Redirections in HTTP

URL redirection, also known as URL forwarding, is a technique to give more than one URL address to a page, a form, or a whole Web site/application. HTTP has a special kind of response, called a HTTP redirect, for this operation.

Redirects accomplish numerous goals:

Temporary redirects during site maintenance or downtime
Permanent redirects to preserve existing links/bookmarks after changing the site’s URLs, progress pages when uploading a file, etc.

302 Found

The HyperText Transfer Protocol (HTTP) 302 Found redirect status response code indicates that the resource requested has been temporarily moved to the URL given by the Location header. A browser redirects to this page but search engines don’t update their links to the resource (in ‘SEO-speak’, it is said that the ‘link-juice’ is not sent to the new URL).

Location

The Location response header indicates the URL to redirect a page to. It only provides a meaning when served with a 3xx (redirection) or 201 (created) status response.

In cases of redirection, the HTTP method used to make the new request to fetch the page pointed to by Location depends on the original method and the kind of redirection:

303 (See Also) responses always lead to the use of a GET (See Also) responses always lead to the use of a GET method.
307 (Temporary Redirect) and 308 (Temporary Redirect) and 308 (Temporary Redirect) and 308 (Temporary Redirect) and 308 (Temporary Redirect) and 308 (Permanent Redirect) don’t change the method used in the original request.
301 (Moved Permanently) and 302 (Found) don’t change the method most of the time, though older user-agents may (so you basically don’t know).

All responses with one of these status codes send a Location header.

In cases of resource creation, it indicates the URL to the newly created resource.

实践

环境

操作系统：

Windows 10 x64

集成开发环境：

Eclipse IDE for Enterprise Java and Web Developers (includes Incubating components)

Version: 2021-09 (4.21.0)

Build id: 20210910-1417

服务器：

apache-tomcat-9.0.55

客户端：

谷歌浏览器：版本 96.0.4664.93（正式版本）（64 位）

新建项目

新建 Dynamic Web Project：

在这里插入图片描述

使用 `HttpServletResponse.sendRedirect(String)` 进行重定向

RedirectServlet 类：

package com.mk.servlet;

import java.io.IOException;

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

@WebServlet(urlPatterns = "/redirect")
public class RedirectServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println(response.getClass().getCanonicalName());
        
        /*
         * Sends a temporary redirect response to the client using the specified
         * redirect location URL. This method can accept relative URLs; the servlet
         * container must convert the relative URL to an absolute URL before sending
         * the response to the client. If the location is relative without a leading
         * '/' the container interprets it as relative to the current request URI.
         * If the location is relative with a leading '/' the container interprets
         * it as relative to the servlet container root.
         */
        response.sendRedirect("index");
    }
}

IndexServlet 类：

package com.mk.servlet;

import java.io.IOException;

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

@WebServlet(urlPatterns = "/index")
public class IndexServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.sendRedirect("index.html");
    }
}

src/main/webapp/index.html 文件：

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>首页</title>
    </head>
    <body>
        <p>首页</p>
    </body>
</html>

测试

启动服务器，启动谷歌浏览器，打开开发者工具的网络选项卡，访问 http://localhost:8080/hello-servlet/redirect，请求成功之后，可以看到：

我们从 http://localhost:8080/hello-servlet/redirect 重定向到 http://localhost:8080/hello-servlet/index；
又从 http://localhost:8080/hello-servlet/index 重定向到 http://localhost:8080/hello-servlet/index.html。

同理，如果你直接访问 http://localhost:8080/hello-servlet/index，最终也会被重定向至 http://localhost:8080/hello-servlet/index.html。

在这里插入图片描述

使用状态码和响应头进行重定向

重定向的本质是服务器向客户端（浏览器）发送 302 状态码和 Location 响应头。

修改 RedirectServlet 类，使用状态码和响应头进行重定向：

package com.mk.servlet;

import java.io.IOException;

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

@WebServlet(urlPatterns = "/redirect")
public class RedirectServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println(response.getClass().getCanonicalName());
        
        String url = "index";
        
        response.setStatus(HttpServletResponse.SC_FOUND);
        response.setHeader("Location", url);
    }
}

修改 IndexServlet 类，使用状态码和响应头进行重定向：

package com.mk.servlet;

import java.io.IOException;

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

@WebServlet(urlPatterns = "/index")
public class IndexServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String scheme = request.getScheme();
        String serverName = request.getServerName();
        int serverPort = request.getServerPort();
        String contextPath = request.getContextPath();
        
        String url = scheme + "://" + serverName + ":" + serverPort + "/" + contextPath + "/index.html";
        
        response.setStatus(HttpServletResponse.SC_FOUND);
        response.setHeader("Location", url);
    }
}