【问题排查】fastjson线上排坑记

前言

版本上线时发现fastjson的toString方法的返回的字符串与与之前版本的toString方法返回的字符串不相同，这导致依赖toString进行md5计算所得到的结果不相同，更进一步导致其他依赖该md5值的插件发现和之前的md5值不相等而重启，导致数据存在丢失情况。

源码

从项目中抽取出该模块代码，并进行了适当修改，但未改变整个处理逻辑，源码如下。

package main;


import com.alibaba.fastjson.JSONObject;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class Main {
    public static void main(String[] args) {
        JSONObject obj = new JSONObject();

        obj.put("the_plugin_id", "the_plugin_id");
        obj.put("the_plugin_name", "the_plugin_name");
        obj.put("the_plugin_version", "the_plugin_version");
        obj.put("the_plugin_md5", "the_plugin_md5");
        obj.put("the_extend_info1", "the_extend_info1");
        obj.put("the_extend_info2", "the_extend_info2");
        obj.put("the_extend_info3", "the_extend_info3");
        obj.put("the_extend_info4", "the_extend_info4");

        System.out.println(obj.toString());
        System.out.println("md5 ==> " + getMD5String(obj.toString()));
    }

    private static final char hexDigits[] = {'0', '1', '2', '3', '4', '5',
            '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
            'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
            'w', 'x', 'y', 'z'};

    static public String getMD5String(String source) {

        String retString = null;

        if (source == null) {
            return retString;
        }

        try {
            StringBuffer sb = new StringBuffer();
            MessageDigest md = MessageDigest.getInstance("MD5");
            md.update(source.getBytes(), 0, source.length());
            byte[] retBytes = md.digest();
            for (byte b : retBytes) {
                sb.append(hexDigits[(b >> 4) & 0x0f]);
                sb.append(hexDigits[b & 0x0f]);
            }

            retString = sb.toString();
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }

        return retString;
    }
}

原因猜想

• 首先怀疑是由于fastjson版本不一致的问题导致toString方法返回的字符串不相同，待比对jar后发现均依赖fastjson1.2.3版本，排除由于fastjson版本问题导致。
• 再者怀疑是由于上线时将JDK从1.7替换到1.8导致，即是由于JDK升级引起该问题，下面是验证过程。

分析验证

为验证是否是由于JDK升级导致该问题，分别使用不同JDK运行上述程序，得到结果如下。

• JDK1.7运行结果

{"the_extend_info1":"the_extend_info1","the_plugin_version":"the_plugin_version","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4","the_plugin_name":"the_plugin_name","the_plugin_id":"the_plugin_id","the_plugin_md5":"the_plugin_md5"}
md5 ==> 87d74d87982fe1063a325c5aa97a9ef5

格式化JSON字符串如下

{"the_extend_info1":"the_extend_info1","the_plugin_version":"the_plugin_version","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4","the_plugin_name":"the_plugin_name","the_plugin_id":"the_plugin_id","the_plugin_md5":"the_plugin_md5"}

• JDK1.8运行结果

{"the_plugin_md5":"the_plugin_md5","the_plugin_id":"the_plugin_id","the_plugin_name":"the_plugin_name","the_extend_info1":"the_extend_info1","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4","the_plugin_version":"the_plugin_version"}
md5 ==> fc8f7f526f5f37141f2fea3a03950f52

格式化JSON字符串如下

{"the_plugin_md5":"the_plugin_md5","the_plugin_id":"the_plugin_id","the_plugin_name":"the_plugin_name","the_extend_info1":"the_extend_info1","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4","the_plugin_version":"the_plugin_version"}

对比JDK1.7和JDK1.8下运行结果可知toString方法返回的结果并不相同，这也就导致md5计算的不相同，进一步导致其他依赖性的问题。

更进一步

当使用JSONObject obj = new JSONObject();创建JSONObject时，跟踪源码可以看到其会调用JSONObject(int, boolean)型构造函数，并且会使用HashMap维护插入的键值对，这是关键所在。

HashMap在JDK1.7和JDK1.8中底层有不同的逻辑，JDK1.8的桶中会维护链表 + 红黑树结构，该结果是对JDK1.7的优化，JDK1.7中维护链表结构，在桶中元素较多而未达到再哈希的条件时查找效率会比较低下，而JDK1.8当桶中元素个数达到一定数量时会将链表转化为红黑树，这样便能提高查询效率，有兴趣的读者可查阅JDK1.7和JDK1.8的源码，JDK1.8源码分析传送门。

解决方案

由前面分析可知，直接使用JSONObject obj = new JSONObject()的方法生成JSONObject对象时，其底层会使用HashMap维护键值对，而HashMap是和JDK版本相关的，所以最好的解决方案应该是能和JDK版本解耦的，而在JSONObject的构造函数中，可以自定义传入Map，这样就由指定Map维护插入的键值对。可使用LinkedHashMap来维护插入键值对，并且还会维护插入的顺序。这样便能保证在不同JDK版本下使用toString方法得到的字符串均相同。

方案验证

使用JSONObject obj = new JSONObject(new LinkedHashMap<String, Object>());代替之前的JSONObject obj = new JSONObject();即可。

• JDK1.7运行结果

{"the_plugin_id":"the_plugin_id","the_plugin_name":"the_plugin_name","the_plugin_version":"the_plugin_version","the_plugin_md5":"the_plugin_md5","the_extend_info1":"the_extend_info1","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4"}
md5 ==> 5c7725cd161d53f1e25a6a5c55b62c1f

格式化JSON字符串如下

{"the_plugin_id":"the_plugin_id","the_plugin_name":"the_plugin_name","the_plugin_version":"the_plugin_version","the_plugin_md5":"the_plugin_md5","the_extend_info1":"the_extend_info1","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4"} 

• JDK1.8运行结果

{"the_plugin_id":"the_plugin_id","the_plugin_name":"the_plugin_name","the_plugin_version":"the_plugin_version","the_plugin_md5":"the_plugin_md5","the_extend_info1":"the_extend_info1","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4"}
md5 ==> 5c7725cd161d53f1e25a6a5c55b62c1f

格式化JSON字符串如下

{"the_plugin_id":"the_plugin_id","the_plugin_name":"the_plugin_name","the_plugin_version":"the_plugin_version","the_plugin_md5":"the_plugin_md5","the_extend_info1":"the_extend_info1","the_extend_info2":"the_extend_info2","the_extend_info3":"the_extend_info3","the_extend_info4":"the_extend_info4"} 

对比在不同JDK下运行的结果，可以发现toString方法获得的字符串是完全相同的，md5值也是完全相同的，即验证了方案的正确性。

总结

在遇到问题时，特别是现网问题时，需要冷静分析，大胆猜想，小心求证，一点点找到突破口，这次的排坑过程大致如上所记录。