servlet ---- servlet入门

执行过程

Tomcat Server处理一个http请求的过程

假设来自客户的请求为：http://localhost:8080/wsota/wsota_index.jsp

请求被发送到本机端口8080，被在那里侦听的Coyote HTTP/1.1 Connector获得
Connector把该请求交给它所在的Service的Engine来处理，并等待来自Engine的回应
Engine获得请求localhost/wsota/wsota_index.jsp，匹配它所拥有的所有虚拟主机Host
Engine匹配到名为localhost的Host（即使匹配不到也把请求交给该Host处理，因为该Host被定义为该Engine的默认主机）
localhost Host获得请求/wsota/wsota_index.jsp，匹配它所拥有的所有Context
Host匹配到路径为/wsota的Context（如果匹配不到就把该请求交给路径名为""的Context去处理）
path="/wsota"的Context获得请求/wsota_index.jsp，在它的mapping table中寻找对应的servlet
Context匹配到URL PATTERN为*.jsp的servlet，对应于JspServlet类
构造HttpServletRequest对象和HttpServletResponse对象，作为参数调用JspServlet的doGet或doPost方法
10)Context把执行完了之后的HttpServletResponse对象返回给Host
11)Host把HttpServletResponse对象返回给Engine
12)Engine把HttpServletResponse对象返回给Connector
13)Connector把HttpServletResponse对象返回给客户browser

servlet接口关系图

HttpServlet抽象类中实现了service

    @Overridepublic void service(ServletRequest req, ServletResponse res)throws ServletException, IOException{HttpServletRequest  request;HttpServletResponse response;if (!(req instanceof HttpServletRequest &&res instanceof HttpServletResponse)) {throw new ServletException("non-HTTP request or response");}request = (HttpServletRequest) req;response = (HttpServletResponse) res;service(request, response);}protected void service(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException{String method = req.getMethod();if (method.equals(METHOD_GET)) {long lastModified = getLastModified(req);if (lastModified == -1) {// servlet doesn't support if-modified-since, no reason// to go through further expensive logicdoGet(req, resp);} else {long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);if (ifModifiedSince < lastModified) {// If the servlet mod time is later, call doGet()// Round down to the nearest second for a proper compare// A ifModifiedSince of -1 will always be lessmaybeSetLastModified(resp, lastModified);doGet(req, resp);} else {resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);}}} else if (method.equals(METHOD_HEAD)) {long lastModified = getLastModified(req);maybeSetLastModified(resp, lastModified);doHead(req, resp);} else if (method.equals(METHOD_POST)) {doPost(req, resp);} else if (method.equals(METHOD_PUT)) {doPut(req, resp);} else if (method.equals(METHOD_DELETE)) {doDelete(req, resp);} else if (method.equals(METHOD_OPTIONS)) {doOptions(req,resp);} else if (method.equals(METHOD_TRACE)) {doTrace(req,resp);} else {//// Note that this means NO servlet supports whatever// method was requested, anywhere on this server.//String errMsg = lStrings.getString("http.method_not_implemented");Object[] errArgs = new Object[1];errArgs[0] = method;errMsg = MessageFormat.format(errMsg, errArgs);resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);}}

所以需要重写doGet和doPost

@WebServlet(name = "ServletDemo2", value = "/ServletDemo2")
public class ServletDemo2 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {System.out.println("hello servlet");}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {this.doGet(request,response);}
}

Servlet 生命周期

init : 请求第一次到达servlet 时，对象创建并初始化成功。
service : servlet 一直存在，并提供服务
destroy：服务器宕机，停机销毁servlet对象时触发

@WebServlet(name = "ServletDemo03", value = "/ServletDemo03")
public class ServletDemo03 extends HttpServlet {@Overridepublic void init() throws ServletException {System.out.println("servlet 对象创建了，初始化成");}@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {System.out.println("接收到客户端请求");}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doGet(request,response);}@Overridepublic void destroy() {System.out.println("对象销毁了。。。");}
}

servlet线程安全问题

Servlet 是单实例
一个浏览器代表一个线程，多个浏览器代表多个线程。
定义类成员要谨慎。如果是共用的，并且只会在初始化时赋值，其他时间都是获取，那么是没有问题的。
如果不是共用的，或者每次使用都有可能对其赋值，就要考虑线程安全问题，可将其定义到doGet或doPost方法内或者使用同步功能即可

@WebServlet(name = "ServletDemo4", value = "/ServletDemo4")
public class ServletDemo4 extends HttpServlet {private String username;@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {username = request.getParameter("username");try {Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}PrintWriter writer = response.getWriter();writer.println("username = " + username );writer.close();}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doGet(request,response);}
}

定义到局部

    //private String username;@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String username = request.getParameter("username");try {Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}PrintWriter writer = response.getWriter();writer.println("username = " + username );writer.close();}

加同步锁

    private String username;@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {synchronized (this){username = request.getParameter("username");try {Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}PrintWriter writer = response.getWriter();writer.println("username = " + username );writer.close();}}

映射方式

//@WebServlet(name = "ServletDemo5", value = "/ServletDemo5")
//@WebServlet(name = "ServletDemo5", value = "/ServletDemo5/zs/*")
@WebServlet(name = "ServletDemo5", value = "*.do")
public class ServletDemo5 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {PrintWriter writer = response.getWriter();writer.println("hello servlet");writer.close();}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doGet(request,response);}
}

