6、Servlet入门、ServletConfig、GenericServlet、HttpServlet、表单的action属性与项目上下文路径

Servlet

HelloWorld

package org.westos.demo;import javax.servlet.*;
import java.io.IOException;
import java.util.Enumeration;/*** @author lwj* @date 2020/8/15 9:29*/
public class MyServlet implements Servlet {private ServletConfig servletConfig;public MyServlet() {System.out.println("构造方法执行了");}@Overridepublic void init(ServletConfig servletConfig) throws ServletException {//servletConfig 配置对象，是个接口，服务器创建子类对象传递给init方法System.out.println(servletConfig);//org.apache.catalina.core.StandardWrapperFacade@51faaa70//用途：获取该Servlet的配置信息Enumeration<String> initNames = servletConfig.getInitParameterNames();while (initNames.hasMoreElements()) {String s = initNames.nextElement();String initParameter = servletConfig.getInitParameter(s);System.out.println(s + ":" + initParameter);//name:John}String servletName = servletConfig.getServletName();System.out.println(servletName);//MyServletServletContext servletContext = servletConfig.getServletContext();System.out.println(servletContext);//org.apache.catalina.core.ApplicationContextFacade@6a4b14b7//ServletContext，全局上下文对象，代表你整个Web应用//ServletContext对象包含在ServletConfig中，ServletConfig对象在初始化Servlet时由Web容器提供给Servlet//该Servlet使用ServletContext接口中定义的方法与其Servlet容器进行通信//init()方法只调用一次,默认情况下第一次访问Servlet时，才会创建，执行初始化initSystem.out.println("init方法调用了");this.servletConfig = servletConfig;}@Overridepublic ServletConfig getServletConfig() {return servletConfig;}@Overridepublic void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {System.out.println("请求到达，开始处理请求...");ServletConfig servletConfig = getServletConfig();servletResponse.getWriter().write("Hello Welcome.");}@Overridepublic String getServletInfo() {return null;}@Overridepublic void destroy() {System.out.println("destroy方法调用过了,正常关闭服务器时，在销毁servlet之前执行destroy方法");}
}

配置Servlet

<?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_4_0.xsd"version="4.0"><servlet><servlet-name>MyServlet</servlet-name><servlet-class>org.westos.demo.MyServlet</servlet-class><!--配置Servlet的初始化参数--><init-param><param-name>name</param-name><param-value>John</param-value></init-param><load-on-startup>1</load-on-startup><!--默认值-1，在第一次访问Servlet时调用--><!--只需要给一个非负数，启动服务器就创建Servlet，该数越小，优先级越高，0最高，当多个Servlet的启动优先级别相等时，默认按顺序启动--></servlet><servlet-mapping><servlet-name>MyServlet</servlet-name><url-pattern>/myServlet</url-pattern></servlet-mapping>
</web-app>

Servlet是单例多线程。

GenericServlet

Defines a generic, protocol-independent servlet。

定义一般的，与协议无关的Servlet。

package javax.servlet;import java.io.IOException;
import java.io.Serializable;
import java.util.Enumeration;public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {private static final long serialVersionUID = 1L;private transient ServletConfig config;public GenericServlet() {}public void destroy() {}public String getInitParameter(String name) {return this.getServletConfig().getInitParameter(name);}public Enumeration<String> getInitParameterNames() {return this.getServletConfig().getInitParameterNames();}public ServletConfig getServletConfig() {return this.config;}public ServletContext getServletContext() {return this.getServletConfig().getServletContext();}public String getServletInfo() {return "";}public void init(ServletConfig config) throws ServletException {this.config = config;this.init();}public void init() throws ServletException {}public void log(String message) {this.getServletContext().log(this.getServletName() + ": " + message);}public void log(String message, Throwable t) {this.getServletContext().log(this.getServletName() + ": " + message, t);}public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;public String getServletName() {return this.config.getServletName();}
}

只保留service()作为抽象方法。

package org.westos.demo;import javax.servlet.*;
import java.io.IOException;/*** @author lwj* @date 2020/8/15 11:05*/
public class MyGenericServlet extends GenericServlet {@Overridepublic void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {ServletConfig servletConfig = getServletConfig();}
}

HttpServlet

public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {HttpServletRequest request;HttpServletResponse response;try {request = (HttpServletRequest)req;response = (HttpServletResponse)res;} catch (ClassCastException var6) {throw new ServletException(lStrings.getString("http.non_http"));}this.service(request, response);//调用service
}

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {String method = req.getMethod();long lastModified;if (method.equals("GET")) {lastModified = this.getLastModified(req);if (lastModified == -1L) {this.doGet(req, resp);} else {long ifModifiedSince;try {ifModifiedSince = req.getDateHeader("If-Modified-Since");} catch (IllegalArgumentException var9) {ifModifiedSince = -1L;}if (ifModifiedSince < lastModified / 1000L * 1000L) {this.maybeSetLastModified(resp, lastModified);this.doGet(req, resp);} else {resp.setStatus(304);}}} else if (method.equals("HEAD")) {lastModified = this.getLastModified(req);this.maybeSetLastModified(resp, lastModified);this.doHead(req, resp);} else if (method.equals("POST")) {this.doPost(req, resp);} else if (method.equals("PUT")) {this.doPut(req, resp);} else if (method.equals("DELETE")) {this.doDelete(req, resp);} else if (method.equals("OPTIONS")) {this.doOptions(req, resp);} else if (method.equals("TRACE")) {this.doTrace(req, resp);} else {String errMsg = lStrings.getString("http.method_not_implemented");Object[] errArgs = new Object[]{method};errMsg = MessageFormat.format(errMsg, errArgs);resp.sendError(501, errMsg);}}

关键：doGet()和doPost()，所以我们只要重写doGet()和doPost()方法。

当请求到来执行service()方法时，会执行到我们重写的方法。多态访问成员方法：编译看左，运行看右。

package org.westos.demo;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;/*** @author lwj* @date 2020/8/15 11:10*/
public class MyHttpServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {System.out.println("get请求来了");}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {System.out.println("post请求来了");}
}

<?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_4_0.xsd"version="4.0"><servlet><servlet-name>MyHttpServlet</servlet-name><servlet-class>org.westos.demo.MyHttpServlet</servlet-class></servlet><servlet-mapping><servlet-name>MyHttpServlet</servlet-name><url-pattern>/httpServlet</url-pattern></servlet-mapping>
</web-app>

表单的action选项 ※

在发布项目时，当你配置了上下文路径，/20200815-servlet这种，那么在表单的action选项中就需要加上上下文路径，/20200815-servlet/httpServlet，以后会使用动态代码来拿上下文路径。

如果你的上下文路径是/，那么在表单的action选项中就不需要加上下文路径，只需要/httpServlet即可。

测试HttpServlet的Get请求和POST请求。

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>login</title>
</head>
<body><form action="/httpServlet" method="get">姓名：<input type="text" name="username"> <br>密码：<input type="password" name="password"> <br><input type="submit" value="提交"></form><hr><form action="/httpServlet" method="post">姓名：<input type="text" name="username"> <br>密码：<input type="password" name="password"> <br><input type="submit" value="提交"></form>
</body>
</html>

get请求来了
post请求来了

表单action选项是正确的。

项目目录结构：