狂神说JavaJavaWeb入门到实战--Servlet详解

Posted 2021-11-04 闲言_

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狂神视频地址

https://www.bilibili.com/video/BV12J411M7Sj?p=2

1.Servlet 简介

Servlet 就是sun公司开发动态web的一门技术
sun 在这些API 中提供了一个接口：Servlet，如果你想开发一个Servlet 程序，只需要完成两个步骤

编写一个类，实现Servlet 接口
把开发号好的Java类部署到服务器中

把实现了Servlet接口的Java 程序叫做：Servlet

2.HelloServlet

Servlet 接口有两个默认的实现类：HttpServlet、GenericServlet

关于mavne 父子工程的理解
父项目中会有

<modules>
    <module>servlet-01</module>
</modules>

子项目中会有

 <parent>
  <artifactId>servlet</artifactId>
  <groupId>cn.bloghut</groupId>
  <version>1.0-SNAPSHOT</version>
</parent>

父项目中的jar包子项目可以直接使用

子项目中的jar包父项目不能使用

Zi extends Fu

编写一个普通类，实现Servlet接口，这里我们直接继承httpServlet

public class HelloServlet extends HttpServlet {
    
}

public class HelloServlet extends HttpServlet {

    //由于get或者post只是请求实现的不同的方法，可以相互调用，业务逻辑都一样

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

//        ServletOutputStream sos = resp.getOutputStream();
        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);
    }
}

3.编写Servlet的映射

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

<!DOCTYPE web-app PUBLIC
        "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
        "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
          http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
         version="3.0">

  <servlet>
    <servlet-name>hello</servlet-name>
    <servlet-class>cn.bloghut.servlet.HelloServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>
  
  
</web-app>

配置tomcat

测试

4.Servlet 原理

Servlet 是由web服务器调用，web服务器在收到浏览器请求之后，会调用相关API对请求处理。

5.Mapping 问题：

一个请求（Servlet）可以指定一个映射路径

<servlet>
  <servlet-name>hello</servlet-name>
  <servlet-class>cn.bloghut.servlet.HelloServlet</servlet-class>
</servlet>

<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>/hello</url-pattern>
</servlet-mapping>

一个请求（Servlet）可以指定多个映射路径

<servlet>
  <servlet-name>hello</servlet-name>
  <servlet-class>cn.bloghut.servlet.HelloServlet</servlet-class>
</servlet>

<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>/hello1</url-pattern>
</servlet-mapping>

<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>/hello2</url-pattern>
</servlet-mapping>

<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>/hello3</url-pattern>
</servlet-mapping>

一个请求（Servlet）可以指定通用映射路径

<!--默认请求路径-->
<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>/*</url-pattern>
</servlet-mapping>

指定一些后缀或者前缀等

<servlet-mapping>
  <servlet-name>hello</servlet-name>
  <url-pattern>*.do</url-pattern>
</servlet-mapping>

优先级问题
指定了固有的映射路径优先级最高，如果找不到就会走默认的处理请求；

6.ServletContext

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

6.1.共享数据

我在这个Servlet中保存的数据

放置数据的Servlet

/**
 * @author by 闲言
 * @classname HelloServlet
 * @description 放置数据
 * @date 2021/9/11 14:23
 */
public class setData extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("hello");

        ServletContext servletContext = this.getServletContext();

        String username = "闲言";//数据
        servletContext.setAttribute("username",username);//将一个数据保存在ServletContext中  名为：username  值为 闲言
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}


读取数据的Servlet
/**
 * @author by 闲言
 * @classname GetServlet
 * @description 获取数据
 * @date 2021/9/16 21:32
 */
public class GetData extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();

        String username = (String) context.getAttribute("username");

        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html");
        PrintWriter writer = resp.getWriter();
        writer.write(username);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

6.2获取初始化参数

<context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/school</param-value>
</context-param>

public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取上下文对象
        ServletContext context = this.getServletContext();
        //获取初始参数
        String url = context.getInitParameter("url");
        
        System.out.println("url: "+url);
    }

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

6.3 请求转发

路径不会发生变化

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    ServletContext context = this.getServletContext();
    //请求转发  /get 转发的路径
    RequestDispatcher dispatcher = context.getRequestDispatcher("/get");
    //调用forward 实现请求转发
    dispatcher.forward(req,resp);

