HttpSession的內容都放在一個單獨的Map中，模擬遠端分散式Session。

1.使用HttpServletRequestWrapper建立自定義Request
2.使用動態代理包裝自定義Request返回的HttpSession物件
3.建立過濾器，使用自定義Request替換原有的Request物件。
4.在Servlet中得到的HttpSession物件，寫入和讀取內容都假設透過遠端Session伺服器。

建立自定義的Request，返回動態代理的HttpSession

import java.lang.reflect.InvocationHandler;

import java.lang.reflect.Method;

import java.lang.reflect.Proxy;

import java.util.HashMap;

import java.util.Map;

import java.util.concurrent.ConcurrentHashMap;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletRequestWrapper;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpServletResponseWrapper;

import javax.servlet.http.HttpSession;

public class RemoteSessionRequest extends HttpServletRequestWrapper {

    public RemoteSessionRequest(HttpServletRequest request) {

        super(request);

    }

    @Override

    public HttpSession getSession() {

        return RemoteSessionHandler.getInstance(super.getSession());

    }

}

class RemoteSessionHandler implements InvocationHandler {

    //模擬遠端Session伺服器，Key表示SessionId,Value表示該Session的內容

    private static Map<String, Map<String, Object>> map = new ConcurrentHashMap<String, Map<String, Object>>();

    private HttpSession session = null;

    private RemoteSessionHandler(HttpSession httpSession) {

        this.session = httpSession;

    };

    public static HttpSession getInstance(HttpSession httpSession) {

        InvocationHandler handler = new RemoteSessionHandler(httpSession);

        return (HttpSession) Proxy.newProxyInstance(httpSession.getClass().getClassLoader(), httpSession.getClass().getInterfaces(), handler);

    }

    @Override

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

        if ("setAttribute".equals(method.getName())) {

            String id = session.getId();

            Map<String, Object> m = map.get(id);

            if (m == null) {

                m = new HashMap<String, Object>();

                map.put(id, m);

            }

            m.put((String) args[0], args[1]);

            System.out.println("[存入]key:" + args[0] + ",value:" + args[1]);

            return null;

        } else if ("getAttribute".equals(method.getName())) {

            String id = session.getId();

            Map<String, Object> m = map.get(id);

            if (m == null) {

                return null;

            }

            Object result = m.get(args[0]);

            System.out.println("[取出]key:" + args[0] + ",value:" + result);

            return result;

        }

        return method.invoke(session, args);

    }

}

使用過濾器替換原有的Request

import java.io.IOException;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.annotation.WebFilter;

import javax.servlet.http.HttpServletRequest;

@WebFilter("/*")

public class SessionFilter implements Filter {

    @Override

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

        chain.doFilter(new RemoteSessionRequest((HttpServletRequest) request), response);

    }

    @Override

    public void destroy() {

        // TODO Auto-generated method stub

    }

    @Override

    public void init(FilterConfig arg0) throws ServletException {

        // TODO Auto-generated method stub

    }

}

在Servlet中按照原有方式使用HttpSession。

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        HttpSession session = request.getSession();

        session.setAttribute("name", "Hello");

        session.getAttribute("name");

        session.getAttribute("other");

    }

結果可以看到，他已經模擬從遠端伺服器存取資料

[存入]key:name,value:Hello
[取出]key:name,value:Hello
[取出]key:other,value:null

HttpServletRequestWrapper模擬實現分散式Session

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