简单Demo实现Javaweb的搭建在Tomcat

环境准备

本机：MacBook Pro 系统：MacOS 13.4

1. Tomcat 下载：Apache Tomcat® - Welcome!

   这里我下载的是9.0.75版本

启动Tomcat容器碰到Bug:Tomcat started.但是就是没法访问到相关页面，同时jps也没有相关的进程，说明其实没启动起来。同时通过chown改变权限也没法解决问题。

Solution:

查看相关的版本对应记录，以及README的Release notes，发现apache-tomcat-10.1.9需要jdk11及以上，我本地是jdk8，重新下载tar.gz 9.0.75。

重新解压并且startup，可以访问http://localhost:8080了

2. 记得需要配置本地的JAVA_HOME路径，我采用命令vim ~/.bash_profile

    export JAVA_HOME="/Library/Java/JavaVirtualMachines/jdk1.8.0_361.jdk/Contents/Home"
   

程序编写

1. idea maven生成javaweb模版

2. pom中导入依赖

   <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/maven-v4_0_0.xsd">
     <modelVersion>4.0.0modelVersion>
     <groupId>com.jxzgroupId>
     <artifactId>MyJavaWebartifactId>
     <packaging>warpackaging>
     <version>1.0-SNAPSHOTversion>
     <name>MyJavaWeb Maven Webappname>
     <url>http://maven.apache.orgurl>
   
     <dependencies>
       <dependency>
         <groupId>javax.servletgroupId>
         <artifactId>servlet-apiartifactId>
         <version>2.5version>
       dependency>
   
       <dependency>
         <groupId>javax.servlet.jspgroupId>
         <artifactId>javax.servlet.jsp-apiartifactId>
         <version>2.3.3version>
       dependency>
   
       <dependency>
         <groupId>junitgroupId>
         <artifactId>junitartifactId>
         <version>3.8.1version>
         <scope>testscope>
       dependency>
     dependencies>
   
     <build>
       <finalName>MyJavaWebfinalName>
     build>
   project>
   

3. main目录下新建java包，并创建com/jxz/servlet目录，在其中放入最简单的HelloServlet，其继承自HttpServlet

   package com.jxz.servlet;
   
   import javax.servlet.ServletException;
   import javax.servlet.http.HttpServlet;
   import javax.servlet.http.HttpServletRequest;
   import javax.servlet.http.HttpServletResponse;
   import java.io.IOException;
   import java.io.PrintWriter;
   
   /**
    * @Author jiangxuzhao
    * @Description
    * @Date 2023/6/3
    */
   public class HelloServlet extends HttpServlet {
       @Override
       protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           PrintWriter pw = resp.getWriter(); // 获取输出流
           pw.print("hello jxz"); // 打印
       }
   
       @Override
       protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
           super.doGet(req, resp);
       }
   }
   

   其中的原理分析：

   我们只实现了Get和Post方法，因为这是最普遍的两个Http请求投，我们也可以看到，其实doPost方法也只是复用了一下doGet方法，因为处理是一样的。那么为什么这样子就可以处理http请求了呢？HelloServlet -> public abstract class HttpServlet -> public abstract class GenericServlet -> implements Servlet

       /**
        * Called by the servlet container to allow the servlet to respond to 
        * a request.
        *
        * 
   This method is only called after the servlet's init()
        * method has completed successfully.
        * 
        * 
     The status code of the response always should be set for a servlet 
        * that throws or sends an error.
        *
        * 
        * 
   Servlets typically run inside multithreaded servlet containers
        * that can handle multiple requests concurrently. Developers must 
        * be aware to synchronize access to any shared resources such as files,
        * network connections, and as well as the servlet's class and instance 
        * variables. 
        * More information on multithreaded programming in Java is available in 
        * 
        * the Java tutorial on multi-threaded programming.
        *
        *
        * @param req 	the ServletRequest object that contains
        *			the client's request
        *
        * @param res 	the ServletResponse object that contains
        *			the servlet's response
        *
        * @exception ServletException 	if an exception occurs that interferes
        *					with the servlet's normal operation 
        *
        * @exception IOException 		if an input or output exception occurs
        *
        */
   
       public void service(ServletRequest req, ServletResponse res)
   	throws ServletException, IOException;
   

