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Servlet – 基础

HTTP协议

HTTP(hypertext transport protocol),即超文本传输协议.这个协议详细规定了浏览器(Browser)和万维网服务器(WebServer)之间互相通信的规则.其主要特点可简单概括如下:

1) 简单快速 : 客户端向服务器请求服务时,只需传送请求方法和路径, 因此使得HTTP服务器的程序规模小,通信速度快;

2) 灵活 : HTTP允许传输任意类型的数据对象(传输类型由 Content-Type 控制);

3) 无连接 : 无连接的含义是限制每次连接只处理一个请求;

4) 无状态 : 无状态是指协议对于事务处理没有记忆能力(如果后续处理需要前面的信息,则必须重传.这样可能导致每次连接传送的数据量增大.但如果在服务器不需要先前信息时它的应答就会非常快快).

HTTP请求

一个HTTP请求通常包含三部分(中间已空行隔开):

请求行:        (方法 /统一资源标识符URI/协议/版本) 请求头:        (Accept/Accept-Language等) 空行:     (CRLF) 请求体:        (携带的数据信息, GET请求没有)

HTTP请求可以使用HTTP标准中定义的所有请求类型, HTTP1.1支持7种请求类型, 但在互联网应用中最为常用的只有 GET 与 POST .

HTTP-GET

GET /WeChat/cc3200/get_status.do HTTP/1.1 Host: aliyun User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: zh-CN,zh;q=0.8,en-US;q=0.5,en;q=0.3 Accept-Encoding: gzip, deflate Connection: keep-alive

请求头解析

请求头	描述
User-Agent	浏览器与操作系统信息
Accept	当前浏览器可以接收的文档类型
Accept-Language	当前浏览器支持的语言
Accept-Encoding	当前浏览器支持的压缩格式:服务器会把数据压缩后再发送到网络中传输
Accept-Charset	当前浏览器支持的编码
Connection	当前浏览器支持的连接方式( `keep-alive` 即保持一段时间的连接,默认为3000ms)
Cookie	如果不是第一次访问该网址,可能会在请求中把上次服务器响应的Cookie数据一并发送过去

HTTP-POST

POST /WeChat/cc3200/get_status.do HTTP/1.1 Content-Length: 36 Cache-Control: max-age=0 Origin: http://localhost:8080 Content-Type: application/x-www-form-urlencoded Referer: http://localhost:8080/test/ ...  user_name=feiqing&user_password=pass

请求头解析

请求头	描述
Referer	表明请求来自哪个页面
Content-Type	application/x-www-form-urlencoded：表单数据类型,说明会使用URL编码来格式化数据
Content-Length	请求体长度
user_name=feiqing&user_password=pass	请求体: 请求携带的数据

HTTP响应

一个HTTP响应通常也包含三部分(中间已空行隔开):

响应行:        (协议/状态码/描述) 响应头:        (Server/Content-Length/Set-Cookie等) 空行:     (CRLF) 响应体:        (携带的数据)

HTTP响应是由服务器发送给浏览器的数据,浏览器会根据HTTP响应来解析并显示内容:

HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Content-Length: 8 Date: Sun, 17 Apr 2016 12:39:11 GMT  <html>     ... </html>

响应头解析

响应头	描述
Server	服务器信息
Content-Length	响应实体长度
Set-Cookie	响应给客户端的Cookie
Expires: -1; / Cache-Control: no-cache; / Pragma: no-cache;	设置浏览器不要缓存数据
Refresh	自动刷新页面

在HTML文件中可用 <meta/> 标签来设置响应头信息:

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

响应状态码

状态码说明了响应的真正含义:

状态	描述
200	请求成功
404	请求资源没找到
500	服务器内部错误
302	重定向: 表示服务器要求浏览器重新再发一个请求到服务器指定的一个 `Location`
304	缓存未过期(服务器资源未曾修改), 详细可参考理解HTTP/304响应

Tomcat是一个免费开源的Serlvet容器,它是Apache基金会的 Jakarta 项目中的一个核心项目,由Apache,Sun和其它一些公司及个人共同开发而成. 由于有了Sun的参与和支持, 因此最新的Servlet和Jsp规范总能在Tomcat中得到体现.主页: http://tomcat.apache.org/ .

