JavaWeb基础系列（一）Servlet

浅浅的花香味﹌ 2021-09-26 22:36 416阅读 0赞

# 一、什么是Servlet #

Servlet 运行在服务端的Java小程序，是sun公司提供一套规范（接口），用来处理客户端请求、响应给浏览器的动态资源。但servlet的实质就是java代码，通过java的API动态的向客户端输出内容。

# 二、Servlet快速入门 #

## 2.1、实现步骤： ##

（1）创建类实现Servlet接口 （2）覆盖尚未实现的方法—Service方法 （3）在web.xml进行Servlet的配置 示例：开发环境为IDEA 创建Servlet并覆盖Service方法：

package com.servlet;
    
    import javax.servlet.*;
    import java.io.IOException;
    
    /** * Created by long on 2018/6/12. */
    public class QuickStartServlet implements Servlet{ 
        @Override
        public void init(ServletConfig servletConfig) throws ServletException {
    
        }
        @Override
        public ServletConfig getServletConfig() {
            return null;
        }
        @Override
        public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
            System.out.println("QuickStartServlet");
        }
        @Override
        public String getServletInfo() {
            return null;
        }
        @Override
        public void destroy() {
    
        }
    }

在web.xml进行Servlet的配置：

<?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_3_1.xsd" version="3.1">
        <servlet>
            <servlet-name>QuickStartServlet</servlet-name>
            <servlet-class>com.servlet.QuickStartServlet</servlet-class>
        </servlet>
        <servlet-mapping>
            <servlet-name>QuickStartServlet</servlet-name>
            <url-pattern>/QuickStartServlet</url-pattern>
        </servlet-mapping>
    </web-app>

项目运行起来： 输入网址：http://localhost:8080/QuickStartServlet 运行结果为：打印QuickStartServlet

## 2.2、Servlet的API和生命周期 ##

public class QuickStartServlet implements Servlet{ 
        @Override
        //Servlet对象创建的时候执行
        //servletConfig：代表该Servlet对象的配置信息
        //获取该Servlet的Name：servletConfig.getServletName();
        //获取该Servlet的初始化参数：servletConfig.getParameter("key");
        //获得ServletContext对象：servletConfig.getServletContext();
        public void init(ServletConfig servletConfig) throws ServletException {
            System.out.println("init running");
        }
        @Override
        //每次请求都会执行
        public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
            System.out.println("QuickStartServlet running");
        }
        @Override
        //Servlet销毁的时候执行
        public void destroy() {
            System.out.println("destory running");
        }
        @Override
        public String getServletInfo() {
            return null;
        }
        @Override
        public ServletConfig getServletConfig() {
            return null;
        }
    }

Servlet生命周期：  
（1）Servlet何时创建：  
默认第一次访问Servlet时创建该对象  
如果加上`<load-on-startup>1（数字表示优先级）</load-on-startup>` Servlet对象在服务器启动时就创建。  
（2）Servlet何时销毁：  
服务器关闭，Servlet就销毁了  
（3）每次访问必然执行的方法：  
service(ServletRequest servletRequest, ServletResponse servletResponse)方法  
ServletRequest：代表请求，认为ServletRequest内部封装的是http请求的内容  
servletResponse：代表响应，认为要封装的是响应的内容

## 2.3、继承HttpServlet ##

但在实际开发中，我们不会直接去实现Servlet接口，因为那样需要覆盖的方法太多，我们一般创建类继承HttpServlet：实现步骤：  
（1）创建类继承HttpServlet类  
（2）覆盖doGet和doPost  
（3）在web.xml中进行servlet的配置  
代码示例：

package com.servlet;
    
    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet; 
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.io.IOException;
    
    /** * Created by long on 2018/6/13. */
    @WebServlet(
            name = "QuickServlet2",
            urlPatterns = {
       "/hello"}
    )
    public class QuickServlet2 extends javax.servlet.http.HttpServlet { 
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            response.getWriter().write("doGet Running...");
        }
    
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            doPost(request,response);
        }
    }

我们注意到，这个代码里面没有service()方法，既然子类没有service()方法，于是呢,当发生请求的时候去父类HttpServlet里面执行service()方法。这个service方法里面会判断`if(method.equals("GET"))`等等识别是Get还是Post方法，由于子类重写了doGet和doPost方法，所以呢，会去咱们自己定义的Servlet中执行doGet和doPost方法。因此输出doGet Running…

