append()方法的坑

我们经常会用到StringBuilder或StringBuffer类的append()方法，然而这里有个坑。

public static void main(String[] args) {

        String ab = null;
        StringBuffer stringBuffer = new StringBuffer();
        stringBuffer.append("测试");

        if(null== ab) {
            stringBuffer.append(ab);
        }

        System.out.println(ab);
        System.out.println(stringBuffer.toString());

    }
输出：
null
测试null

多了一个null字符串。

下面是撸源码时间←_←

java.lang.StringBuffer#append(java.lang.String)

    public synchronized StringBuffer append(String str) {
        super.append(str); //关键从这里进入
        return this;
    }

java.lang.AbstractStringBuilder#append(java.lang.String)

    /**
     * Appends the specified string to this character sequence.
     * 

     * The characters of the {@code String} argument are appended, in
     * order, increasing the length of this sequence by the length of the
     * argument.   这里： If {@code str} is {@code null}, then the four
     * characters {@code "null"} are appended.
     * 如果入参str是null，那么就会把“null”字符串给append进去。
     * 

     * Let n be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at
     * index k in the new character sequence is equal to the character
     * at index k in the old character sequence, if k is less
     * than n; otherwise, it is equal to the character at index
     * k-n in the argument {@code str}.
     *
     * @param   str   a string.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(String str) {
        if (str == null) str = "null"; //就是这一步
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

上面的源码注释说的明明白白，jdk给多此一举的转换了[捂脸]！！！

所以如果你在组装返回给前端的数据时，使用到了append()方法，那么当你像下面的栗子一样使用append()方法时，请注意判空，负责会发生神马。。。。。。我神马都不知道，我神马都没说[捂脸]！！！：

append(coupon.getName());

改为：
String couponName = coupon.getName();
if(StringUtils.isNotBlank(couponName )){
    append(couponName );
}