Tomcat目录结构

Servlet – 基础

bin: 存放可执行脚本文件(如startup.bat/startup.sh等)
conf: 存放Tomcat相关配置文件:
- server.xml: 整个Tomcat运行环境配置(如端口号/虚拟主机等)
- web.xml: 部署描述符文件(定义了默认JSP/Servlet处理规则,是所有web项目中WEB-INF/web.xml的父文件)
- context.xml: 对所有应用的统一配置.
lib：Tomcat类库, 该目录中的jar包所有项目共享.
logs : Tomcat日志目录.
webapps：存放WEB应用,其每个子目录都是一个项目;
work：运行时生成的文件.

server.xml

<Server port="8005" shutdown="SHUTDOWN">   <Service name="Catalina">     <Connector port="8080" protocol="HTTP/1.1"                connectionTimeout="20000"                redirectPort="8443" />     <Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />      <Engine name="Catalina" defaultHost="localhost">       <Realm className="org.apache.catalina.realm.LockOutRealm">         <Realm className="org.apache.catalina.realm.UserDatabaseRealm"                resourceName="UserDatabase"/>       </Realm>        <Host name="localhost"  appBase="webapps"             unpackWARs="true" autoDeploy="true">         <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"                prefix="localhost_access_log." suffix=".txt"                pattern="%h %l %u %t "%r" %s %b" />       </Host>     </Engine>   </Service> </Server>

元素解析

元素	描述
`<Server/>`	根元素,整个Tomcat的配置信息
`<Service/>`	服务(在 `<Server/>` 中只能有一个 `<Service/>` )
`<Connector/>`	连接
`<Engine/>`	引擎,是 `<Service/>` 组件核心
`<Host/>`	每个 `<Host/>` 元素表示一台虚拟主机.每台虚拟主机都有自己的主机名和项目目录
`<Context/>`	每个 `<Context/>` 元素表示一个应用.如果应用在 `<Host/>` 的appBase指定的目录下,那么可以不配置 `<Context/>` 元素,如果是外部应用,那么就必须配置 `<Context/>`

Tomcat配置

1. 配置端口号

编辑 %CATALANA_HOME%/conf/server.xml 文件中的 <Connector/> 元素

<!-- A "Connector" represents an endpoint by which requests are received      and responses are returned. Documentation at :      Java HTTP Connector: /docs/config/http.html (blocking & non-blocking)      Java AJP  Connector: /docs/config/ajp.html      APR (HTTP/AJP) Connector: /docs/apr.html      Define a non-SSL HTTP/1.1 Connector on port 8080 --> <Connector port="8080" protocol="HTTP/1.1"            connectionTimeout="20000"            redirectPort="8443" />

2. 配置外部应用

配置外部应用之后, 项目就可以不用拷贝到webapps目录下,自定义项目存放位置,其配置方式有两种:

1: 修改server.xml

在 <Host/> 元素中添加 <Context/> 元素

<Host name="localhost"  appBase="webapps"         unpackWARs="true" autoDeploy="true">     <Context path="/test/" docBase="/home/www/test"/>      <!-- Access log processes all example.          Documentation at: /docs/config/valve.html          Note: The pattern used is equivalent to using pattern="common" -->     <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"            prefix="localhost_access_log." suffix=".txt"            pattern="%h %l %u %t "%r" %s %b" />    </Host>

如果指定path为空(path=”“), 则默认访问的项目就是 /home/www/test , 而不再是webapps下的ROOT.

2: 编辑conf/catalana/localhost目录:

新增test.xml文件

<Context docBase="/home/www/test"/>

存放到 %CATALANA_HOME%/conf/catalana/localhost 目录下, 文件名即为应用名.

Servlet

Servlet技术核心就是 Servlet 接口,所有Servlet实现类都必须实现 Servlet 接口,Servlet容器(如Tomcat)会把Servlet类加载到内存并生成唯一实例,当有请求到来时调用其方法.

实现Servlet方式有三种:
1. 实现 javax.servlet.Servlet 接口
2. 继承 javax.servlet.GenericServlet 类
3. 继承 javax.servlet.http.HttpServlet 类

Servlet

Servlet 接口定义如下

public interface Servlet {      public void init(ServletConfig config) throws ServletException;      public ServletConfig getServletConfig();      public void service(ServletRequest req, ServletResponse res)     throws ServletException, IOException;      public String getServletInfo();      public void destroy(); }

方法	描述
init	在第一次请求该Servlet(默认)或容器启动时, Servlet容器就会调用 `init()` , 且只调用一次
service	每次请求Servlet都会调用该方法
destroy	销毁Servlet时(卸载应用/关闭容器时), 调用该方法

