java web-01-servlet

本文主要是介绍java web-01-servlet，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

1、helloservlet

在浏览器中输出 hello servlet！

1.1、创建一个maven项目，连接Tomcat

public class HelloServlet extends HttpServlet {

    //Art+Ins
    //由于get或者post只是请求实现的不同的方式，可以相互调用，业务逻辑都一样
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        PrintWriter writer = resp.getWriter();//响应流
        writer.println("hello servlet!!!");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

1.2、在web.xml文件中写添加映射

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <!--注册servlet-->
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.feng.servlet.HelloServlet</servlet-class>
    </servlet>
    <!--mapping：映射-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

</web-app>

为什么需要映射：我们写的是java程序，但是要通过浏览器访问，而浏览器需要连接web服务器，所以我们需要在web服务器中注册我们写的Servlet，还需给他一个浏览器能够访问的路径

如果项目没有web，直接项目右键添加即可！

2、ServletContext

web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用；

2.1、共享数据

public class SetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //this.getInitParameter();初始化参数
        //this.getServletConfig();Servlet配置
        //this.getServletContext();Servlet上下文

        ServletContext context = this.getServletContext();
        String name = "bitwind";
        //将一个数据保存在了ServletContext中
        context.setAttribute("username",name);//setAttribute : 设置属性

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取ServletContext中的值
        ServletContext context = this.getServletContext();
        String name = (String) context.getAttribute("username");
        resp.getWriter().println("name = "+name);
        System.out.println(name);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

添加映射

启动Tomcat，访问！

注意：要先访问GetServlet类，否则name=null，要先存进ServletContext，才有得拿！

2.2、获取初始化参数

public class GetInitParameter extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        //this.getInitParameter();初始化参数
        //this.getServletConfig();Servlet配置
        //this.getServletContext();Servlet上下文
        ServletContext context = this.getServletContext();
        String name = context.getInitParameter("Tom");
        resp.getWriter().println(name);
        System.out.println(name);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

在web.xml中设置初始化参数

<!--初始化参数-->
<context-param>
    <param-name>Tom</param-name>
    <param-value>这是javaweb，这是servlet，这是初始化参数！</param-value>
</context-param>

添加映射

启动Tomcat，访问！

2.3、请求转发

public class GetRequestDispatcher extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        System.out.println("这是GetRequestDispatcher！！！");
        RequestDispatcher dispatcher = context.getRequestDispatcher("/init");//这是web设置的url路径
        dispatcher.forward(req,resp);

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

添加映射

启动Tomcat，访问！

2.4、读取资源文件

target*是用来存放项目构建后的文件和目录、jar包、war包、编译的class*文件

Properties

在java目录下新建properties文件
在resources目录新建properties文件

username=张三
password=123456

发现：都被打包到了同一个路径下：classes，我们俗称这个路径为classpath；

public class GetProperties extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        //第一条“/”不能省略,代表真当前web项目
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(is);//获取文件流
        String user = prop.getProperty("username");
        String pwd = prop.getProperty("password");
        resp.getWriter().print(user+"："+pwd);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

注意："/WEB-INF/classes/db.properties"这是target 下的项目同名的目录路径！

添加映射

启动Tomcat，访问！

注意：要用maven模板新建的项目，这个target 目录下才有项目同名的包！

3、HttpServletRespouse

web服务器接收到客户端的http请求，针对这个请求，分别创建一个代表请求的HttpServletRequest对象，代表响应的一个HttpServletResponse；

如果要获取客户端请求过来的参数：找HttpServletRequest
如果要给客户端响应一些信息：找HttpServletResponse

3.1、下载文件

要获取下载文件的路径
下载的文件名是什么？
设置想办法让浏览器能够支持下载我们需要的东西
获取下载文件的输入流
创建缓冲区
获取OutputStream对象
将FileOutputStream流写入buffer缓冲区
使用OutputStream将缓冲区中的数据输出到客户端！

response下载文件

public class HSResponse extends HelloServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String realPath = "文件路径\\文件名.文件类型";
        //2. 下载的文件名是什么？
        String fileName = realPath.substring(realPath.lastIndexOf("\\") + 1);

        resp.setHeader("Content-Disposition","attachment;filename="+ URLEncoder.encode(fileName,"UTF-8"));

        FileInputStream fileInputStream = new FileInputStream(realPath);
        int len = 0;
        byte[] buffer = new byte[1024];

        ServletOutputStream outputStream = resp.getOutputStream();
        while ((len = fileInputStream.read(buffer)) > 0){
            outputStream.write(buffer,0,len);
        }

        fileInputStream.close();
        outputStream.close();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

添加映射

启动Tomcat，访问！

3.2、实现重定向

一个web资源收到客户端请求后，它会通知客户端去访问另一个web资源，这个过程叫重定向：

void sendRedirect(java.lang.String s) throws java.io.IOException;

public class Send extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.sendRedirect("/init");//路径发生变化
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

4、HttpServletRequest

HttpServletRequest代表客户端的请求，用户通过Http协议访问服务器，HTTP请求中的所有信息会被封装到HttpServlet，通过这个HttpServlet的方法，获得客户端的所有信息；

4.1、获取前端传递的参数和请求转发

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>HSRequest</title>
</head>
<body>

<div>
    <form action="${pageContext.request.contextPath}/request" method="post">
        用户名：<input type="text" name="username"><br>
        密码：<input type="password" name="password"><br>
        爱好：
        <input type="checkbox" name="hobbys" value="java">java
        <input type="checkbox" name="hobbys" value="JavaScript">JavaScript
        <input type="checkbox" name="hobbys" value="Oracle">Oracle
        <input type="checkbox" name="hobbys" value="Python">Python
        <br>
        <input type="submit">
    </form>
</div>

</body>
</html>

public class HSRequest extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String[] hobbys = req.getParameterValues("hobbys");
        System.out.println(username+"\t"+password+"\t"+ Arrays.toString(hobbys));
        req.getRequestDispatcher("/success.jsp").forward(req,resp);

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>成功页面</title>
</head>
<body>
<h1>登录成功</h1>
<a href="${pageContext.request.contextPath}/index.jsp">首页</a>
</body>
</html>

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