            for (int i = 0; i < cache.length; i++)
                cache[i] = new Character((char)i);
        }
    }

• Character的缓存范围是：0~127

Byte

public static Byte valueOf(byte b) {
        final int offset = 128;
        return ByteCache.cache[(int)b + offset];
    }
private static class ByteCache {
        private ByteCache(){}
        
        static final Byte cache[] = new Byte[-(-128) + 127 + 1];
        //范围是：-128~127
        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Byte((byte)(i - 128));
        }
    }

• Byte的缓存范围是：-128~127

Short

public static Short valueOf(short s) {
        final int offset = 128;
        int sAsInt = s;
        if (sAsInt >= -128 && sAsInt <= 127) { // must cache
            return ShortCache.cache[sAsInt + offset];
        }
        return new Short(s);
    }
private static class ShortCache {
        private ShortCache(){}

        static final Short cache[] = new Short[-(-128) + 127 + 1];
        //-128~127
        static {
            for(int i = 0; i < cache.length; i++)
                cache[i] = new Short((short)(i - 128));
        }
    }

• Short的缓存范围是：-128~127

总结

• 整形才有缓存范围：比如Byte,Short,Character,Integer,Long。
• 浮点型没缓存范围：比如Float,Double