应用场景:在数据驱动的web开发中，经常要重复从数据库中取出相同的数据，这种重复极大的增加了数据库负载。缓存是解决这个问题的好办法。但是ASP.NET中的虽然已经可以实现对页面局部进行缓存，但还是不够灵活。此时Memcached或许是你想要的。

一.   什么是Memcached

Memcached 是一个高性能的分布式内存 对象缓存系统，用于动态Web应用以减轻数据库负载。它通过在内存中缓存数据和对象 来减少读取数据库的次数，从而提供动态、数据库驱动网站的速度。

Memcached基于一 个存储键/值对的hashmap。其守护进程是用C写的，但是客户端可以用任何语言来编写(本文使用JAVA作为例子)，并通过memcached协议与守护进程通信。

二.   Memcached 特征

(1)     协议简单: 不使用复杂的xml格式，而是使用文本格式 

(2)     基于libevent的事件处理机制 

(3)     内置内存存储方式: 数据存在在内存中，所以重启机器会导致数据丢失 

（4）Memcached相互不通信的分布式: Memcached 服务器之间不会进行通信，数据都是通过客户端的分布式算法存储到各个服务器中 

三.Memcached的安装(windows)

下载Memcached 服务端: http://memcached.org/ （官网寻找适用的版本）

1 解压缩文件到D:\memchche\memcached-win32-1.4.4-14
2 命令行输入 D:\memchche\memcached-win32-1.4.4-14\memcached.exe -d install 
3 命令行输入 D:\memchche\memcached-win32-1.4.4-14\memcached.exe -d start，该命令启动 Memcached ，默认监听端口为 11211

四.Memcached在java在java项目中的应用

(一)StaticValue 类中设置常量

package com.jssso.htgl.util;

public class StaticValue {
    public static final String COOKIES_ROLE_KKEY = "COOKIES_ROLE_KKEY";
    public static final Long MEMCAED_ROLE_KKEY = 120l; //分钟
}

(二)MyCache 为一个单例模式,这样会更加安全,每次调用的时候会创建一个私有实例

1.add方法为向memcached中添加查到的集合,在方法中

MEMCAED_ROLE_KKEY为设置的分钟数,可以从类StaticValue 中取到设置的值(注意此处设置的时间为120分钟:120l)

2.getCache方法为从memcached中取出缓存的数据

package com.zxrytj.netty;

import java.util.Date;
import java.util.List;

import com.danga.MemCached.MemCachedClient;
import com.danga.MemCached.SockIOPool;
import com.jssso.htgl.util.StaticValue;

public class MyCache {
    private static MemCachedClient client=new MemCachedClient(); 
    private static MyCache myCache=new MyCache();
    private MyCache(){};
    public  static MyCache getinstance()
    {
        return myCache;
    }
    static 
    {      
        String [] addr ={"127.0.0.1:11211"};  
        Integer [] weights = {3};  
        SockIOPool pool = SockIOPool.getInstance();  
        pool.setServers(addr);  
        pool.setWeights(weights);  
        pool.setInitConn(5);  
        pool.setMinConn(5);  
        pool.setMaxConn(200);  
        pool.setMaxIdle(1000*30*30);  
        pool.setMaintSleep(30);  
        pool.setNagle(false);  
        pool.setSocketTO(30);  
        pool.setSocketConnectTO(0);  
        pool.initialize();  
    } 
    public boolean add(String key,List list)//add
    {
    /**
     * date为memcached过期时间
     * MEMCAED_ROLE_KKEY,分钟
     */
          Date date = new Date(StaticValue.MEMCAED_ROLE_KKEY * 60 * 1000);//2小时,单位毫秒
         // System.out.println( "XXXXXXXX"+(List)client.get(key));
          return  client.add(key, list,date);
    }
    public  List getCache(String key)
    {   
        List list=null;
        boolean b=client.keyExists(key);
        if(b==true)
        {
             list=(List) client.get(key);
              //System.out.println( "XXXXXXXXlist.size()"+list.size());
        }    
        return list;
    }
    
