-128~127的数字 会缓存起来

该范围内的自动装箱 调用 		Integer.valueOf(i)

Integer int1 = 3; 相当于 
Integer int1 = Integer.valueOf(3);

 public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }
    
 
// -XX:AutoBoxCacheMax=256
System.out.println(sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high"));

high 可通过参数配置 


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() {}
    }

Long,Short,Byte,Character 都有类似缓存 ， 只有 IntegerCache 可以配置 high 值


测试代码 IntegerCacheTest.java

public class IntegerCacheTest {
	
	public static void main(String[] args) throws Exception {
		
		// -XX:AutoBoxCacheMax=256
		System.out.println(sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high"));
		
		// 自动装箱 
		Integer int0 = Integer.valueOf(3);
		Integer int1 = 3;
		Integer int2 = 3;
		Integer int3 = new Integer(3);
		System.out.println(int1==int2);
		System.out.println(int1==int3);
		System.out.println(int1.equals(int3));
		
		Integer int4 = 666;
		Integer int5 = 6666;
		Integer int6 = new Integer(666);


		System.out.println(int4==int5);
		System.out.println(int4==int6);
		System.out.println(int4.equals(int6));

	}

}