Long.ValueOf("str") 和Long.parseLong("str")真的有性能差別嗎？

問題

 用SonarQube做代碼掃描，發現Boxing/Unboxing to  parse a primitive,tag標記得是性能問題。

 驗證

    隨後翻了源代碼
    Long.valueOf(str)源碼

public static Long valueOf(String s) throws NumberFormatException
    {
        return Long.valueOf(parseLong(s, 10));
    }


    paserLong()
    
    
public static long parseLong(String s, int radix)
              throws NumberFormatException
    {
        if (s == null) {
            throw new NumberFormatException("null");
        }
        if (radix < Character.MIN_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " less than Character.MIN_RADIX");
        }
        if (radix > Character.MAX_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " greater than Character.MAX_RADIX");
        }
        long result = 0;
        boolean negative = false;
        int i = 0, len = s.length();
        long limit = -Long.MAX_VALUE;
        long multmin;
        int digit;
        if (len > 0) {
            char firstChar = s.charAt(0);
            if (firstChar < '0') { // Possible leading "+" or "-"
                if (firstChar == '-') {
                    negative = true;
                    limit = Long.MIN_VALUE;
                } else if (firstChar != '+')
                    throw NumberFormatException.forInputString(s);
                if (len == 1) // Cannot have lone "+" or "-"
                    throw NumberFormatException.forInputString(s);
                i++;
            }
            multmin = limit / radix;
            while (i < len) {
                // Accumulating negatively avoids surprises near MAX_VALUE
                digit = Character.digit(s.charAt(i++),radix);
                if (digit < 0) {
                    throw NumberFormatException.forInputString(s);
                }
                if (result < multmin) {
                    throw NumberFormatException.forInputString(s);
                }
                result *= radix;
                if (result < limit + digit) {
                    throw NumberFormatException.forInputString(s);
                }
                result -= digit;
            }
        } else {
            throw NumberFormatException.forInputString(s);
        }
        return negative ? result : -result;
    }

 再看Long.parseLong(str)

  public static long parseLong(String s) throws NumberFormatException {
        return parseLong(s, 10);
    }

 parseLong 和Long.valueOf(str) 中調用的是一致的。

所以差別，就在於Long.valueOf(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);
    }

也就是將long基本類型加了一個封裝類。

做了一個小測試，分別用Long.valueOf和Long.parseLong()來轉換，結果時間都相等

public static void main(String[] args) throws Exception{
        Long  start= System.currentTimeMillis();
        Long t=Long.valueOf("1222222222222222223");
        Long  end= System.currentTimeMillis();
        System.out.println("test1  "+(end-start));
        Long  start1= System.currentTimeMillis();
        long t1=Long.parseLong("1222222222222222223");
        Long  end1= System.currentTimeMillis();
        System.out.println("test2  "+(end1-start1));
    }

 結果
test1  0
test2  0

看樣子，兩個沒有什麼差別

結論
 通過自己的方法驗證，其實兩個字符串轉換，性能沒有差別。
 以上都是個人愚見！