【Spring in action】SpringMVC的准备工作：Servlet基础

前言：

基于Spring思想的web框架SpringMVC。需要了解SpringMVC需要知道Servlet的工作方式，很简单。两篇文章，一篇各种原理组件，一篇实战搞定。高手请忽略。

正文：

一、java的Servlet的使用：

1、写一个Servlet继承HttpServlet：

1》publicclass MyServletextends HttpServlet

package com.demo;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyServlet extends HttpServlet {

/**
*
*/
private static final long serialVersionUID = 1L;

public MyServlet() {
super();
// TODO Auto-generated constructor stub
}

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// TODO Auto-generated method stub
//		super.doGet(req, resp);
//		logic HttpServletRequest请求,HttpServletResponse 响应
PrintWriter out = resp.getWriter();
out.print("hello servlet!");
out.close();
//
//		String str = this.getServletContext().getInitParameter("servlet_name");
//		System.out.println(str);
//		resp.sendRedirect("http://www.baidu.com");
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// TODO Auto-generated method stub
// super.doPost(req, resp);
doGet(req, resp);
}

}

2》在WEB-INF下写一个web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
<display-name>Servlet1</display-name>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file>
</welcome-file-list>
<servlet>
<servlet-name>MyServlet</servlet-name>
<servlet-class>com.demo.MyServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>MyServlet</servlet-name>
<url-pattern>/myServlet</url-pattern>
</servlet-mapping>
</web-app>

总结1：

在Servlet1中：新建一个servlet-mapping对象：外网访问地址http://localhost:8080/Servlet1/myServlet ,由此处约定。

关键代码新建一个servlet对象：MyServlet-->com.demo.MyServlet。外网根据MyServlet输出hello servlet!

PS：常见问题：

1、严重: Allocate exception for servlet MyServlet

java.lang.ClassNotFoundException: com.demo

原因： web.xml需要完整路径

例如错误代码：

<servlet>

<servlet-name>MyServlet</servlet-name>

<servlet-class>com.demo</servlet-class>

</servlet>

我的路径为com.demo.MyServlet，而我只是写了包名。

总结2:

servlet生命周期：构造方法-init初始化-doget／dopost-destory

2、写一个生命周期看效果：LoginServletextends HttpServlet

package com.demo;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
* Servlet implementation class LoginServlet
*/

public class LoginServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public LoginServlet() {

// TODO Auto-generated constructor stub
System.out.println("Constructor...");
}

@Override
public void init() throws ServletException {
// TODO Auto-generated method stub
System.out.println("init...");
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
System.out.println("doGet...");
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
//		System.out.println("doPost...");
PrintWriter out = response.getWriter();
out.print("<h1>hello servlet</h1>");
//		request.getRequestDispatcher("hello.html").forward(request, response);
request.getRequestDispatcher("hello.html").include(request, response);
}

@Override
public void destroy() {
// TODO Auto-generated method stub
System.out.println("destroy...");
}

}

修改web.xml，添加LoginServlet：

<servlet>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>com.demo.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/LoginServlet</url-pattern>
</servlet-mapping>

直接执行该Servlet，执行结果：

九月 07, 2017 2:37:08 下午 org.apache.catalina.startup.Catalina start

信息: Server startup in 4956 ms
Constructor...
init...
doFilter...
doGet...

3、过滤器：对特定请求进行过滤。

作用：客户端和服务端请求资源之间，起到过滤作用：session管理、权限管理、日志管理等。

1》在web.xml下配置：和上面的servlet平级处配置：

<filter>
<filter-name>MyFlter</filter-name>
<filter-class>com.filter.MyFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>MyFlter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

2》写一个MyFilter继承自Filter

package com.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class MyFilter implements Filter{

@Override
public void destroy() {
// TODO Auto-generated method stub

}

@Override
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)
throws IOException, ServletException {
// TODO Auto-generated method stub
System.out.println("doFilter...");
//过滤传递
arg2.doFilter(arg0, arg1);
}

@Override
public void init(FilterConfig arg0) throws ServletException {
// TODO Auto-generated method stub

}

}

执行结果，由于设置/*，所有请求均会执行：

doFilter...
结果：不管调用什么，例如jdbclogin.html也会调用fliter过滤器

如果指定包下过滤：则重新配置如下：（admin下面生效）

<!-- /admin/* admin文件夹下的才需要过滤 -->

<filter-mapping>

<filter-name>MyFlter</filter-name>

<url-pattern>/admin/*</url-pattern>

</filter-mapping>

例如：
http://localhost:8080/Servers1/login.html不会执行过滤器 http://localhost:8080/Servers1/admin/admin.html会执行过滤器