}

6.4 读取配置文件

Properties
在resources 目录下新建properties
在java 目录下新建properties
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；
思路：需要一个文件流

properties文件
username=xy123
password=123

代码

public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream stream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/cn/bloghut/servlet/aa.properties");
//        InputStream stream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties pt = new Properties();
        pt.load(stream);
        String username = pt.getProperty("username");
        String password = pt.getProperty("password");
        resp.getWriter().println(username+":"+password);
    }

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

效果

maven资源导出失败问题

<build>
    <resources>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.xml</include>
                <include>**/*.properties</include>
            </includes>
        </resource>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
        </resource>
    </resources>
</build>

6.5HttpServletResponse

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

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

简单分类

负责向浏览器发送数据的方法

public ServletOutputStream getOutputStream() throws IOException;
public PrintWriter getWriter() throws IOException;

负责向浏览器写一些响应头的方法

public void setCharacterEncoding(String charset);
public void setContentLength(int len);
public void setContentLengthLong(long len);
public void setContentType(String type);

public void setDateHeader(String name, long date);
public void addDateHeader(String name, long date);
public void setHeader(String name, String value);
public void addHeader(String name, String value);
public void setIntHeader(String name, int value);
public void addIntHeader(String name, int value);

响应的状态码常量

public static final int SC_CONTINUE = 100;
public static final int SC_SWITCHING_PROTOCOLS = 101;
public static final int SC_OK = 200;
public static final int SC_CREATED = 201;
public static final int SC_ACCEPTED = 202;
public static final int SC_NON_AUTHORITATIVE_INFORMATION = 203;
public static final int SC_NO_CONTENT = 204;
public static final int SC_RESET_CONTENT = 205;
public static final int SC_PARTIAL_CONTENT = 206;
public static final int SC_MULTIPLE_CHOICES = 300;
public static final int SC_MOVED_PERMANENTLY = 301;
public static final int SC_MOVED_TEMPORARILY = 302;
public static final int SC_FOUND = 302;
public static final int SC_SEE_OTHER = 303;
public static final int SC_NOT_MODIFIED = 304;
public static final int SC_USE_PROXY = 305;
public static final int SC_TEMPORARY_REDIRECT = 307;
public static final int SC_BAD_REQUEST = 400;
public static final int SC_UNAUTHORIZED = 401;
public static final int SC_PAYMENT_REQUIRED = 402;
public static final int SC_FORBIDDEN = 403;
public static final int SC_NOT_FOUND = 404;
public static final int SC_METHOD_NOT_ALLOWED = 405;
public static final int SC_NOT_ACCEPTABLE = 406;
public static final int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
public static final int SC_REQUEST_TIMEOUT = 408;
public static final int SC_CONFLICT = 409;
public static final int SC_GONE = 410;
public static final int SC_LENGTH_REQUIRED = 411;
public static final int SC_PRECONDITION_FAILED = 412;
public static final int SC_REQUEST_ENTITY_TOO_LARGE = 413;
public static final int SC_REQUEST_URI_TOO_LONG = 414;
public static final int SC_UNSUPPORTED_MEDIA_TYPE = 415;
public static final int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
public static final int SC_EXPECTATION_FAILED = 417;
public static final int SC_INTERNAL_SERVER_ERROR = 500;
public static final int SC_NOT_IMPLEMENTED = 501;
public static final int SC_BAD_GATEWAY = 502;
public static final int SC_SERVICE_UNAVAILABLE = 503;
public static final int SC_GATEWAY_TIMEOUT = 504;
public static final int SC_HTTP_VERSION_NOT_SUPPORTED = 505;

下载文件

1.向浏览器输出消息
2.下载文件
2.1.要获取文件的路径
2.2下载的文件名是啥
2.3设置想办法让浏览器能够支持下载我们需要的东西
2.4获取下载文件的输入流
2.创建缓冲区
2.5获取OutputStream对象
2.6将FileOutputStream流写入到buff缓冲区
2.7使用OutputStream将缓冲区中的数据输出到客户端！

 @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //要获取文件的路径
//        String realPath = this.getServletContext().getRealPath("/1.png");
        String realPath = 以上是关于狂神说JavaJavaWeb入门到实战--Servlet详解的主要内容，如果未能解决你的问题，请参考以下文章 
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