Java的ServletContext、HttpServletResponse

1、ServletContext：
（1）ServletContext在一个web应用中只有一个，但是Servlet能有多个；

（2）生命周期
创建：

当web应用已经部署到服务器时，服务器开启时被加载；
当web应用还未部署到服务器时，服务器开启后将web应用部署到服务器后被加载；
销毁：

服务器关闭时被销毁；
服务器处于开启状态，将web应用从服务器移除时被销毁；
（3）获取：

ServletConfig.getServletContext();等价于getServletContext();
getServletContext()；

两个是不同的方法：
第一个是ServletConfig接口的方法，第二个是ServletContext类的方法；
但是获取的是同一个ServletContext对象，因为只有一个；一般用后一个方法比较多；
（4）获取全局的初始化参数：[注：param参数]
举个例子：在<display-name>WEB02</display-name>后面配置

<context-param>
<param-name>aa</param-name>
<param-value>aaa</param-value>
</context-param>

然后调用ServletContext的getInitParameter("aa")方法，参数为<param-name>aa</param-name>的参数名字

package com.oracle.web;

import java.io.IOException;

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

public class ContexServlet extends HttpServlet {
        
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = getServletContext();
        String  inti = context.getInitParameter("aa");
        System.out.println(inti);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

（5）获取web应用的绝对路径：举个例子
绝对路径是从根目录开始，相对路径是从当前目录开始
在WEB02的WebContent下建立a.txt → ServletContext.getRealPath("a.txt")→D:javaEE omcat-7.0.52(x86)webappsWEB02a.txt
在WEB02的WebContent的WEB-INF下建立b.txt → ServletContext.getRealPath("WEB-INF/b.txt")→D:javaEE omcat-7.0.52(x86)webappsWEB02WEB-INF.txt
在WEB02的Java Resource的src下建立c.txt → ServletContext.getRealPath("WEB-INF/classes/c.txt")→D:javaEE omcat-7.0.52(x86)webappsWEB02WEB-INFclassesc.txt
在WEB02下建立d.txt，无法访问；

package com.oracle.web;

import java.io.IOException;

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

public class ContexServlet extends HttpServlet {
        
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = getServletContext();
        String  inti = context.getInitParameter("aa");
        System.out.println(inti);
        //获取web应用中的资源的绝对路径
        String  patha = context.getRealPath("a.txt");
        String  pathb = context.getRealPath("WEB-INF/b.txt");
        String  pathc =  context.getRealPath("WEB-INF/classes/c.txt");
        System.out.println(patha);
        System.out.println(pathb);
        System.out.println(pathc);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

（6）域对象：Servlet有四大域对象
存储数据的区域就是域对象[数据可以共享]；
域对象的通用方法：

setAttribute(str,obj)注意向下转型

getAttribute(str);

removeAttribute(str);

举个例子：在Servlet02设置值

package com.oracle.web;

import java.io.IOException;

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

public class Servlet02 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext context = getServletContext();
        context.setAttribute("name", "lisi");
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

在Servlet03取值；

package com.oracle.web;

import java.io.IOException;

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

public class Servlet03 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = getServletContext();
        String name= (String) context.getAttribute("name");
        System.out.println(name);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

先访问下Servlet02

再访问Servlet03

则在console打印：

2、HttpServletResponse
（1）write方法是将内容写到response的缓冲区；
（2）设置响应行：setStatus(sc);sc = 200/302/304/404/500
（3）设置响应头：addHeather(name,value)不常用；添加

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HeaderServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setStatus(500);
        response.addHeader("qipa", "fushouer");
        response.getWriter().write("hello dandan...");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

 setHearder(name,value)常用；设置

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class RefreshServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //定时刷新功能
        response.setHeader("refresh", "5;url=http://www.baidu.com");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

（4）重定向：请求两次；url地址改变；
第一种方法：设置状态码为302，设置响应头为("location","/xxx地址")；

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet01 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //重定向
        //1、设置状态码302
        response.setStatus(302);
        //2、设置Location头
        response.setHeader("Location", "/WEB03/Servlet");
        response.getWriter().write("hello 123"); 
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().write("hello dandan...");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

第二种方法：调用方法response.sendRedirect("/WEB03/Servlet");[/不要掉下];

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet01 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //重定向 //在实际开发中不会设置头
        response.sendRedirect("/WEB03/Servlet");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().write("hello dandan...");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

（5）中文乱码问题：[键入下列语句]
response.setContentType("text/html;charset=UTF-8");

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet02 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //response解决中文乱码问题
        //response.setContentType("text/html;charset=UTF-8");
        response.getWriter().write("Hello世界");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

package com.oracle.Servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet02 extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //response解决中文乱码问题
        response.setContentType("text/html;charset=UTF-8");
        response.getWriter().write("Hello世界");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}