Servlet缓存

在运行ajax的图片浏览器的时候，仅有第一次运行的情况是正确的，能正确显示图片。

JS的代码为：

var store = new Ext.data.JsonStore({
        url: '../ImagesServlet',
        root: 'images',
        fields: ['name', 'url',{name:'size', type: 'float'}, {name:'lastmod', type:'date', dateFormat:'timestamp'}]
    });

JS调用ImageServlet,取得json类型的数据。然后根据json的数据显示图片。

ImageServlet代码为：

/**
 * 
 */
package org.eimhe;
import java.io.File;
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;
/**
 * @author liheng
 * ImagesServlet.java
 * 2009-2-23
 */
public class ImagesServlet extends HttpServlet{
	
 private boolean is_first = true;
 private StringBuffer jsonString = new StringBuffer();
 protected void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException
 {
	 processRequest(request,response);
 }
 
 protected void doPost(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException{
	 processRequest(request,response);
 }
 
 protected void processRequest(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException
 {
	 //json "[images:{{name:'',url:'',size:'',lastmod:''},{}}]"
	 ServletContext ctx = getServletContext();
	 String cs = ctx.getRealPath("/upload");
	 //String cs = "c:/upload";
	 File f = new File(cs);
	 /**
	 * if(jsonString != null)                                         
	 * 	 {                                                            
	 * 		 System.out.println("the cache is: "+jsonString.toString());
	 * 		 jsonString = new StringBuffer();                           
	 * 		 is_first = true;                                           
	 * 	 }  
	 */                                                          
	 
		 
	 jsonString.append("{\"images\":[");  //append json string head
	 
	 if(f.isDirectory())
	 {
		 File[] fs = f.listFiles(new picFilter());
		 for(int i=0,len=fs.length;i<len;i++){
			 if(fs[i].isFile())
			 {
				 if(is_first)
				 {
					 is_first = false;
				 }
				 else
				 {
					 jsonString.append(",");
				 }
				 jsonString.append("{\"name\":\""+fs[i].getName()+"\",\"url\":\"../upload/"+fs[i].getName()+"\",\"size\":"+fs[i].length()+",\"lastmod\":"+fs[i].lastModified()+"}");
				 //jsonString.append("{\"name\":\""+fs[i].getName()+"\",\"url\":\""+fs[i].getPath()+"\",\"size\":"+fs[i].length()+",\"lastmod\":"+fs[i].lastModified()+"}");
			 }
                         
			 else if(fs[i].isDirectory())
			 {
				 getJsonStringForDirectory(fs[i],fs[i].getName());
			 }
		 }
	 }
	 jsonString.append("]}"); //append json string bottom
         
         response.setContentType("text/html");
         response.setHeader("Pragma","No-cache");  
         response.setDateHeader("Expires",0);  
         response.setHeader("Cache-Control","no-cache"); 
	 PrintWriter out = response.getWriter();
	 out.print(jsonString.toString());
	 out.close();
 }
 
 protected void getJsonStringForDirectory(File f,String directoryName)
 {
	 File[] ifile = f.listFiles(new picFilter());
	 for(int i=0,len=ifile.length;i<len;i++)
	 {
		 if(ifile[i].isFile())
		 {
			 if(is_first)
			 {
				 is_first = false;
			 }
			 else
			 {
				 jsonString.append(",");
			 }
			 jsonString.append("{\"name\":\""+ifile[i].getName()+"\",\"url\":\"../upload/"+directoryName+"/"+ifile[i].getName()+"\",\"size\":"+ifile[i].length()+",\"lastmod\":"+ifile[i].lastModified()+"}");
		 }
		 else if(ifile[i].isDirectory())
		 {
			 getJsonStringForDirectory(ifile[i],directoryName+"/"+ifile[i].getName());
		 }
	 }
 }
 
}

刷新的时候，图片老是不能正常显示，于是怀疑是现实的json的格式有问题，发现没刷新一次，json的数据就多一些。原来是jsonString 和is_first都缓存了数据，在第二次运行的时候没有设置为默认值，导致json的格式错误。

去掉代码中注释的那块，图片能正常显示啦。