20.Servlet体系结构

Servlet只是一个接口规范，我们通常使用一些定义好的实现类来使用，而不是自己从头实现一个Servlet接口。

Servlet的实现类

看API文档，可以看到Servlet有两个实现类：

Servlet -- 接口
|
GenericServlet -- 抽象类、实现类，实现了接口Servlet
|
HttpServlet -- 抽象类、实现类，继承了GenericServlet

‍

GenericServlet

GenericServlet：将Servlet接口中其他的方法做了默认空实现，只将service()​方法作为抽象。这是因为我们大多数时候只需用到service()​方法，其他的都很少用，因此GenericServlet对其他方法做了空实现，但如果想要复写也可以自己实现。

‍

将来定义Servlet类时，可以继承GenericServlet，实现service()​方法即可。

public class GenericServletDemo1 extends GenericServlet {
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
  
    }
}

‍

‍

‍

‍

GenericServlet 源码：

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();
    }
}

‍

‍

‍

‍

HTTPServlet介绍

我们写Servlet是为了做什么？ 不外乎就是处理用户的请求，并且大部分是HTTP请求，例如客户通过表单送来了账户和密码，要对登录做校验。

收到请求后，我们首先需要判断是get还是post方式，然后定义方法分别处理post请求和get请求：

String method = req.getMethod();    //返回HTTP请求的方式，get还是post
if ("GET".equals(method)){
    //get方式获取数据
    doGet();
}else if("POST".equals(method)){
    //post方式获取数据
    doPost();
}

public void doGet(){
    //.....处理请求
}

public void doPost(){
    //.....处理请求
}

‍

‍

而HTTPServlet，就是封装好了这个操作，不用自己写代码去判断是get还是post，对http协议的做了一层简单的封装。API文档里的描述：

Provides an abstract class to be subclassed to create an HTTP servlet suitable for a Web site. A subclass of HttpServlet must override at least one method, usually one of these:

• doGet, if the servlet supports HTTP GET requests
• doPost, for HTTP POST requests
• doPut, for HTTP PUT requests
• doDelete, for HTTP DELETE requests
• ​init​ and destroy​, to manage resources that are held for the life of the servlet
• ​getServletInfo​, which the servlet uses to provide information about itself

因此，GenericServlet较少使用，我们主要是使用HTTPServlet。

‍

‍

HttpServlet 的service方法源码：HTTP共7种请求方式，每个都做了判断

public abstract class HttpServlet extends GenericServlet {
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {
            .....doGet();
        } 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);
        }

    }
}

‍

‍

‍

HTTPServlet的使用

定义类继承HttpServlet，然后复写doGet/doPost方法：

package com.peterjxl;

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("/httpDemo1")
public class ServletHttpDemo1Hello extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("ServletHttpDemo1Hello doGet");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("ServletHttpDemo1Hello doPost");
    }
}

重启Tomcat，访问http://localhost:8080/hello/httpDemo1​；浏览器默认是用get，所以复写了doGet方法后，默认会执行get方法，输出："ServletHttpDemo1Hello doGet"​

‍

然后我们来测试post。在web目录下新建ServletHttpDemo1Post.jsp，并用表单发送post请求

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
    <head>
        <title>测试post请求</title>
    </head>
    <body>
        <form action="/hello/httpDemo1" method="post">
            <span>用户名：</span><input type="text" name="username" placeholder="请输入用户名">
            <input type="submit" value="点击发送post请求">
        </form>
    </body>
</html>

访问：http://localhost:8080/hello/ServletHttpDemo1Post.jsp​ ，随便输入一个用户名，点击按钮；然后可以看到后台输出ServletHttpDemo1Hello doPost​

‍

如果使用的method是get，那么请求参数会拼接在url中，这里不再演示

<form action="/hello/httpDemo1" method="get">