Servlet（Server Applet），全稱JavaServlet，未有中文譯文。是用Java編寫的服務器端程序。其主要功能在於交互式地瀏覽和修改數據，生成動態Web內容。狹義的Servlet是指Java語言實現的一個接口，廣義的Servlet是指任何實現了這個Servlet接口的類，一般情況下，人們將Servlet理解為後者。

Servlet運行於支持Java的應用服務器中。從原理上講，Servlet可以響應任何類型的請求，但絕大多數情況下Servlet只用來擴展基於HTTP協議的Web服務器。[百度百科]

** 如何使用servlet?

#1.編寫一個繼承與HttpServlet的java類

#2.重寫doGet和doPost方法

#3.配置web.xml文件,配置servlet節點

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>

……

運行結果：

** URL匹配

註意：

#1.路徑要不就是/開始，要不就是*開始，而且兩種不能混合在一起用。

#2.當多個servlet被同時匹配，精確匹配優先

#3.默認路徑：當路徑匹配不到任何自定義的servlet的url路徑，則去tomcat下找DefaultServlet到對象的應用下查找對應的靜態文件，如果還找不到就返回404 not found[先找動態資源，再找靜態文件]

** servlet的生命周期

構造方法：第一次訪問servlet時候調用

init方法：創建完servlet時候調用

service方法：每次請求的時候調用

destroy方法：當servlet被銷毀時候被調用

註意：

#1. service方法本來是用來分發調用doPost方法還是doGet方法，如果我們重寫了service方法就不會調用doGet或者doPost方法。

#2. Init 方法有有參數和無參數兩種方法，有參數的其實還是會調用無參的，所以servlet方法初始化代碼寫在無參數的init方法即可

#3. Servlet對象在tomcat中是單實例多線程的[servlet肯定用了多線程]

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 {

   

    /**

     * 構造方法

    */

    public MyFirstServlet() {

       System.out.println("1. 我被創建了......");

    }

   

    /**

     * init方法

    */

    @Override

    public void init(ServletConfig config) throws ServletException {

       System.out.println("2. init方法被調用......");

    }

   

    /**

     * service方法

     */

    @Override

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

       System.out.println("3. service方法被調用......");

    }

   

    @Override

    public void destroy() {

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

    }

   

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

       System.out.println("doGet 方法被調用......");

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

    }

 

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

       System.out.println("doPost 方法被調用......");

       doGet(request, response);

    }

}

 

** servlet的自動加載

在web.xml中加入load-on-startup即可[中間的數值越大優先級越低]

<servlet>

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

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

    <!-- 讓servlet在服務器啟動的時候就創建 -->

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

  </servlet>

** servlet的線程安全問題

由於servlet是單實例多線程的，所以有可能引發多線程問題！

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);
         }

}

結果是：[出現了想要的問題]

可以通過加鎖來來解決問題

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);

         }

}

java web-----servlet