HelloServlet

/**  * @author jifang.  * @since 2016/4/17 8:32.  */ public class HelloServlet implements Servlet {      private ServletConfig config;      public void init(ServletConfig config) throws ServletException {         System.out.println("init()...");         this.config = config;         System.out.println("config: <" + config + ">");     }      public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {         System.out.println("service()...");         System.out.println("req: <" + req + ">, res: <" + res + ">");         res.getWriter().print("<h1>HelloServlet</h1>");     }      public void destroy() {         System.out.println("destroy()...");     }      public ServletConfig getServletConfig() {         return this.config;     }      public String getServletInfo() {         return null;     } }

web.xml

<web-app xmlns="http://java.sun.com/xml/ns/javaee"          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"          xsi:schemaLocation="http://java.sun.com/xml/ns/javaee  http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"           version="3.0"          metadata-complete="true">     <display-name>JavaWeb</display-name>     <servlet>         <servlet-name>HelloServlet</servlet-name>         <servlet-class>com.fq.web.servlet.HelloServlet</servlet-class>     </servlet>     <servlet-mapping>         <servlet-name>HelloServlet</servlet-name>         <url-pattern>/hello_servlet.do</url-pattern>     </servlet-mapping> </web-app>

1.url-pattern

<url-pattern/>用来指定Servlet的访问路径,必须以/开头.

可以在<servlet-mapping/>配置多个<url-pattern/>, 此时一个Servlet实例就绑定多个URL.
可以在<url-pattern/>中使用通配符*,可以使一个Servlet绑定一组URL, 但*不能出现在中间位置,也不能只有*通配符, 另外, 通配符只是一种模糊匹配URL的方式,如果存在更具体的<url-pattern/>,那么会优先选择精确匹配.

2.配置在容器启动时创建Servlet实例

<servlet>     <servlet-name>HelloServlet</servlet-name>     <servlet-class>com.fq.web.servlet.HelloServlet</servlet-class>     <load-on-startup>1</load-on-startup> </servlet>

<load-on-startup/> 元素可以让容器在启动时就创建该Servlet实例(调用 init() 方法),注意 <load-on-startup/> 元素的值必须是 >=0 的整数,它代表容器启动时创建Servlet实例的顺序.

GenericService

GenericService 抽象类实现了 Servlet 接口并完成以下工作:

1. 将 init() 方法中的 ServletConfig 赋给一个实例变量, 使他可以通过 getServletConfig() 来获取.

2. 为Servlet接口的所有方法提供默认实现.

3. 提供方法来包装 ServletConfig .

Generic部分代码

public abstract class GenericServlet      implements Servlet, ServletConfig, java.io.Serializable {     private transient ServletConfig config;      public GenericServlet() { }      public void destroy() {     }      public String getInitParameter(String name) {         ServletConfig sc = getServletConfig();         if (sc == null) {             throw new IllegalStateException(                 lStrings.getString("err.servlet_config_not_initialized"));         }          return sc.getInitParameter(name);     }      public Enumeration<String> getInitParameterNames() {         ServletConfig sc = getServletConfig();         if (sc == null) {             throw new IllegalStateException(                 lStrings.getString("err.servlet_config_not_initialized"));         }          return sc.getInitParameterNames();     }         public ServletConfig getServletConfig() {     return config;     }      public ServletContext getServletContext() {         ServletConfig sc = getServletConfig();         if (sc == null) {             throw new IllegalStateException(                 lStrings.getString("err.servlet_config_not_initialized"));         }          return sc.getServletContext();     }      public String getServletInfo() {     return "";     }      public void init(ServletConfig config) throws ServletException {     this.config = config;     this.init();     }      public void init() throws ServletException {      }      public abstract void service(ServletRequest req, ServletResponse res)     throws ServletException, IOException;      public String getServletName() {         ServletConfig sc = getServletConfig();         if (sc == null) {             throw new IllegalStateException(                 lStrings.getString("err.servlet_config_not_initialized"));         }          return sc.getServletName();     } }

HttpServlet

HttpServlet 是 GenericServlet 的子类,它提供了对HTTP协议的支持.覆盖了 GenericServlet 的 service() 方法,并新增了接受 HttpServletRequest / HttpServletResponse 参数的 service() 方法:

@Override public 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 logic             doGet(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 less                 maybeSetLastModified(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);     } }

原始的 service() 将请求/响应向下转型为 HttpServletRequest / HttpServletResponse , 并调用新的 service() . 由于 HttpServlet 在新的 service() 方法中已经做了很多工作, 因此在继承 HttpServlet 实现自动以Servlet时, 则只需覆盖 doGet() / doPost() 等即可, 而没有必要覆盖 service() (极少数情况需要覆盖 doHead() 等)

注意: Request/Response向下转型总会成功:因为在调用 service() 方法时,Servlet容器总会预计使用HTTP,从而直接创建并传递 HttpServletRequest / HttpServletResponse 实例.

