一遍博客带你上手Servlet

概念

Servlet其实就是Java提供的一门动态web资源开发技术。本质就是一个接口。

快速入门

1. 创建web项目，导入servlet依赖坐标（注意依赖范围scope，是provided，只在编译和测试时使用）
   
   pom文件：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>tomcat-demo2</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.1.0</version>
            <scope>provided</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.tomcat.maven</groupId>
                <artifactId>tomcat7-maven-plugin</artifactId>
                <version>2.2</version>
<!--                <configuration>-->
<!--                    <port>80</port>&lt;!&ndash; 访问端口号 &ndash;&gt;-->
<!--                    <path>/</path>&lt;!&ndash; 项目访问路径 &ndash;&gt;-->
<!--                </configuration>-->
            </plugin></plugins>
    </build>

    <packaging>war</packaging>

</project>

1. 创建：定义一个类，实现servlet接口，重写接口中的方法
   
   在Java目录下创建类，类名后加入implements Servlet实现接口，再CTRL+o重写父类方法。

2. 配置：在类上使用@WebServlet注解，配置该servlet的访问路径
   
   再在类名写写明路径的注解。2、3步代码如下：

package com.itheijma.web;

import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import java.io.IOException;

@WebServlet("/demo1")
public class ServletDemo1 implements Servlet {

    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("servlet hello world");

    }

    public String getServletInfo() {
        return null;
    }

    public void destroy() {

    }

    public void init(ServletConfig servletConfig) throws ServletException {

    }

    public ServletConfig getServletConfig() {
        return null;
    }
}

1. 访问，启动tomcat，在浏览器中访问
   
   随后在网页中访问demo1，控制台出现要输出的信息，说明servlet编写成功。
   
   

Servlet执行流程

浏览器根据路径向服务器发出请求，服务器再根据我们自定义的Servlet接口创建Servlet对象，再调用service方法，执行我们的代码。

Servlet生命周期

对象的生命周期指一个对象从被创建到销毁的整个过程。

servlet运行在servlet容器中，其生命周期由容器来管理，分为四个阶段：

1. 加载和实例化：默认情况下，当servlet第一次被访问时，由容器创建servlet对象。
2. 初始化：在servlet实例化之后，容器调用servlet的init（）方法初始化这个对象。完成一些如加载配置文件、创建连接等初始化的工作。注：该方法只被调用一次，因为一个servlet对象只被创建一次。
3. 请求处理：每次请求servlet时，servlet容器都会调用servlet的service（）方法对请求进行处理。
4. 服务终止：当需要释放内存或容器关闭时，容器就会调用servlet实例的destory（）方法完成资源的释放。

package com.itheijma.web;

import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import java.io.IOException;

@WebServlet(urlPatterns = "/demo2",loadOnStartup = 1)
public class ServletDemo2 implements Servlet {

    /**
     * 初始化方法
     * 1. 调用时机：默认情况下，servlet被第一次访问时调用
     *  * loadOnStartup
     * 2. 调用次数：1次
     * @param servletConfig
     * @throws ServletException
     */
    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println("init....");

    }

    /**
     * 提供服务方法
     * 1. 调用时机：每一次servlet被访问时调用
     * 2. 调用次数：多次
     *
     * @param servletRequest
     * @param servletResponse
     * @throws ServletException
     * @throws IOException
     */
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("servlet hello world");

    }

    /**
     * 销毁方法
     * 1. 调用时机：内存释放或服务器关闭的时候，servlet对象会被销毁，调用
     * 2. 调用次数：一次
     */
    public void destroy() {
        System.out.println("destory.....");

    }

    public String getServletInfo() {
        return null;
    }

    public ServletConfig getServletConfig() {
        return null;
    }
}

Servlet体系结构

Servlet——Servlet体系根接口

GenericServlet——Servlet抽象实现类

HttpServlet——对HTTP协议封装的Servlet实现类

因为我们开发的是B/S架构的web项目，都是针对HTTP协议的，所以我们自定义的Servlet，会继承HttpServlet

package com.itheijma.web;

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

@WebServlet("/demo3")
public class ServletDemo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("get...");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("post...");
    }
}

urlPattern配置

Servlet想要被访问，必须配置其访问路径（urlPattern）

package com.itheijma.web;

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

/**
 * urlPattern: 一个Servlet可以配置多个访问路径
 */
@WebServlet(urlPatterns = {"/demo4","/demo5"})
public class ServletDemo4 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("demo4 get.....");
    }

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

    }
}

package com.itheijma.web;

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

/**
 * urlPattern
 *  *目录匹配 /user/*
 */
@WebServlet(urlPatterns = "/user/*")
public class ServletDemo6 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("demo6 get.....");
    }

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

    }
}

package com.itheijma.web;

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

/**
 * urlPattern
 *  *扩展名匹配： *.do
 */
@WebServlet(urlPatterns = "*.do")
public class ServletDemo7 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("demo7 get.....");
    }

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

    }
}

package com.itheijma.web;

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

/**
 * urlPattern
 *  *任意匹配 /*  /
 */
@WebServlet(urlPatterns = "/")
public class ServletDemo8 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("demo8 get.....");
    }

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

    }
}