4、获得请求头header信息：

package com.demo;

import java.io.IOException;
import java.util.Enumeration;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
* Servlet implementation class HeaderServlet
*/
@WebServlet("/HeaderServlet")
public class HeaderServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public HeaderServlet() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
Enumeration<String> enum1 = request.getHeaderNames();
while (enum1.hasMoreElements()) {
String string = (String) enum1.nextElement();
String value = request.getHeader(string);
System.out.println("name:"+string +"  value:"+value);
}
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}

}

此处不需要修改web.xml配置，原因是此处通过注解进行了mapping映射：

@WebServlet("/HeaderServlet")
执行结果：

doFilter...
name:host value:localhost:8080
name:connection value:keep-alive
name:upgrade-insecure-requests value:1
name:accept value:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
name:user-agent value:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/603.1.30 (KHTML, like Gecko) Safari/522.0
name:accept-language value:zh-cn
name:cache-control value:no-cache
name:accept-encoding value:gzip, deflate

PS：其中如何设置返回每一秒刷新一次：例如显示不断变化的时间日期：在doGet中输出：

response.setHeader("refresh", "1");
PrintWriter out = response.getWriter();
out.println(new Date().toString());

5、持久化技术：Cookies和Session：

给客户端设置Cookies和从客户端取Cookies：
1》设置Cookies：

package com.demo;

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

/**
* Servlet implementation class CookieServlet
*/
@WebServlet("/CookieServlet")
public class CookieServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public CookieServlet() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
Cookie c = new Cookie("geek99", "www.geek99.com");
c.setMaxAge(60*60*24*7);
response.addCookie(c);
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}

}

执行结果：日志打印仅一个过滤器表示执行了。

doFilter...
2》从客户端获取Cookies：

package com.demo;

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

/**
* Servlet implementation class GetCookieServlet
*/
@WebServlet("/GetCookieServlet")
public class GetCookieServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public GetCookieServlet() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
Cookie[] cs = request.getCookies();
for (int i = 0; i < cs.length; i++) {
System.out.println("key:"+cs[i].getName()+"value:"+cs[i].getValue());
}
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}

}

执行结果：

doFilter...
key:geek99value:www.geek99.com
其中：设置Cookies是通过response返回给客户端，获取Cookies是通过请求request获取。

给客户端设置Session和取出Session：
1》设置Session：

package com.demo;

import java.io.IOException;
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 javax.servlet.http.HttpSession;

/**
* Servlet implementation class SetSessionServlet
*/
@WebServlet("/SetSessionServlet")
public class SetSessionServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public SetSessionServlet() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
HttpSession session = request.getSession();
session.setAttribute("session_name", "session_value");
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}

}

执行结果：

doFilter...
2》获取Session：

package com.demo;

import java.io.IOException;
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 javax.servlet.http.HttpSession;

/**
* Servlet implementation class GetSessionServlet
*/
@WebServlet("/GetSessionServlet")
public class GetSessionServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public GetSessionServlet() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
HttpSession session = request.getSession();
String value = (String) session.getAttribute("session_name");
System.out.println("name:"+value);
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}

}

执行结果：

doFilter...
name:session_value
总结：区别于Cookies：Sesion的设置和获取都是通过request进行的。因此，Cookies是设置在浏览器本地的。不随着浏览器关闭而被清除。Session浏览器关闭则清除，同一浏览器不同页签可使用。

6、共享变量

变量的作用域

ServletContext整个应用程序

HttpSession当前浏览器

HttpServletRequest当前请求

1》设置共享变量和获取共享变量：

package com.shareVar;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
* Servlet implementation class SetShareVar
* @WebServlet("/SetShareVar")
*/

public class SetShareVar extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public SetShareVar() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
//1.ServletContext
//2.HttpSession
//3.HttpServletRequest
ServletContext ctx = this.getServletContext();
//1、ServletContext共享变量
ctx.setAttribute("ctx_name", "ctx_value");
HttpSession session = request.getSession();
//2、HttpSession共享变量
session.setAttribute("session_name", "session_value");
request.setAttribute("request_name", "request_value");
//3.上一行代码注释：HttpServletRequest携带信息到／getShareVar 的mapping指向的页面
request.getRequestDispatcher("getShareVar").forward(request, response);
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}

}

getShareVar如下：

package com.shareVar;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
* Servlet implementation class getShareVar
* @WebServlet("/getShareVar")
*/

public class getShareVar extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public getShareVar() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
ServletContext ctx = this.getServletContext();
String ctxValue = (String) ctx.getAttribute("ctx_name");
HttpSession session = request.getSession();
String sessionValue = (String)session.getAttribute("session_name");
String requestValue = (String)request.getAttribute("request_name");