}

二,在Servlet中的调用add,getCache方法

通过判断cookie是否过期向memcached中取数据,为说明问题把关键部分摘出来加以说明

1.cookie存在就直接向memcached中取数据

2.cookie不存在就调用add方法向memcached中添加数据集合

    Cookie[] cookies = request.getCookies();//这样便可以获取一个cookie数组
            String ckvalue="";//声明一个cookie的value,判断cookie是否存在
            List listcahe=null;//这里声明一个集合存放数据
//该for循环判断cookie是否存在
        for(Cookie cookie : cookies)
        {
            if(cookie.getName().equalsIgnoreCase(StaticValue.COOKIES_ROLE_KKEY))//cookie的key值存在,ckvalue存在,直接从memcache中取值
            {
                 ckvalue=cookie.getValue();
                 System.out.println("ckvalue存在"+ckvalue);
                 MyCache cache=MyCache.getinstance();
                 listcahe=cache.getCache(ckvalue);//从memcached中取值赋值给listcache
                
            }
            
            
        }
//cookie不存在
        if("".equals(ckvalue))//ckvalue不存在
        { 
            System.out.println("ckvalue不存在"+ckvalue);
            MyCache cache=MyCache.getinstance();
             String uuid = UUID.randomUUID().toString();
             /**
              * COOKIES_ROLE_KKEY为cookie的key
              */        
            Cookie ck=new Cookie(StaticValue.COOKIES_ROLE_KKEY,uuid);
            ck.setMaxAge(-1);//过期时间,单位为秒,-1关闭浏览器cookie消失
            ck.setPath("/js_sso_server/");
            response.addCookie(ck);
            listcahe = roleService.getApp(rname, rdes, pageIndex, pageSize);//cookie不存在,从数据库获取数据
            cache.add(uuid,listcahe);
        }

下面是Servlet中的完整代码,可以不看(自己可以练手的可以复制试试)

package com.jssso.htgl.action;

import java.io.IOException;
import java.io.PrintWriter;
import java.net.URLDecoder;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.UUID;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import net.sf.json.JSONArray;
import net.sf.json.JSONObject;

import com.jssso.htgl.entity.Role;
import com.jssso.htgl.service.RoleService;
import com.jssso.htgl.service.impl.RoleServiceImpl;
import com.jssso.htgl.util.GetChche;
import com.jssso.htgl.util.StaticValue;
import com.zxrytj.netty.MyCache;

@SuppressWarnings("serial")
public class RoleServlet extends HttpServlet {

    

    @Override
    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    };

    @Override
    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        String option = request.getParameter("option");
        if (option != null) {
            if ("selectApp".equals(option)) {
                selectApp(request, response);
            } else if ("saveApp".equals(option)) {
                saveApp(request, response);
            } else if ("updateApp".equals(option)) {
                updateApp(request, response);
            } else if ("deleteApp".equals(option)) {
                deleteApp(request, response);
            } else if ("index".equals(option)) {
                goView("/WEB-INF/view/jsp/htgl/role.jsp", request, response);
                return;
            } else if("jsonRole".equals(option)){
                jsonRole(request, response);
            }
        }
    }

    /**
     * 删除应用地址
     * 
     * @param request
     * @param response
     * @throws IOException
     */
    private void deleteApp(HttpServletRequest request,
            HttpServletResponse response) throws IOException {
        String urlId = request.getParameter("rid");
         RoleService roleService = new RoleServiceImpl();
        boolean flag = roleService.deleteApp(urlId);
        PrintWriter out = response.getWriter();
        if (flag) {
            out.print("success");
        } else {
            out.print("error");
        }
        out.flush();
        out.close();
    }

    /**
     * 修改应用地址
     * 
     * @param request
     * @param response
     * @throws IOException
     */
    private void updateApp(HttpServletRequest request,
            HttpServletResponse response) throws IOException {
        String urlId = request.getParameter("rid");
        String name = request.getParameter("rname");
        String url = request.getParameter("des");
        Long rid = null;
        if (urlId != null) {
            rid = Long.parseLong(urlId);
        }
        Role role = new Role(rid, name, url);
         RoleService roleService = new RoleServiceImpl();
        boolean flag = roleService.updateApp(role);
        PrintWriter out = response.getWriter();
        if (flag)
            out.print("success");
        else
            out.print("error");
        out.flush();
        out.close();
    }

    /**
     * 添加应用地址
     * 
     * @param request
     * @param response
     * @throws IOException
     */
    private void saveApp(HttpServletRequest request,
            HttpServletResponse response) throws IOException {
        String rname = request.getParameter("rname");
        String rdes = request.getParameter("des");
         RoleService roleService = new RoleServiceImpl();
        boolean flag = roleService.saveApp(rname, rdes);
        PrintWriter out = response.getWriter();
        if (flag)
            out.print("success");
        else
            out.print("error");
        out.flush();
        out.close();
    }

