EL表达式&JSTL&Filter&Upload&Download

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哆啦A梦

Java Web

JSP内置对象作用域

page:对应的内置对象是pageContext、只在当前页面有效
request:对应的内置对象是request、在一次请求内有效
session:对应的内置对象是Session、在一次会话内有效
appliaction：对应的内置对象是Appliaction、对应整个Web应用

  • 作用范围

page < request < session < appliaction

EL表达式

替代JSP页面中数据访问时的复杂编码、方便取出域中对象保存的数据

  • 表达式语法
    ${变量名}
  • EL表达式默认查找顺序

pageContext ——> request ——> session ——>appliaction

  • EL表达式指定查找顺序

pageContext:${pageScope.name}

request:${requestScope.name}

session:${sessionScope.name}

appliaction:${appliactionScope.name}

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public class User {

    private Integer id;
    private String name;
    private Double score;
    private Student student;

    public User(Integer id, String name, Double score, Student student) {
        this.id = id;
        this.name = name;
        this.score = score;
        this.student = student;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Double getScore() {
        return score;
    }

    public void setScore(Double score) {
        this.score = score;
    }

    public Student getStudent() {
        return student;
    }

    public void setStudent(Student student) {
        this.student = student;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", score=" + score +
                ", student=" + student +
                '}';
    }
}
public class Student {
}
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<%@ page import="com.dream.xiaobo.EL.User" %>
<%@ page import="com.dream.xiaobo.EL.Student" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>EL</title>
</head>
<body>
    <%
        Student student = new Student();
        User user = new User(1,"小博",99.9, student);
        pageContext.setAttribute("user",user);
    %>
    <table>
        <tr>
            <th>编号</th>
            <th>姓名</th>
            <th>分数</th>
            <th>地址</th>
        </tr>
        <tr>
            <td>${pageContext.user.id}</td>
            <td>${pageContext.user.name}</td>
            <td>${pageContext.user.score}</td>
            <td>${pageContext.user.student}</td>
        </tr>
    </table>
</body>
</html>
  • EL表达式支持运算符

&& –> and
|| –> or
! –> not
== ->> eq
< –> lt
–> gt
<= –> le
= –> ge
empty（判断是否为空） –> 空是变量为null、长度为0的String、size为0的集合

JSTL

使用标签完成一些逻辑处理、不在出现JSP脚本穿插情况

  • JSTL使用

1、导入两个jar包（jstl.jar standard.jar）、放在web/WEB-INF

2、在JSP页面引入

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<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

3、直接使用

  • JSTL优点

提供统一标签
用于编写各种动态功能

  • JSTL常用标签

set:向域中对象添加数据

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<c:set var="name" value="set值" scope="request"></c:set>
    ${requestScope.name}

out:输出域对象中的数据

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<c:out value="${name}" default="未定义"></c:out>

remove:删除域对象中的数据

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<c:remove var="name" scope="page"></c:remove>

catch:异常捕获

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<c:catch var="exception">
            <%
                int a = 10/0;
            %>
        </c:catch>
        ${exception}

forEach:迭代标签

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<c:forEach items="${map}" var="user">
            <tr>
                <td>${user.id}</td>
                <td>${user.name}</td>
                <td>${user.score}</td>
            </tr>
</c:forEach>

Filter

  • 功能

拦截传入的请求和传出的响应
修改或以某种方式处理正在客户端和服务器之间交换的数据流

  • Filter接口的实现
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public class MyFilter implements Filter {

@Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
    }
  }
  • Filter配置方法

web.xml

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<filter>
    <filter-name>myFilter</filter-name>
    <filter-class>com.dream.xiaobo.filter.MyFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>myFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

注解

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@WebFilter("*")

注解可以简化配置、但是web.xml配置可以控制Filter的执行先后顺序、而注解无法控制Filter执行先后顺序

  • Filter基本的使用场景

统一处理资源乱码

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request.setCharacterEncoding("UTF-8");
response.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");

屏蔽敏感词汇

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String name = request.getParameter("name");
name.replaceAll("敏感词","***");
request.setAttribute("name",name);
filterChain.doFilter(request, response);