# 三、案例：用户登陆功能 #

Html部分代码：

<!DOCTYPE html>
    <html>
        <head>
            <meta charset="UTF-8">
            <title></title>
        </head>
        <body>
            <form action="/login" method="post">
                用户名:<input type="text" name="username"/><br />
                密码：<input type="password" name="password" /><br />
                <input type="submit" value="登录" />
            </form>
        </body>
    </html>

数据库配置：

package com.naruto.utils;
    
    import java.sql.Connection;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.sql.Statement;
    
    import javax.sql.DataSource;
    
    import com.mchange.v2.c3p0.ComboPooledDataSource;
    
    public class DataSourceUtils { 
    
        private static DataSource dataSource = new ComboPooledDataSource();
    
        private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>();
    
        // 直接可以获取一个连接池
        public static DataSource getDataSource() {
            return dataSource;
        }
    
        // 获取连接对象
        public static Connection getConnection() throws SQLException {
    
            Connection con = tl.get();
            if (con == null) {
                con = dataSource.getConnection();
                tl.set(con);
            }
            return con;
        }
    
        // 开启事务
        public static void startTransaction() throws SQLException {
            Connection con = getConnection();
            if (con != null) {
                con.setAutoCommit(false);
            }
        }
    
        // 事务回滚
        public static void rollback() throws SQLException {
            Connection con = getConnection();
            if (con != null) {
                con.rollback();
            }
        }
    
        // 提交并且 关闭资源及从ThreadLocall中释放
        public static void commitAndRelease() throws SQLException {
            Connection con = getConnection();
            if (con != null) {
                con.commit(); // 事务提交
                con.close();// 关闭资源
                tl.remove();// 从线程绑定中移除
            }
        }
    
        // 关闭资源方法
        public static void closeConnection() throws SQLException {
            Connection con = getConnection();
            if (con != null) {
                con.close();
            }
        }
    
        public static void closeStatement(Statement st) throws SQLException {
            if (st != null) {
                st.close();
            }
        }
    
        public static void closeResultSet(ResultSet rs) throws SQLException {
            if (rs != null) {
                rs.close();
            }
        }
    
    }

c3p0-config.xml：

<?xml version="1.0" encoding="UTF-8"?>
    <c3p0-config>
        <default-config>
            <property name="user">root</property>
            <property name="password">root</property>
            <property name="driverClass">com.mysql.jdbc.Driver</property>
            <property name="jdbcUrl">jdbc:mysql://192.168.1.104:3306/web13?characterEncoding=utf-8</property>
        </default-config> 
    </c3p0-config>

实体类：

package com.naruto.domain;
    
    /** * Created by long on 2018/6/14. */
    public class User { 
        private int id;
        private String username;
        private  String password;
        private String email;
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        public String getPassword() {
            return password;
        }
    
        public void setPassword(String password) {
            this.password = password;
        }
    
        public String getUsername() {
            return username;
        }
    
        public void setUsername(String username) {
            this.username = username;
        }
    
        public String getEmail() {
            return email;
        }
    
        public void setEmail(String email) {
            this.email = email;
        }
    
        @Override
        public String toString() {
            return "User{" +
                    "id=" + id +
                    ", username='" + username + '\'' +
                    ", password='" + password + '\'' +
                    ", email='" + email + '\'' +
                    '}';
        }
    }

Servlet代码：

package com.servlet;
    
    
    import com.naruto.domain.User;
    import com.naruto.utils.DataSourceUtils;
    import org.apache.commons.dbutils.QueryRunner;
    import org.apache.commons.dbutils.handlers.BeanHandler;
    
    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.io.IOException;
    import java.sql.SQLException;
    
    /** * Created by long on 2018/6/13. */
    @WebServlet(
            name = "QuickServlet2",
            urlPatterns = {
       "/login"}
    )
    public class QuickServlet2 extends javax.servlet.http.HttpServlet { 
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            //获取用户名密码
            String username = request.getParameter("username");
            String password = request.getParameter("password");
    
            QueryRunner runner = new QueryRunner(DataSourceUtils.getDataSource());
            String sql = "select * from user where username=? and password=?";
            User user = null;
            try {
               user = runner.query(sql, new BeanHandler<User>(User.class), username, password);
            } catch (SQLException e) {
                e.printStackTrace();
            }
            //用户登陆成功
            if(user != null){
                response.getWriter().write(user.toString());
            }
            else{
                response.getWriter().write("Login Error");
            }
        }
    
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            doPost(request,response);
        }
    }

运行结果：  
![这里写图片描述][70]  
输入用户名密码结果：  
![这里写图片描述][70 1]

