Apache的Commons-configuration自动加载特性

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　　在一些项目可能配置文件经常变化，配置文件的类型可能为xml，或者properties文件等，我们系统会自动检测到配置文件的变化，自动更新到内存等中，可以采用Apache 的commons-confiugrations实现相关的功能。如在Tomcat中web.xml配置文件的变化，tomcat会自动加载。
　　在commons-configuration中自动保存代码如下：

PropertiesConfiguration config = new PropertiesConfiguration("usergui.properties");
config.setAutoSave(true);
config.setProperty("colors.background", "#000000); // the configuration is saved after this call

　　自动加载代码如下：

PropertiesConfiguration config = new PropertiesConfiguration("usergui.properties");
config.setReloadingStrategy(new FileChangedReloadingStrategy());

　　FileChangedReloadingStrategy中重点代码如下：

public boolean reloadingRequired() {
if (!reloading) {
long now = System.currentTimeMillis();
if (now > lastChecked + refreshDelay) {
lastChecked = now;
if (hasChanged())
reloading = true;
}
}
return reloading;
}
　　在web项目写一个servlet类测试属性文件变化，并自动加载。

package com.easyway.app.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.configuration.ConfigurationException;
import org.apache.commons.configuration.PropertiesConfiguration;
import org.apache.commons.configuration.reloading.FileChangedReloadingStrategy;
public class ConfigurationServlet extends HttpServlet {
/**
* Constructor of the object.
*/
public ConfigurationServlet() {
super();
}
/**
* Destruction of the servlet. <br>
*/
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
}
/**
* The doGet method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to get.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
//从上下文中获取
PropertiesConfiguration propconfig=(PropertiesConfiguration)this.getServletContext().getAttribute("propconfig");
//打印信息到页面
String username=propconfig.getString("username");
out.println("用户的名称为：username ="+username);
out.flush();
out.close();
}
/**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
PropertiesConfiguration propconfig=(PropertiesConfiguration)this.getServletContext().getAttribute("propconfig");
String username=propconfig.getString("username");
out.println("username ="+username);
out.flush();
out.close();
}
/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occure
*/
public void init() throws ServletException {
//读取配置的文件的名称
ServletContext servletContext = this.getServletContext();
String filename=servletContext.getInitParameter("appconfig");
System.out.println("filename"+filename);
//创建自动加载的机制
PropertiesConfiguration propconfig=null;
try {
propconfig = new PropertiesConfiguration(filename);
//设置编码
propconfig.setEncoding("UTF-8");
//设置自动冲加载机制
propconfig.setReloadingStrategy(new FileChangedReloadingStrategy());
//设置到ServletContext便于使用时候获取
servletContext.setAttribute("propconfig", propconfig);
} catch (ConfigurationException e) {
e.printStackTrace();
}
}
}

　　web.xml配置如下：

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<context-param>
<param-name>appconfig</param-name>
<param-value>jdbc.properties</param-value>
</context-param>
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>configurationServlet</servlet-name>
<servlet-class>com.easyway.app.servlet.ConfigurationServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>configurationServlet</servlet-name>
<url-pattern>/configurationServlet</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>


<?xml version="1.0" encoding="UTF-8"?>
<origin dataSource="ds">
<columns>
<column code="ID" name="" type="java.lang.Integer" length="" />
<column code="FIRSTNAME" name="" type="java.lang.String" length="" />
<column code="LASTNAME" name="" type="java.lang.String" length="" />
<column code="SEX" name="" type="java.lang.String" length="" />
</columns>
</origin>

XMLConfiguration config = new XMLConfiguration(String); //关于XML的读取在“commons-configuration的使用”中讲过
String id = config.getString("columns.column.[@code]");   //拿到第一个Column的code
String type = config.getString("columns.column.[@type]"); //拿到第一个Column的type
Object value = ConvertUtils.convert(id,Class.forName(type)); //转换了
　　背景：开发一个使用xml配置文件来处理跨数据库的数据操作。例如，我本地有一个mysql数据库叫A，里面有一个表叫test,test有两个字段，fistName和lastName；另一个服务器上有一个Oracle数据库叫B，里面有两个表，test1,test2,他两的结构是一样的，都有，Name这个字段；现在就是要把A中的表test里的fistName和lastName做字符串连接，然后存到B中的表test1,test2的Name字段。
  

commons-configuration的使用
　　得到XMLConfiguration 的几种方式

XMLConfiguration config  = new XMLConfiguration(String);
XMLConfiguration config = new XMLConfiguration();
config.load(String);
　　
我自己是不喜欢操作xml的，所以就没有使用dom4j和jdom的习惯。使用commons-configuration令我最爽的地方是指针式的访问方式，例如，在如下xml中，如果你得到code属性的值，只需要写如下Java代码

List codes = config.getList("columns.column.[@code]");
  <?xml version="1.0" encoding="UTF-8"?>
<origin dataSource="ds">
<columns>
<column code="ID" name="" type="java.lang.Integer" length="" />
<column code="FIRSTNAME" name="" type="java.lang.String" length="" />
<column code="LASTNAME" name="" type="java.lang.String" length="" />
<column code="SEX" name="" type="java.lang.String" length="" />
</columns>
</origin>
　　
如果你的得到的字符串中有逗号，那么他会以","分割，只会得到第一个字符串。所以你必须用"\"转义,例如

String array = config.getString("config.array"); //array="10",并不是10,20,30,40
String scalar = config.getString("config.scalar"); // scalar="3,1415"
String text = config.getString("config.cite.[@text]"); // text ="To be or not to be, this is the question!"

<config>
<array>10,20,30,40</array>
<scalar>3\,1415</scalar>
<cite text="To be or not to be\, this is the question!"/>
</config>
　　
默认情况下，config以","分割字符串，其实还可以使用如下方法(AbstractConfiguration 是XMLConfiguration 的父类)

AbstractConfiguration.setListDelimiter(char listDelimiter) //设置限定符
AbstractConfiguration.setDelimiterParsingDisabled(boolean) //设置限定符是否有效
　　
注意：以上方法的设置要xml文件load之前(换而言之，是不能使用XMLConfiguration config = new XMLConfiguration(String))，否则，此次设置将无效，正确使用如下方式

XMLConfiguration config = new XMLConfiguration();
config.setDelimiterParsingDisabled(true);
try {
config.load(path);
} catch (Exception e) {
// TODO Auto-generated catch block
logger.error("载入文件出错",e);
}
　　
还有一个比较常用的方法，例如你想把column中的属性全部取出到一个叫Bean的Java类中，你就必须定位到column，这时常用以下方式

List<HierarchicalConfiguration>  columns = (List<HierarchicalConfiguration>) config.configurationsAt("origin.columns.column");
List Beans = new ArrayList(columns.size());
for (int i = 0; i < columns.size(); i++) {
HierarchicalConfiguration column = (HierarchicalConfiguration)columns.get(i);
Bean bean = new Bean();
bean.setCode(column.getString("[@code]"));
bean.setName(column.getString("[@name]"));
...
...
beans.add(bean);
}