HttpServlet概述及应用

Servlet采用了接口的设计思想，由Tomcat提供

HttpServlet是⼀个与HTTP协议相关的Servlet，专门用来处理HTTP协议的请求响应。

• 在HttpServlet类的service方法内部，根据HTTP协议请求方式不同，执行不同的doXXX的方法（get请求执行doGet方法，如果是post请求就会执行doPost方法）
• 继承了HttpServlet之后不需要重写service方法，只需要重写doGet和doPost方法即可。

实现Servlet方法详细解读

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

public class MyServlet extends HttpServlet {
     
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
     

        response.getWriter().print("MyServlet doPost");
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
     

        response.getWriter().print("MyServlet doGet");
    }
}

<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>MyServletservlet-name>
        <servlet-class>com.byc.servlet.MyServletservlet-class>
    servlet>
    <servlet-mapping>
        <servlet-name>MyServletservlet-name>
        <url-pattern>/myurl-pattern>        
    servlet-mapping>
web-app>

浏览器客户端只响应了doGet的输出：
servlet接口下的方法service()方法
在tomcat运行的时候就会自动完成(HttpServletRequest req, HttpServletResponse resp)中req、resp对象的创建

String method = req.getMethod();//获取定义的请求方式是post请求还是get请求
/**
post请求只有在表单中存在，get请求则是显示在地址栏中
*/

在HttpServlet中定义doGet方法

/**
     * Called by the server (via the service method) to
     * allow a servlet to handle a GET request.
     *
     * 
Overriding this method to support a GET request also
     * automatically supports an HTTP HEAD request. A HEAD
     * request is a GET request that returns no body in the
     * response, only the request header fields.
     *
     * 
When overriding this method, read the request data,
     * write the response headers, get the response's writer or
     * output stream object, and finally, write the response data.
     * It's best to include content type and encoding. When using
     * a PrintWriter object to return the response,
     * set the content type before accessing the
     * PrintWriter object.
     *
     * 
The servlet container must write the headers before
     * committing the response, because in HTTP the headers must be sent
     * before the response body.
     *
     * 
Where possible, set the Content-Length header (with the
     * {@link javax.servlet.ServletResponse#setContentLength} method),
     * to allow the servlet container to use a persistent connection
     * to return its response to the client, improving performance.
     * The content length is automatically set if the entire response fits
     * inside the response buffer.
     *
     * 
When using HTTP 1.1 chunked encoding (which means that the response
     * has a Transfer-Encoding header), do not set the Content-Length header.
     *
     * 
The GET method should be safe, that is, without
     * any side effects for which users are held responsible.
     * For example, most form queries have no side effects.
     * If a client request is intended to change stored data,
     * the request should use some other HTTP method.
     *
     * 
The GET method should also be idempotent, meaning
     * that it can be safely repeated. Sometimes making a
     * method safe also makes it idempotent. For example,
     * repeating queries is both safe and idempotent, but
     * buying a product online or modifying data is neither
     * safe nor idempotent.
     *
     * 
If the request is incorrectly formatted, doGet
     * returns an HTTP "Bad Request" message.
     *
     * @param req   an {@link HttpServletRequest} object that
     *                  contains the request the client has made
     *                  of the servlet
     *
     * @param resp  an {@link HttpServletResponse} object that
     *                  contains the response the servlet sends
     *                  to the client
     *
     * @exception IOException   if an input or output error is
     *                              detected when the servlet handles
     *                              the GET request
     *
     * @exception ServletException  if the request for the GET
     *                                  could not be handled
     *
     * @see javax.servlet.ServletResponse#setContentType
     */
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
     
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_get_not_supported");
        if (protocol.endsWith("1.1")) {
     
            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
        } else {
     
            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
        }
    }

构建的实体类继承HttpServlet，其中doGet方法作为父类，再在实体类中重写doGet方法，在客户端浏览器中调用子类的doGet方法，由于子类service方法没有重写，所以就直接调用父类的service方法