# 四、ServletContext对象 #

## 4.1、ServletContext对象简介 ##

代表一个web应用的环境（上下文）对象。内部封装的是该web应用的信息。一个web应用只有一个ServletContext对象。  
ServletContext对象的生命周期？  
创建的时间：该web应用被加载到服务器（服务器启动）  
销毁的时间：服务器关闭（应用被卸载）

## 4.2、怎么获得ServletContext对象 ##

方法一：ServletContext servletContext = config.getServletContext();  
方法二：ServletContext servletContext = this.getServletContext();

## 4.2、ServletContext作用 ##

（1）获取web应用的初始化参数

<context-param>
            <param-name>driver</param-name>
            <param-value>com.mysql.jdbc.Driver</param-value>
        </context-param>

获取该参数：

@WebServlet(name = "Servlet3",
            urlPatterns = {
       "/servlet3"}
    )
    public class Servlet3 extends HttpServlet { 
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            //获得ServletContext对象
            ServletContext context = getServletContext();
            //1、获得初始化参数
            String initParameter = context.getInitParameter("driver");
            System.out.print(initParameter);
        }
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
        }
    }

**（2）获得web应用中任何资源的绝对路径**  
方法：String path = context.getRealPath(“资源”);（相对于该web应用的相对地址即相对于WEB-INF）  
在获取src(classes)下的资源是可以用同类加载器–专门加载classes下的文件  
ContextServlet.class.getClassLoader.getResource(“参数”).getPath();  
该参数是一个相对地址，相对classes的地址  
**（3）ServletContext是一个域对象**  
ServletContext域对象的作用范围：整个web应用（所有的web资源可以随意向ServletContext域中存取数据，数据可以共享。  
存值：this.getServletContext.setAttribute(name,value);  
取值：this.getServletContext.getAttribute(name);  
域对象的通用方法：  
setAttribute(String name,Object obj);  
getAttribute(String name);  
removeAttribute(String name);

# 五、案例：记录成功登陆的人次 #

package com.servlet;
    
    
    import com.naruto.domain.User;
    import com.naruto.utils.DataSourceUtils;
    import org.apache.commons.dbutils.QueryRunner;
    import org.apache.commons.dbutils.handlers.BeanHandler;
    
    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.io.IOException;
    import java.sql.SQLException;
    
    /** * Created by long on 2018/6/13. */
    @WebServlet(
            name = "QuickServlet2",
            urlPatterns = {
       "/login"}
    )
    public class QuickServlet2 extends javax.servlet.http.HttpServlet { 
        @Override
        public void init() throws ServletException {
           //在ServletContext域中存一个数据count
            int count = 0;
            this.getServletContext().setAttribute("count",count);
        }
    
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            //获取用户名密码
            String username = request.getParameter("username");
            String password = request.getParameter("password");
    
            QueryRunner runner = new QueryRunner(DataSourceUtils.getDataSource());
            String sql = "select * from user where username=? and password=?";
            User user = null;
            try {
               user = runner.query(sql, new BeanHandler<User>(User.class), username, password);
            } catch (SQLException e) {
                e.printStackTrace();
            }
            //用户登陆成功
            if(user != null){
                //从ServletContext域中取出count进行++运算
                Integer count = (Integer) this.getServletContext().getAttribute("count");
                count++;
                response.getWriter().write(user.toString()+"you are"+count);
                this.getServletContext().setAttribute("count",count);
            }
            else{
                response.getWriter().write("Login Error");
            }
        }
    
        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            doPost(request,response);
        }
    }

运行结果：  
![这里写图片描述][70 2]  
登陆两个之后显示：  
![这里写图片描述][70 3]

--------------------

**Java完整博客目录如下：[博客文章目录汇总][Link 1]**

> **转载请标明出处，原文地址：[https://blog.csdn.net/weixin\_41835916][https_blog.csdn.net_weixin_41835916] 如果觉得本文对您有帮助，请点击顶支持一下，您的支持是我写作最大的动力，谢谢。  
> ![这里写图片描述][70 4]**

[70]: /images/20210923/b45b49dce7ff4b1db7c965abf91b69b1.png
[70 1]: /images/20210923/bf731f9a01bb4f94bbf276e05cc25e27.png
[70 2]: /images/20210923/bfb8c24dbefa420795a38caa0bdb96a0.png
[70 3]: /images/20210923/4cb112472b00423183cf64d8e21c20ed.png
[Link 1]: https://blog.csdn.net/weixin_41835916/article/details/81879208
[https_blog.csdn.net_weixin_41835916]: https://blog.csdn.net/weixin_41835916
[70 4]: /images/20210923/47dc0989b47c4f649373e38e26516e0b.png