[Java Web] Servlet

一、概述

Servlet是使用Java编写的服务端程序，主要功能是交互式的浏览和修改数据，从而生成动态的Web内容。

Servlet运行需要特定的容器，例如由Tomcat为Servlet提供基本运行环境。

当Web服务器接收到HTTP请求时，会将该请求交给Servlet容器。容器首先对请求的URL进行解析，并根据web.xml找到相应的Servlet，如果该Servlet为第一次访问，则调用则初始化之。之后将request和respone传递给该Servlet。Servlet通过request获取客户端的请求信息，处理完成后将返回的信息放入respone。之后控制权回到Web服务器，返回HTTP响应到客户端。

Servlet的状态由其容器来维护，生命周期从服务器运行开始直至关闭，包括加载、初始化、运行和销毁阶段。

二、常用接口及类

service：当Web服务器接收到HTTP请求，就会直接调用该方法。相当于Servlet应用程序的入口。
HttpServlet：提供处理HTTP协议框架的抽象类。
HttpSession：代表HTTP服务端与客户端会话的接口。
ServletConfig：封装了Servlet的配置信息，在Servlet初始化期间被传递。
ServletContext：Servlet环境对象，Servlet引擎通过该对象向Servlet传递环境信息。

三、创建与调用

//MyServlet.java
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;

public class MyServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("GB2312");
        PrintWriter out = resp.getWriter();

        String content = "<DOCTYPE html>" +
                "<html>" +
                "<head>" + "<title>servlet</title>" + "</head>" +
                "<body>" + "servlet has been created and configured" + "</body>" +
                "</html>";
        out.println(content);
        out.flush();
        out.close();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}



//web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
    <servlet>
        <servlet-name>MyServlet</servlet-name>
        <servlet-class>MyServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/MyServlet</url-pattern>
    </servlet-mapping>
</web-app>

View Code

获取Servlet信息。

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;
import java.util.Enumeration;

public class MyServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("UTF-8");
        PrintWriter out = resp.getWriter();

        //获取HTTP头部信息
        Enumeration enumeration = req.getHeaderNames();
        while (enumeration.hasMoreElements()) {
            String name = (String) enumeration.nextElement();
            String value = req.getHeader(name);
            out.println(String.format("%s = %s<br/>", name, value));
        }

        //获取请求对象信息
        out.println(req.getMethod() + "<br/>");
        out.println(req.getProtocol() + "<br/>");
        out.println(req.getRemoteAddr() + "<br/>");
        out.println(req.getRequestURL() + "<br/>");
        out.println(req.getRequestURI() + "<br/>");

        //获取参数信息
        out.println(req.getParameter("user") + "<br/>");
        out.println(req.getParameter("pass") + "<br/>");

        out.flush();
        out.close();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

View Code

JSP调用Servlet。

//方法一
<form action="MyServlet" method="post">
    ...
</form>

//方法二
<a href="MyServlet?user=root&pass=0000">Call MyServlet</a>

View Code

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