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。
