JAVA int自动装箱

int 转 Integer:

Integer int127_1 = 127;
Integer int127_2 = 127;
System.out.println("int127_1 == int127_2 : " + (int127_1 == int127_2));

Integer int128_1 = 128;
Integer int128_2 = 128;
System.out.println("int1000_1 == int1000_2 : " + (int128_1 == int128_2));

运行结果为：

int127_1 == int127_2 : true
int1000_1 == int1000_2 : false

通过反编译的方法，我们发现int在进行自动装箱的时候，调用了Integer.valueOf方法：

Code:
0: bipush 127
2: invokestatic #2 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
5: astore_1
6: bipush 127
8: invokestatic #2 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
11: astore_2
12: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
15: new #4 // class java/lang/StringBuilder
18: dup
19: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
22: ldc #6 // String int127_1 == int127_2 :
24: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/

而我们查看Integer.valueOf()方法：

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

    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() {}
    }

Integer.valueOf()方法，默认当int的值在-128~127之间时，会采用缓存的方式提高性能，返回的是一个数组中的值