   从interface Servlet接口中的定义可以看出：service允许web容器处理请求request，而GenericServlet里面并没有对service()方法进行实现，HttpServlet对service()方法进行了重写，同时这里应用了模板模式，在service中定义了各种http请求方法头method的处理方式，在我们的子类HelloServlet中，让我们重写的doGet或者doPost等方法，子类被调用的时候也就是调用被重写的方法。

       /**
        *
        * Receives standard HTTP requests from the public
        * service method and dispatches
        * them to the doXXX methods defined in 
        * this class. This method is an HTTP-specific version of the 
        * {@link javax.servlet.Servlet#service} method. There's no
        * need to override this method.
        *
        *
        *
        * @param req	the {@link HttpServletRequest} object that
        *			contains the request the client made of
        *			the servlet
        *
        *
        * @param resp	the {@link HttpServletResponse} object that
        *			contains the response the servlet returns
        *			to the client				
        *
        *
        * @exception IOException	if an input or output error occurs
        *				while the servlet is handling the
        *				HTTP request
        *
        * @exception ServletException	if the HTTP request
        *					cannot be handled
        * 
        * @see 				javax.servlet.Servlet#service
        *
        */
   
       protected void service(HttpServletRequest req, HttpServletResponse resp)
   	throws ServletException, IOException
       {
   	String method = req.getMethod();
   
   	if (method.equals(METHOD_GET)) {
   	    long lastModified = getLastModified(req);
   	    if (lastModified == -1) {
   		// servlet doesn't support if-modified-since, no reason
   		// to go through further expensive logic
   		doGet(req, resp);
   	    } else {
   		long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
   		if (ifModifiedSince < (lastModified / 1000 * 1000)) {
   		    // If the servlet mod time is later, call doGet()
                       // Round down to the nearest second for a proper compare
                       // A ifModifiedSince of -1 will always be less
   		    maybeSetLastModified(resp, lastModified);
   		    doGet(req, resp);
   		} else {
   		    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
   		}
   	    }
   
   	} else if (method.equals(METHOD_HEAD)) {
   	    long lastModified = getLastModified(req);
   	    maybeSetLastModified(resp, lastModified);
   	    doHead(req, resp);
   
   	} else if (method.equals(METHOD_POST)) {
   	    doPost(req, resp);
   	    
   	} else if (method.equals(METHOD_PUT)) {
   	    doPut(req, resp);	
   	    
   	} else if (method.equals(METHOD_DELETE)) {
   	    doDelete(req, resp);
   	    
   	} else if (method.equals(METHOD_OPTIONS)) {
   	    doOptions(req,resp);
   	    
   	} else if (method.equals(METHOD_TRACE)) {
   	    doTrace(req,resp);
   	    
   	} else {
   	    //
   	    // Note that this means NO servlet supports whatever
   	    // method was requested, anywhere on this server.
   	    //
   
   	    String errMsg = lStrings.getString("http.method_not_implemented");
   	    Object[] errArgs = new Object[1];
   	    errArgs[0] = method;
   	    errMsg = MessageFormat.format(errMsg, errArgs);
   	    
   	    resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
   	}
       }
   

4. Web.xml中配置servlet，完成路径映射

   <?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"
            metadata-complete="true">
   
   <!--  注册servlet名字和类-->
     <servlet>
       <servlet-name>hello</servlet-name>
       <servlet-class>com.jxz.servlet.HelloServlet</servlet-class>
     </servlet>
   
   <!--配置Servlet的映射路径和对应的servlet名字-->
     <servlet-mapping>
       <servlet-name>hello</servlet-name>
       <url-pattern>/hellojxz</url-pattern>
     </servlet-mapping>
   </web-app>
   

5. Run Configuration中配置web war包到tomcat服务器中

   其中需要配置本地的web服务器：
   

   同时指定生成的Deployment war包

6. 测试访问相关路径 http://localhost:8080/hellojxz
   

除此以外，一个servlet可以指定多个路径映射到相同的servlet，同时可以实现默认的请求路径映射，

  <servlet-mapping>
    <servlet-name>helloservlet-name>
    <url-pattern>/hellojxz2url-pattern>
  servlet-mapping>


  <servlet-mapping>
    <servlet-name>helloservlet-name>
    <url-pattern>/*url-pattern>
  servlet-mapping>
web-app>