简单demo

request.getRequestURI() : /ServletDemo6/vip
path.substring(path.lastIndexOf("/")) : /vip
原价：1000 优惠价：900.0

@WebServlet(name = "ServletDemo6", value = "/ServletDemo6/*")
public class ServletDemo6 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//定义商品金额int money = 1000;//获取访问资源路径String path = request.getRequestURI();System.out.println("request.getRequestURI() : " + path);path = path.substring(path.lastIndexOf("/"));System.out.println("path.substring(path.lastIndexOf(\"/\")) : " + path);//判断访问路径if ("/vip".equals(path)){System.out.println("原价：" + money + "\t" + "优惠价：" + (money*0.9));}else if("/vvip".equals(path)){System.out.println("原价：" + money + "\t" + "优惠价：" + (money*0.5));}else {System.out.println("原价：" + money + "\t");}}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doGet(request,response);}
}

Servlet 创建时机

ServletConfig

ServletConfig 是Servlet的配置参数对象，在Servlet的规范中，允许每个Servlet都提供一些初始化的配置。所以，每个Servlet都有一个自己的ServletConfig
作用：在Servlet的初始化时，把一些配置信息传递给Servlet
生命周期：和Servlet相同

常用方法

public class ServletDemo7 extends HttpServlet {private ServletConfig config;@Overridepublic void init(ServletConfig config) throws ServletException {this.config = config;}@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//根据key获取valueString encodingValue = config.getInitParameter("encoding");System.out.println(encodingValue);//获取Servlet的所有keyEnumeration<String> keys = config.getInitParameterNames();while (keys.hasMoreElements()){String key = keys.nextElement();String value = config.getInitParameter(key);System.out.println(key + "," + value);}//获取Servlet名称String servletName = config.getServletName();System.out.println(servletName);//获取ServletContext对象ServletContext servletContext = config.getServletContext();System.out.println(servletContext);}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doGet(request,response);}
}

    <servlet><servlet-name>ServletDemo7</servlet-name><servlet-class>com.example.demo1.servlet.ServletDemo7</servlet-class><init-param><param-name>encoding</param-name><param-value>UTF-8</param-value></init-param><init-param><param-name>ZS</param-name><param-value>hello servletConfig</param-value></init-param></servlet><servlet-mapping><servlet-name>ServletDemo7</servlet-name><url-pattern>/ServletDemo7</url-pattern></servlet-mapping>

ServletContext

ServletContext 是应用上下文对象（应用域对象）。每个应用中只有一个ServetContext对象。
作用：可以配置和获得应用的全局初始化参数，可以实现Servlet之间的数据共享
生命周期：应用一加载则创建，应用被停止则销毁

域对象
域对象指的是对象有作用域。也就是有作用范围。域对象可以实现数据的共享。不同作用域的对象，共享数据的能力也不一样。
Servlet中，有4个域对象。ServletContext是其中之一，也是最大的作用域，也称application域，可以实现整个应用之间的数据共享

    <!--配置ServletContext--><context-param><param-name>globalEncoding</param-name><param-value>UTF-8</param-value></context-param><context-param><param-name>globalDesc</param-name><param-value>This is ServletContext</param-value></context-param>

@WebServlet(name = "ServletDemo8", value = "/ServletDemo8")
public class ServletDemo8 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {//获取ServletContext对象ServletContext context = getServletContext();//根据key获取valueString value = context.getInitParameter("globalDesc");System.out.println("globalDesc = " + value);//获取应用的虚拟目录String contextPath = context.getContextPath();System.out.println("虚拟目录：" + contextPath);//根据虚拟目录获取绝对路径String realPath = context.getRealPath("/");System.out.println("虚拟目录的绝对路径" + realPath);}@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doGet(request,response);}
}

globalDesc = This is ServletContext
虚拟目录：
虚拟目录的绝对路径/Users/Shared/study/javaWeb/MyWeb/demo1/src/main/webapp/

获取虚拟目录的绝对路径可以方便找到文件的绝对路径

ServletContext 用于数据共享

  private ServletConfig config;@Overridepublic void init(ServletConfig config) throws ServletException {this.config = config;}//获取ServletContext对象ServletContext servletContext = config.getServletContext();//获取ServletContext对象ServletContext context = getServletContext();context.setAttribute("username","zs");context.setAttribute("username","lisi");context.removeAttribute("username");

简单案例

webapp>addStudent.html

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><title>保存学生信息</title>
</head>
<body><form action="/studentServlet" method="get" autocomplete="off">学生姓名：<input type="text" name="username"><br/>学生年龄：<input type="text" name="age"><br/>学生成绩：<input type="text" name="score"><br/><button type="submit">保存</button></form>
</body>
</html>

web.xml

    <!--配置默认主页--><welcome-file-list><welcome-file>/addStudent.html</welcome-file></welcome-file-list><servlet><servlet-name>studentServlet</servlet-name><servlet-class>com.example.demo1.servlet.studentServlet</servlet-class></servlet><servlet-mapping><servlet-name>studentServlet</servlet-name><url-pattern>/studentServlet</url-pattern></servlet-mapping>

studentServlet

public class studentServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {String username = req.getParameter("username");String age = req.getParameter("age");String score = req.getParameter("score");BufferedWriter bw = new BufferedWriter(new FileWriter(getServletContext().getRealPath("/stu.txt"), true));bw.write(username + "," + age + "," + score);bw.newLine();bw.close();PrintWriter pw = resp.getWriter();pw.println("Save Success~");pw.close();}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req,resp);}
}