Servlet入门

#:前言:

很多人说在2020学习Servlet就如同98年入国军，就现在的技术栈来讲在工作中用到Servlet的机会确实很小，不过熟悉Servlet能帮助我们更好的深入了解当前流行技术的一些细节，如当前火热的Spring框架中就有Servlet的存在。

#:Servlet是什么?

Servlet是Java Applet的简称,指用java编写运行在服务器上的小应用程序，用来处理客户端请求并动态生成响应内容。

1.从客户端发起请求到服务器
2.服务器(一般是Tomcat)将请求发送给Servlet
3.Servlet动态生成响应内容发送给服务器
4.服务器将响应返回给客户端

#编写Servlet程序

package com.mike;

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;

/**
 * @author mike
 * @date 2020-11-12
 * @desc 使用注解@WebServlet配置servlet
 */

@WebServlet(name="MyServlet",value="/helloServlet")
public class MyServlet extends HttpServlet { 
	public MyServlet() {
		super();
	}

	@Override
	protected void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		System.out.println("get 请求执行");
	}

	@Override
	protected void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		System.out.println("post 请求执行");
	}
}

1.定义一个类MyServlet继承HttpServlet类并重写其中doGet()及doPost方法,并返回一句话"get 请求执行"
2.使用注解@WebServlet(name="MyServlet",value="/helloServlet")来指定访问此Servlet的路由名称,其中name为Servlet名称，value为路由名称
3.编译后代码后，在tomcat服务器启动状态下打开浏览器输入 http://localhost:8080/helloServlet
4.观察tomacat catalina日志中打印日志

#Servlet生命周期

1.容器加载MyServlet.class字节码文件
2.容器调用其构造方法
3.容器调用init()方法初始化,在此会初始化ServletConfig对象
4.容器调用service()方法，并调用doGet()或doPost()方法处理客户端端请求
5.容器关闭时调用destroy()方法释放资源

#ServletConfig对象
在init()方法调用时读取web.xml文件中init-param配置信息并初始化ServletConfig对象
打开web.xml文件添加一个servlet

   <servlet>
        <!-- servlet-name属性表示当前servlet的内部自定义名称-->
        <servlet-name>config</servlet-name>
        <!-- servlet-class属性表示servlet的真实类名称-->
        <servlet-class>com.mike.ServletConfigDemo</servlet-class>
        <!-- 初始参数：这些参数会在加载web应用的时候，封装到ServletConfig对象中 -->
        <init-param>
            <param-name>name</param-name>
            <param-value>mike</param-value>
        </init-param>
        <init-param>
            <param-name>password</param-name>
            <param-value>123456</param-value>
        </init-param>
        <!-- 让servlet对象自动加载，注意：init-param要在自动加载之前 -->
        <load-on-startup>1</load-on-startup>
    </servlet>

    <!-- 配置servlet和路由映射关系-->
    <servlet-mapping>
        <!-- servlet-name为上文中servlet内部名-->
        <servlet-name>config</servlet-name>
        <!-- 将当前servlet绑定到路由名称可在地址栏访问-->
        <url-pattern>/config</url-pattern>
    </servlet-mapping>

编写ServletConfigDemo类文件

package com.mike;

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

/**
 * @author mike
 * @date 2020-11-15
 * desc 使用ServletConfig对象获取初始化参数
 */
public class ServletConfigDemo extends HttpServlet {
	public ServletConfigDemo() {
		super();
	}

	@Override
	public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		doPost(request,response);
	}

	@Override
	public void doPost(HttpServletRequest request,HttpServletResponse  response)throws ServletException,IOException {
		ServletConfig config = getServletConfig();
		String name = config.getInitParameter("name"); //利用name获取配置值
		String password = config.getInitParameter("password");
		System.out.println("name: " + name + " password:" + password);
		response.setCharacterEncoding("UTF-8");
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		out.println("<HEAD><TITLE>初始化信息</TITLE></HEAD>");
		out.println("姓名：" + name + "<br>");
		out.println("密码：" + password + "<br>");
		out.flush();
		out.close();
	}
}

访问http://localhost:8080/config，观察输出

#ServletContext对象

ServletContext是servlet上下文对象，各Servlet之间可以通过ServletContext共享信息
在web.xml添加context-param配置

 <!--注意，context-param必须放在servlet配置外面，它不属于任何servlet，而属于整个应用配置-->
 <context-param>
        <param-name>name</param-name>
        <param-value>mike</param-value>
    </context-param>
    <context-param>
        <param-name>password</param-name>
        <param-value>8888</param-value>
    </context-param>

新建ServletContextDemo类文件

package com.mike;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * @author mike
 * @date 2020-11-18
 * desc 使用ServletContext获取整个web应用的配置信息
 */
public class ServletContextDemo extends HttpServlet {
	public ServletContextDemo() {
		super();
	}

	@Override
	public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		doPost(request,response);
	}

	public void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		ServletContext context = getServletContext();
		Enumeration<String> names = context.getInitParameterNames();
		while(names.hasMoreElements()) {
			String name = names.nextElement();
			String value = context.getInitParameter(name);
			System.out.println(name+":"+value);
		}
	}
}

访问http://localhost:8080/context 查看输出信息

新建ServletContextDemo2类文件

package com.mike;

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

/**
 * @author mike
 * @date 2020-11-18
 * desc 测试多个servlet利用context通信
 */

@WebServlet(name="ServletContextDemo2",value = "/context2")
public class ServletContextDemo2 extends HttpServlet{
	public ServletContextDemo2() {
		super();
	}

	@Override
	public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException{
		doPost(request,response);
	}

	@Override
	public void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		ServletContext context = getServletContext();
		String name = "xiuyue";
		String password = "1234";
		context.setAttribute("name",name);
		context.setAttribute("password",password);
	}
}

最后新建新建ServletContextDemo3类文件

package com.mike;

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

/**
 * @author mike
 * @date 2020-11-18
 * desc 测试多个servlet利用context通信
 */

@WebServlet(name="ServletContextDemo3",value = "/context3")
public class ServletContextDemo3 extends HttpServlet{
	public ServletContextDemo3() {
		super();
	}

	@Override
	public void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException{
		doPost(request,response);
	}

	@Override
	public void doPost(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		ServletContext context = getServletContext();
		response.setCharacterEncoding("UTF-8");
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		out.println("姓名" + context.getAttribute("name") + "<br>");
		out.println("密码" + context.getAttribute("password"));
		out.close();
	}
}

分别运行http://localhost:8080/context2,http://localhost:8080/context3查看输出信息

姓名xiuyue
密码1234

当运行context2时会修改第一次定义的配置信息，而context3会将修改后信息读取，这就说明ServletContext是对所有Servlet共享的

#Servlet重定向

package com.mike;

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;

/**
 * @author mike
 * @date 2020-11-12
 * @desc 测试servlet重定向
 */

@WebServlet(name="MyServlet",value="/helloServlet")
public class MyServlet extends HttpServlet {
	public MyServlet() {
		super();
	}

	@Override
	protected void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException {
		response.sendRedirect("http://www.baidu.com");
	}

}

访问http://localhost:8080/helloServlet查看结果，网页被重定向到baidu

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原文地址：https://www.cnblogs.com/gy1010/p/14008164.html