HelloHttpServlet

/**  * @author jifang.  * @since 2016/4/20 19:48.  */ @WebServlet(name = "HelloHttpServlet", urlPatterns = "/hello_http_servlet.do") public class HelloHttpServlet extends HttpServlet {      @Override     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         System.out.println("doPost() ...");     }      @Override     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         System.out.println("doGet() ...");     } }

HttpServletRequest

对于每一个HTTP请求, Servlet容器会在调用 service() 方法时创建Request实例并传递给 service 形参, HttpServletRequest 是Request在HTTP环境下的实例,其封装了有关请求的信息:

封装请求头信息;
封装请求正文数据(GET没有正文);
提供请求转发/包含功能;
作为域对象, 可以传递数据.

获取请求头

方法	描述
`String getHeader(String name)`	Returns the value of the specified request header as a String.
`Enumeration<String> getHeaderNames()`	Returns an enumeration of all the header names this request contains.
`long getDateHeader(String name)`	Returns the value of the specified request header as a long value that represents a Date object.
`Enumeration<String> getHeaders(String name)`	Returns all the values of the specified request header as an Enumeration of String objects.
`int getIntHeader(String name)`	Returns the value of the specified request header as an int.
`String getRemoteAddr()`	Returns the Internet Protocol (IP) address of the client or last proxy that sent the request.
`String getMethod()`	Returns the name of the HTTP method with which this request was made, for example, GET, POST, or PUT.
`String getContextPath()`	Returns the portion of the request URI that indicates the context of the request.

获取请求来源

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     String referer = request.getHeader("Referer");     String userAgent = request.getHeader("User-Agent");     composeResponse(referer, userAgent, response); }  private void composeResponse(String referer, String userAgent, HttpServletResponse response) throws IOException {     response.setHeader("Content-Type", "text/html;charset=utf-8");     PrintWriter writer = response.getWriter();     if (!Strings.isNullOrEmpty(referer)) {         writer.print("<h1>来源地址: " + referer + "</h1>");     } else {         writer.print("<h1>来自浏览器地址栏</h1>");     }      writer.print("<hr>");     writer.print("<h1>来源信息: " + userAgent + "</h1>"); }

获取请求参数

方法	描述
`String getParameter(String name)`	Returns the value of a request parameter as a String, or null if the parameter does not exist.
`Map<String,String[]> getParameterMap()`	Returns a java.util.Map of the parameters of this request.
`Enumeration<String> getParameterNames()`	Returns an Enumeration of String objects containing the names of the parameters contained in this request.
`String[] getParameterValues(String name)`	Returns an array of String objects containing all of the values the given request parameter has, or null if the parameter does not exist.

获取微信请求消息

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     Element root;     String xml = request.getParameter("xml");     try {         if (!Strings.isNullOrEmpty(xml)) {             root = new SAXReader().read(new StringReader(xml)).getRootElement();         } else {             String data = CharStreams.toString(new InputStreamReader(request.getInputStream()));             root = new SAXReader().read(new StringReader(data)).getRootElement();         }     } catch (DocumentException | IOException e) {         LOGGER.error("parse wx xml error", e);         throw new RuntimeException();     }      // ... }

请求转发/包含

Request提供了 getRequestDispatcher() 来获取一个 RequestDispatcher , 并由其提供请求转发/请求包含功能.

Request方法	描述
`RequestDispatcher getRequestDispatcher(String path)`	Returns a RequestDispatcher object that acts as a wrapper for the resource located at the given path.

请求转发/请求包含都是由多个Servlet协作完成一个请求, 因此需要从一个Servlet中跳到另一个Servlet中:

RequestDispatcher方法	描述
`void include(ServletRequest request, ServletResponse response)`	Includes the content of a resource (servlet, JSP page, HTML file) in the response.
`void forward(ServletRequest request, ServletResponse response)`	Forwards a request from a servlet to another resource (servlet, JSP file, or HTML file) on the server.

请求转发: 原Servlet只会保留设置的响应头信息.
请求包含: 原Servlet既会保留响应头, 还会保留响应体内容.

