java web-----servlet

Servlet（Server Applet），全称JavaServlet，未有中文译文。是用Java编写的服务器端程序。其主要功能在于交互式地浏览和修改数据，生成动态Web内容。狭义的Servlet是指Java语言实现的一个接口，广义的Servlet是指任何实现了这个Servlet接口的类，一般情况下，人们将Servlet理解为后者。

Servlet运行于支持Java的应用服务器中。从原理上讲，Servlet可以响应任何类型的请求，但绝大多数情况下Servlet只用来扩展基于HTTP协议的Web服务器。[百度百科]

** 如何使用servlet?

#1.编写一个继承与HttpServlet的java类

#2.重写doGet和doPost方法

#3.配置web.xml文件,配置servlet节点

MyFirstServlet.java

package com.ibatis01.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class MyFirstServlet extends HttpServlet {
         protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

                   response.getWriter().append("my frist servlet");
         }
         protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

                   doGet(request, response);
         }
}

Web.xml

……

<servlet>

    <servlet-name>MyFirstServlet</servlet-name>

    <servlet-class>com.ibatis01.servlet.MyFirstServlet</servlet-class>

  </servlet>

  <servlet-mapping>

    <servlet-name>MyFirstServlet</servlet-name>

    <url-pattern>/MyFirst</url-pattern>

  </servlet-mapping>

……

运行结果：

** URL匹配

                    url-pattern                   浏览器输入

精确匹配             /first                 http://localhost:8080/day/first

                                               /itcast/demo1          http://localhost:8080/day/itcast/demo1

模糊匹配             /*                   http://localhost:8080/day/任意路径

                                               /itcast/*               http://localhost:8080/day/itcast/任意路径

                                              *.后缀名              http://localhost:8080/day/任意路径.do

                                               *.do

                                               *.action

                                               *.html(伪静态)：并不是一个真的静态文件

默认路径[缺省路径，尽量少用]       /          http://localhost:8080/day

注意：

#1.路径要不就是/开始，要不就是*开始，而且两种不能混合在一起用。

#2.当多个servlet被同时匹配，精确匹配优先

#3.默认路径：当路径匹配不到任何自定义的servlet的url路径，则去tomcat下找DefaultServlet到对象的应用下查找对应的静态文件，如果还找不到就返回404 not found[先找动态资源，再找静态文件]

** servlet的生命周期

构造方法：第一次访问servlet时候调用

init方法：创建完servlet时候调用

         service方法：每次请求的时候调用

         destroy方法：当servlet被销毁时候被调用

注意：

#1. service方法本来是用来分发调用doPost方法还是doGet方法，如果我们重写了service方法就不会调用doGet或者doPost方法。

#2. Init 方法有有参数和无参数两种方法，有参数的其实还是会调用无参的，所以servlet方法初始化代码写在无参数的init方法即可

#3. Servlet对象在tomcat中是单实例多线程的[servlet肯定用了多线程]

package com.ibatis01.servlet;

 

import java.io.IOException;

 

import javax.servlet.ServletConfig;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

 

publicclass MyFirstServlet extends HttpServlet {

   

    /**

     * 构造方法

    */

    public MyFirstServlet() {

       System.out.println("1. 我被创建了......");

    }

   

    /**

     * init方法

    */

    @Override

    public void init(ServletConfig config) throws ServletException {

       System.out.println("2. init方法被调用......");

    }

   

    /**

     * service方法

     */

    @Override

    protected void service(HttpServletRequest arg0, HttpServletResponse arg1) throws ServletException, IOException {

       System.out.println("3. service方法被调用......");

    }

   

    @Override

    public void destroy() {

       System.out.println("4. 调用destroy方法......");

    }

   

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

       System.out.println("doGet 方法被调用......");

       response.getWriter().append("my frist servlet");

    }

 

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

       System.out.println("doPost 方法被调用......");

       doGet(request, response);

    }

}

 

** servlet的自动加载 

在web.xml中加入load-on-startup即可[中间的数值越大优先级越低]

<servlet>

    <servlet-name>MyFirstServlet</servlet-name>

    <servlet-class>com.ibatis01.servlet.MyFirstServlet</servlet-class>

    <!-- 让servlet在服务器启动的时候就创建 -->

    <load-on-startup>1</load-on-startup>

  </servlet>

** servlet的线程安全问题

由于servlet是单实例多线程的，所以有可能引发多线程问题！

package com.ibatis01.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ThreadServlet extends HttpServlet {
         private int num=0;
         protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
                   try {
                            num++;

                            Thread.sleep(2000);

                            response.getWriter().append(this.toString()+"<br/>").append("num="+num);

                   } catch (InterruptedException e) {

                            e.printStackTrace();

                   }       

         }
         protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
                   doGet(request, response);
         }

}

结果是：[出现了想要的问题]

可以通过加锁来来解决问题

package com.ibatis01.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ThreadServlet extends HttpServlet {
         private int num = 0;
         protected void doGet(HttpServletRequest request, HttpServletResponse response)

                            throws ServletException, IOException {

                   synchronized (this) {
                            try {

                                     num++;

                                     Thread.sleep(2000);

                                     response.getWriter().append(this.toString() + "<br/>").append("num=" + num);

                            } catch (InterruptedException e) {

                                     e.printStackTrace();

                            }

                   }

         }

 

         protected void doPost(HttpServletRequest request, HttpServletResponse response)

                            throws ServletException, IOException {

                   doGet(request, response);

         }

}