    /**
     * 查询应用地址
     * 
     * @param request
     * @param response
     * @throws IOException
     */
    private void selectApp(HttpServletRequest request,
            HttpServletResponse response) throws IOException {
        String rname = request.getParameter("rname");
        String rdes = request.getParameter("des");

        if(rname!=null && !"".equals(rname.trim())){
            try {
                
                rname = URLDecoder.decode(rname, "utf-8");
                
            } catch (Exception e) {

                e.printStackTrace();

            }
        }
        
        Long pageIndex = 1L;
        if (request.getParameter("page") != null) {
            pageIndex = Long.parseLong(request.getParameter("page"));
            if (pageIndex == null || pageIndex == 0) {
                pageIndex = 1L;
            }
        }
        Integer pageSize = 10;
        if (request.getParameter("rows") != null) {
            pageSize = Integer.parseInt(request.getParameter("rows"));
            if (pageSize == null) {
                pageSize = 10;
            }
        }
         RoleService roleService = new RoleServiceImpl();
        
            Cookie[] cookies = request.getCookies();//这样便可以获取一个cookie数组
            String ckvalue="";
            List listcahe=null;
        for(Cookie cookie : cookies)
        {
            if(cookie.getName().equalsIgnoreCase(StaticValue.COOKIES_ROLE_KKEY))//cookie的key值存在,ckvalue存在,直接取值listcacahe
            {
                 ckvalue=cookie.getValue();
                 System.out.println("ckvalue存在"+ckvalue);
                 MyCache cache=MyCache.getinstance();
                 listcahe=cache.getCache(ckvalue);
                
            }
            
            
        }
        if("".equals(ckvalue))//ckvalue不存在
        { 
            System.out.println("ckvalue不存在"+ckvalue);
            MyCache cache=MyCache.getinstance();
             String uuid = UUID.randomUUID().toString();
             /**
              * COOKIES_ROLE_KKEY为cookie的key
              */        
            Cookie ck=new Cookie(StaticValue.COOKIES_ROLE_KKEY,uuid);
            ck.setMaxAge(-1);//过期时间,单位为秒,-1关闭浏览器cookie消失
            /**
             * 获取web-inf路径
             */
            //String path=request.getSession().getServletContext().getRealPath("/WEB-INF/cookie");
            //  System.out.println("&&&&&&&&&&&&&&&&&path:"+path);
            ck.setPath("/js_sso_server/");
            response.addCookie(ck);
            listcahe = roleService.getApp(rname, rdes, pageIndex, pageSize);
            cache.add(uuid,listcahe);
        }
        //GetChche  get=new GetChche();
       // get.getCache(request);
        Long count = roleService.pageCount(rname, rdes);
        Long pageCount = count % pageSize == 0 ? count / pageSize
                : (count / pageSize) + 1;
        Map<String, Object> jsonMap = new HashMap<String, Object>();// 定义map
        jsonMap.put("total", count);
        jsonMap.put("rows", listcahe);
        jsonMap.put("pages", pageCount);
        jsonMap.put("from", (pageIndex - 1) * pageSize + 1);
        jsonMap.put("to", pageIndex * pageSize);
        PrintWriter out = response.getWriter();
        out.print(JSONObject.fromObject(jsonMap).toString());
        out.flush();
        out.close();
    }
    /**
     * 查询应用地址
     * 
     * @param request
     * @param response
     * @throws IOException
     */
    private void jsonRole(HttpServletRequest request,
            HttpServletResponse response) throws IOException {
        
        Long pageIndex = 1L;
        if (request.getParameter("page") != null) {
            pageIndex = Long.parseLong(request.getParameter("page"));
            if (pageIndex == null || pageIndex == 0) {
                pageIndex = 1L;
            }
        }
        Integer pageSize = 10;
        if (request.getParameter("rows") != null) {
            pageSize = Integer.parseInt(request.getParameter("rows"));
            if (pageSize == null) {
                pageSize = 10;
            }
        }
         RoleService roleService = new RoleServiceImpl();
        List<Role> json = roleService.getApp(null,null, pageIndex, pageSize);
        
        PrintWriter out = response.getWriter();
        
        out.print(JSONArray.fromObject(json).toString());
        out.flush();
        out.close();
    }

    private void goView(String url, HttpServletRequest request,
            HttpServletResponse response) {
        RequestDispatcher dispatcher = request.getRequestDispatcher(url);
        try {
            dispatcher.forward(request, response);
        } catch (ServletException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}