System.out.println("ctxValue:"+ctxValue);
System.out.println("sessionValue:"+sessionValue);
System.out.println("requestValue:"+requestValue);
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doGet(request, response);
}

}

web.xml配置略；

执行/setShare打印日志如下：
ctxValue:ctx_value

sessionValue:session_value

requestValue:request_value

打开新浏览器如下：其中ctx的值携带了，而session，request_value并没有。

ctxValue:ctx_value
sessionValue:null
requestValue:null

7、监听：

监听web工程状态的改变

1）整个工程状态的改变

servletContextListener ServletContextAttributeListener

2）session属性的变化

httpSessionListener httpSessionAttributeListener

3）request属性的变化

servletRequestListener servletRequestAttributeListener

1》对整个Servlet监听：前置性的工作可置于此处contextInitialized

implements ServletContextListener

写一个MyListener继承自ServletContextListener（还有其他的可以供继承），MyListener

package com.listener;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

/**
* Application Lifecycle Listener implementation class MyListener
*
*/

public class MyListener implements ServletContextListener {

/**
* Default constructor.
*/
public MyListener() {
// TODO Auto-generated constructor stub
}

/**
* @see ServletContextListener#contextDestroyed(ServletContextEvent)
*/
public void contextDestroyed(ServletContextEvent arg0)  {
// TODO Auto-generated method stub
}

/**
* @see ServletContextListener#contextInitialized(ServletContextEvent)
*/
public void contextInitialized(ServletContextEvent arg0)  {
System.out.println("contextInitialized...");
ServletContext sctx = arg0.getServletContext();
sctx.setAttribute("onLineCount", 0);
}

}

在web.xml下配置：和上面的servlet平级处配置：

<listener>
<listener-class>com.listener.MyListener</listener-class>
</listener>

启动初始化服务的时候，会执行监听。打印：

contextInitialized...
attributeAdded...

2》对Session进行监听：
写一个类SessionListenerimplements HttpSessionListener ：

package com.listener;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class SessionListener implements HttpSessionListener {

@Override
public void sessionCreated(HttpSessionEvent arg0) {
// TODO Auto-generated method stub
ServletContext ctx = arg0.getSession().getServletContext();
Integer numSessions = (Integer) ctx.getAttribute("numSessions");
if (numSessions == null) {
numSessions = new Integer(1);
} else {
int count = numSessions.intValue();
numSessions = new Integer(count + 1);
}
ctx.setAttribute("numSessions", numSessions);
System.out.println("当前在线人数：" + numSessions);
}

@Override
public void sessionDestroyed(HttpSessionEvent arg0) {
// TODO Auto-generated method stub
ServletContext ctx = arg0.getSession().getServletContext();
Integer numSessions = (Integer) ctx.getAttribute("numSessions");
if (numSessions == null) {
numSessions = new Integer(0);
} else {
int count = numSessions.intValue();
numSessions = new Integer(count - 1);
}
ctx.setAttribute("numSessions", numSessions);
System.out.println("当前在线人数：" + numSessions);
}

}

在web.xml中添加：

<listener>
<listener-class>com.listener.SessionListener</listener-class>
</listener>

每打开一个浏览器，调用前面的/GetSessionServlet 会触发导致在线人数加1.

当前在线人数：0
doFilter...
当前在线人数：1
name:null
doFilter...
当前在线人数：2

3》对attribute的监听：

package com.listener;

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;

/**
* Application Lifecycle Listener implementation class MyServletContextAttributeListener
*
*/

public class MyServletContextAttributeListener implements ServletContextAttributeListener {

/**
* Default constructor.
*/
public MyServletContextAttributeListener() {
// TODO Auto-generated constructor stub
}

/**
* @see ServletContextAttributeListener#attributeAdded(ServletContextAttributeEvent)
*/
public void attributeAdded(ServletContextAttributeEvent arg0)  {
// TODO Auto-generated method stub
System.out.println("attributeAdded...");
}

/**
* @see ServletContextAttributeListener#attributeRemoved(ServletContextAttributeEvent)
*/
public void attributeRemoved(ServletContextAttributeEvent arg0)  {
// TODO Auto-generated method stub
System.out.println("attributeRemoved...");
}

/**
* @see ServletContextAttributeListener#attributeReplaced(ServletContextAttributeEvent)
*/
public void attributeReplaced(ServletContextAttributeEvent arg0)  {
// TODO Auto-generated method stub
System.out.println("attributeReplaced...");
}

}

添加web.xml配置略。

第一次执行/SetShareVar提示

attributeAdded...
第二次执行/SetShareVar提示
attributeReplaced...
销毁时执行
attributeRemoved...