[libxml2]_[XML处理]_[使用libxml2的xpath特性修改xml文件内容]

场景：

1.在软件需要保存一些配置项时,使用数据库的话比较复杂,查看内容也不容易.纯文本文件对utf8字符支持也不好.

2.这时候使用xml是最佳选择,使用跨平台库libxml2。

3.基于xpath的保存方式对保存局部内容非常方便。

4.参考例子xpath2.c

     #include <stdio.h>
     #include <assert.h>
     #include <string>
     #include <iostream>
     #include <map>  

     #include "libxml/tree.h"
     #include "libxml/parser.h"
     #include "libxml/xpath.h"
     #include "libxml/xpathInternals.h"
     #include "libxml/xmlsave.h"  

     using namespace std;  

     static void _UpdateXpathNodes(xmlNodeSetPtr nodes, const xmlChar* value)
     {
         int size;
         int i;  

         assert(value);
         size = (nodes) ? nodes->nodeNr : ;
         for(i = size - ; i >= ; i--)
         {
             assert(nodes->nodeTab[i]);
             xmlNodeSetContent(nodes->nodeTab[i], value);
             if (nodes->nodeTab[i]->type != XML_NAMESPACE_DECL)
             {
                 nodes->nodeTab[i] = NULL;
             }
        }
     }  

     static int _UpdateWithXpath(xmlXPathContextPtr xpathCtx,const char* key,const char* value)
     {
         xmlXPathObjectPtr xpathObj;  

         xpathObj = xmlXPathEvalExpression(BAD_CAST key, xpathCtx);
         if(!xpathObj)
         {
             fprintf(stderr,"Error: unable to evaluate xpath expression \"%s\"\n", key);
             return -;
         }
         _UpdateXpathNodes(xpathObj->nodesetval, BAD_CAST value);
         xmlXPathFreeObject(xpathObj);
         return ;
     }  

     static int _UpdateXml(const char* path,map<string,string>& keyValue)
     {
         xmlDocPtr doc;
         xmlXPathContextPtr xpathCtx;
         doc = xmlParseFile(path);
         if (!doc)
         {
             fprintf(stderr, "Error: unable to parse file \"%s\"\n", path);
             return(-);
         }  

         /* Create xpath evaluation context */
         xpathCtx = xmlXPathNewContext(doc);
         //1.注意,这里根Node有声明xmlns,那么必须加下边这句，相应的xpath要加前缀 /c:container/c:rootfiles
         //xmlXPathRegisterNs(xpathCtx,BAD_CAST"c",BAD_CAST"urn:oasis:names:tc:opendocument:xmlns:container");
         if(!xpathCtx)
         {
             fprintf(stderr,"Error: unable to create new XPath context\n");
             xmlFreeDoc(doc);
             return(-);
         }
         //3.update
         map<string,string>::iterator iter;
         map<string,string>::iterator end = keyValue.end();
         for(iter = keyValue.begin();iter!= end;iter++)
         {
             cout << "word: " << iter->first << ", count: " << iter->second << endl;
             _UpdateWithXpath(xpathCtx,iter->first.c_str(),iter->second.c_str());
         }  

         xmlXPathFreeContext(xpathCtx);
         //4.save
         xmlSaveCtxtPtr saveCtxtPtx = xmlSaveToFilename(path,"UTF-8",XML_SAVE_FORMAT);
         if(!saveCtxtPtx)
         {
             xmlFreeDoc(doc);
             return -;
         }  

         if(- == xmlSaveDoc(saveCtxtPtx,doc))
         {
             xmlFreeDoc(doc);
             return -;
         }
         xmlSaveClose(saveCtxtPtx);
         //xmlDocDump(stdout, doc);
         //5.free
         xmlFreeDoc(doc);
         return ;
     }  

     int UpdateXml(const char* path,map<string,string>& keyValue)
     {
         /* Init libxml */
         xmlInitParser();
         int res  = _UpdateXml(path,keyValue);
         xmlCleanupParser();
         return res;
     }  

     int main(int argc, char *argv[])
     {
         printf("Hello, world\n");
         map<string,string> m;
         m["/doc/parent/discarded/@info"] = string("info attri");
         m["/doc/parent/discarded[2]"] = string("change second discarded text 中文");
         int ret = UpdateXml("xpath2.res",m);
         assert(!ret);
         ret = UpdateXml("xpath2.res",m);
         assert(!ret);
         ret = UpdateXml("xpath2.res",m);
         assert(!ret);
         return ;
     }  

xpath2.res

    <?xml version="1.0" encoding="UTF-8"?>
    <doc>
      <parent>
        <discarded info="test">discarded</discarded>
        <preserved/>
        This text node must be discarded
        <discarded>test</discarded>
        <preserved>
          content1
          <child1/>
          <child2>content2</child2>
          <preserved>too</preserved>
          <child2>content3</child2>
          <preserved/>
          <child2>content4</child2>
          <preserved/>
          <child2>content5</child2>
          content6
        </preserved>
      </parent>
    </doc>  

xpath2.res

    <?xml version="1.0" encoding="UTF-8"?>
    <doc>
      <parent>
        <discarded info="info attri">discarded</discarded>
        <preserved/>
        This text node must be discarded
        <discarded>change second discarded text 中文</discarded>
        <preserved>
          content1
          <child1/>
          <child2>content2</child2>
          <preserved>too</preserved>
          <child2>content3</child2>
          <preserved/>
          <child2>content4</child2>
          <preserved/>
          <child2>content5</child2>
          content6
        </preserved>
      </parent>
    </doc>