使用Java编写HTTP代理解决跨域资源共享问题

package servlet;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.jsoup.Connection.Method;
import org.jsoup.Connection.Response;
import org.jsoup.Jsoup;

//下面一行将访问该应用的所有请求交给这个程序处理，比较老的环境可能不支持
@WebServlet("/*")
public class Proxy extends HttpServlet {
    private static final long serialVersionUID = 1L;
    
    //需要代理访问的主机地址（API所在主机地址）
    private static final String REMOTE_HOST = "http://example.com";
    
    //这份代码所在的网站被部署在服务器的什么路径下
    private static final String LOCAL_PATH = "/proxy";
    
    //默认的请求头
    private static Map<String, String> headers = new HashMap<String, String>();

    public Proxy() {
        super();
        headers.put("Host", "vjudge.net");
        headers.put("Connection", "keep-alive");
        headers.put("Content-Length","0");
        headers.put("Accept", "*/*");
        headers.put("Origin", REMOTE_HOST);
        headers.put("X-Requested-With", "XMLHttpRequest");
        headers.put("User-Agent",
                "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36");
        headers.put("Referer", REMOTE_HOST);
        headers.put("Accept-Encoding", "gzip, deflate, br");
        headers.put("Accept-Language", "zh-CN,zh;q=0.8,en;q=0.6");
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doIt(request, response, false);
    }
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doIt(request, response, true);
    }

    private void doIt(HttpServletRequest request, HttpServletResponse response, boolean post) throws IOException {
        //下面的两行是重点，有了他们就可以带上Cookies跨域获取资源了
        //第一行允许来源null，也就是本地网页访问这里，第二行允许带上Cookies
        response.addHeader("Access-Control-Allow-Origin", "null");
        response.addHeader("Access-Control-Allow-Credentials", "true");
        
        //客户端传来的Cookies
        Cookie[] clientCookies = request.getCookies();
        //发给服务器的Cookies
        HashMap<String, String> serverCookie = new HashMap<String, String>();
        //客户端传来的数据
        Map<String, String> data = new HashMap<String, String>();
        //发给服务器的数据
        Set<Entry<String, String[]>> para = request.getParameterMap().entrySet();
        //客户端访问的URI
        String uri = request.getRequestURI();
        
        //把客户端传来的Cookies转成jsoup使用的格式准备发给远程服务器
        if (clientCookies != null) {
            for (Cookie clienCookie : clientCookies) {
                String key = clienCookie.getName();
                String value = clienCookie.getValue();
                serverCookie.put(key, value);
            }
        }
        
        //把客户端传来的数据转成jsoup使用的格式准备发给远程服务器
        for (Entry<String, String[]> t : para) {
            String key = t.getKey();
            String value = t.getValue()[0];
            data.put(key, value);
        }
        
        //转发给服务器
        Response serverResponse = Jsoup.connect(uri.replace(LOCAL_PATH, REMOTE_HOST)).headers(headers)
                .cookies(serverCookie).data(data).method(post ? Method.POST : Method.GET)
                .ignoreContentType(true).ignoreHttpErrors(true)
                .execute();
        
        if(serverResponse.statusCode() != 200){
            //出现错误，转发给客户端
            response.sendError(serverResponse.statusCode(),serverResponse.statusMessage());
        } else {
            //服务器传回来的Cookies放入response里
            for (Entry<String, String> t : serverResponse.cookies().entrySet()) {
                String key = t.getKey();
                String value = t.getValue();
                Cookie cookie = new Cookie(key, value);
                cookie.setPath(LOCAL_PATH);
                //我所使用的API，注销是以logout结尾的，我就偷懒直接写死了
                cookie.setMaxAge(uri.endsWith("logout") ? -1 : 60 * 60 * 24 * 365);
                response.addCookie(cookie);
            }
            //返回服务器传来的数据
            response.getWriter().append(serverResponse.body());
        }
    }
}