控制资源访问权限

文件上传

  • 流程

1、input设置为file

2、form表单method设置为post、get请求发送的是文件名给服务端、而不是文件本身、所以必须用post

3、form表单enctype设置为multipart/form-data,以二进制的形式传输数据

  • 底层大致流程
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<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>首页</title>
  </head>
  <body>
  <form action="uploadServlet" enctype="multipart/form-data" method="post">
      <input type="text" name="desc" /></br>
      <input type="file" name="img" /></br>
      <input type="submit" value="上传">
  </form>
  </body>
</html>
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@WebServlet("/uploadServlet")
public class UploadServlet extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        ServletInputStream inputStream = req.getInputStream();
        InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);

        String path = req.getServletContext().getRealPath("file/xiaobo.txt");

        OutputStream outputStream = new FileOutputStream(path);
        OutputStreamWriter outputStreamWriter = new OutputStreamWriter(outputStream);
        BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);

        String temp = "";
        while((temp = bufferedReader.readLine()) != null){
            System.out.println(temp);
            bufferedWriter.write(temp);
        }
        bufferedWriter.close();
        outputStreamWriter.close();
        outputStream.close();
        bufferedReader.close();
        inputStreamReader.close();
        inputStream.close();
    }
}
  • 组件实现流程

web/WEB-INF下导入jar
commons-fileupload-1.3.1
commons-io-2.5

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<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>首页</title>
  </head>
  <body>
  <form action="upload" enctype="multipart/form-data" method="post">
      <input type="text" name="desc" /></br>
      <input type="file" name="img" /></br>
      <input type="submit" value="上传">
  </form>
  </body>
</html>

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@WebServlet("/upload")
public class MyUpload extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        try {
            DiskFileItemFactory diskFileItemFactory = new DiskFileItemFactory();
            ServletFileUpload fileUpload = new ServletFileUpload(diskFileItemFactory);
            List<FileItem> fileItems = fileUpload.parseRequest(req);
            for(FileItem fileItem: fileItems){
                if(fileItem.isFormField()){
                    String name = fileItem.getName();
                    String fieldName = fileItem.getFieldName();
                    System.out.println(name + ":" + fieldName);
                }else{
                    String name = fileItem.getName();
                    InputStream inputStream = fileItem.getInputStream();
                    String path = req.getServletContext().getRealPath("file/" + name);
                    System.out.println(path);
                    OutputStream outputStream = new FileOutputStream(path);
                    int temp = 0;
                    while((temp = inputStream.read()) != -1) {
                        System.out.println(temp);
                        outputStream.write(temp);
                    }
                    outputStream.close();
                    inputStream.close();
                    System.out.println("上传成功");
                }
            }

        } catch (FileUploadException e) {
            e.printStackTrace();
        }
    }
}

文件下载

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<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>下载</title>
</head>
<body>
    <a href="/download?type=png">1.png</a>
    <a href="/download?type=txt">xiaobo.txt</a>
</body>
</html>
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@WebServlet("/download")
public class MyDownload extends HttpServlet {


    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        String type = req.getParameter("type");

        String fileName = "";

        switch (type){
            case "png":
                fileName = "微信截图_20210618143454.png";
                break;
            case "txt":
                fileName = "xiaobo.txt";
                break;
        }
        //设置响应⽅式
        resp.setContentType("application/x-msdownload");
        //设置下载之后的⽂件名
        resp.setHeader("Content-Disposition","attachment;filename="+fileName);
        //获取输出流
        OutputStream outputStream = resp.getOutputStream();
        String path = req.getServletContext().getRealPath("file/"+fileName);
        InputStream inputStream = new FileInputStream(path);
        int temp = 0;
        while((temp=inputStream.read())!=-1){
            outputStream.write(temp);
        }
        inputStream.close(); outputStream.close();
    }
}

正确的开始、微小的长进、然后持续、嘿、我是小博、带你一起看我目之所及的世界……

-------------本文结束 感谢您的阅读-------------

本文标题:EL表达式&JSTL&Filter&Upload&Download

文章作者:小博

发布时间:2021年06月18日 - 17:04

最后更新:2021年06月18日 - 17:06

原始链接:https://codexiaobo.github.io/posts/3944027832/

许可协议: 署名-非商业性使用-禁止演绎 4.0 国际 转载请保留原文链接及作者。