[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 : 0;  
        for(i = size - 1; i >= 0; 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"
", key);  
            return -1;  
        }  
        _UpdateXpathNodes(xpathObj->nodesetval, BAD_CAST value);  
        xmlXPathFreeObject(xpathObj);  
        return 0;  
    }  
      
    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"
", path);  
            return(-1);  
        }  
      
        /* 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
");  
            xmlFreeDoc(doc);   
            return(-1);  
        }  
        //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 -1;  
        }  
      
        if(-1 == xmlSaveDoc(saveCtxtPtx,doc))  
        {  
            xmlFreeDoc(doc);  
            return -1;  
        }  
        xmlSaveClose(saveCtxtPtx);  
        //xmlDocDump(stdout, doc);  
        //5.free  
        xmlFreeDoc(doc);   
        return 0;  
    }  
      
    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
");  
        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 0;  
    }  

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>