注意: 请求转发时, 可能会因为原Servlet设置了过多的响应体内容导致抛出异常 java.lang.IllegalStateException: Cannot forward after response has been committed

域对象传递数据

由于请求转发/请求包含都只是一次请求, 因此在多个Servlet之间都是共用一个 Reqeust , 因此可以利用Request的在多个Servlet之间共享数据:

方法	描述
`Object getAttribute(String name)`	Returns the value of the named attribute as an Object, or null if no attribute of the given name exists.
`Enumeration<String> getAttributeNames()`	Returns an Enumeration containing the names of the attributes available to this request.
`void setAttribute(String name, Object o)`	Stores an attribute in this request.
`void removeAttribute(String name)`	Removes an attribute from this request.

HttpServletResponse

同Request, Servlet容器会在每次调用 service() 方法时创建Response实例并传递给 service() 形参, HttpServletResponse 是Response绑定在HTTP环境下的实例, 其隐藏了将响应发送给浏览器的复杂性:

设置响应状态码;
设置响应头信息;
设置响应正文;

设置响应状态码

方法	描述
`void setStatus(int sc)`	Sets the status code for this response.
`void sendError(int sc)`	Sends an error response to the client using the specified status code and clears the buffer.
`void sendError(int sc, String msg)`	Sends an error response to the client using the specified status and clears the buffer.

关于状态码的描述, 详见HTTP协议部分介绍, 在此就不再赘述.

响应404

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     // response.sendError(404, "nothing!!");     response.setStatus(404); }

设置响应头信息

方法	描述
`void setHeader(String name, String value)`	Sets a response header with the given name and value.
`void addHeader(String name, String value)`	Adds a response header with the given name and value.
`void setIntHeader(String name, int value)`	Sets a response header with the given name and integer value.
`void addIntHeader(String name, int value)`	Adds a response header with the given name and integer value.
`void addDateHeader(String name, long date)`	Adds a response header with the given name and date-value.
`void setDateHeader(String name, long date)`	Sets a response header with the given name and date-value.
`void sendRedirect(String location)`	Sends a temporary redirect response to the client using the specified redirect location URL and clears the buffer.

关于HTTP响应头的描述, 详见HTTP协议部分介绍, 在此就不再赘述.

设置禁用浏览器缓存(Cache-Control, pragma, expires)

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     response.setHeader("Cache-Control", "no-cache");     response.setHeader("pragma", "no-cache");     response.setDateHeader("expires", -1); }

设置重定向(302, Location)

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     response.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY);     response.setHeader("Location", "http://www.baidu.com"); }

HttpServletResponse还提供了另外一种重定向的方式, 直接使用 sendRedirect() 方法, 避免了以上的步骤:

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     response.sendRedirect("http://www.baidu.com"); }

设置响应正文

Response提供了如下两个方法来获取输出流对象以响应HTTP正文内容

方法	描述
`ServletOutputStream getOutputStream()`	Returns a ServletOutputStream suitable for writing binary data in the response.
`PrintWriter getWriter()`	Returns a PrintWriter object that can send character text to the client.

OutputStream传输二进制数据流(字节数据), 常用作文件下载; Writer传输字符数据, 常用作响应HTTP正文内容(如HTML/XML等).

注意: 在一个请求中,不能同时使用这两个流, 否则会抛出 IllegalStateException .

字符响应流

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     PrintWriter writer = response.getWriter();     writer.print("<html>");     writer.print("<h1>content</h1>");     writer.print("</html>"); }

缓冲区

PrintWriter 的默认缓冲区大小为8K, 因此当响应数据大小＜8K时, 数据存放在缓冲区, 而不会立刻发送到浏览器, 直到Servlet执行结束,因此如果希望马上发送给浏览器, 需要调用Response的 flushBuffer() 方法手动刷新缓冲区.

ServletConfig

在容器初始化Servlet时, 会将一个 ServletConfig 实例传给 init() 方法,其封装了 @WebServlet /部署描述符传递给Servlet的配置信息:

方法	描述
`String getInitParameter(String name)`	Gets the value of the initialization parameter with the given name.
`Enumeration<String> getInitParameterNames()`	Returns the names of the servlet’s initialization parameters as an Enumeration of String objects.
`ServletContext getServletContext()`	Returns a reference to the ServletContext in which the caller is executing.

java

public void init(ServletConfig config) throws ServletException {     this.config = config;      Enumeration<String> names = config.getInitParameterNames();     while (names.hasMoreElements()) {         String name = names.nextElement();         String value = config.getInitParameter(name);          System.out.println(name + " -> " + value);     } }

web.xml

<servlet>         <servlet-name>HelloServlet</servlet-name>         <servlet-class>com.fq.web.servlet.HelloServlet</servlet-class>         <init-param>             <param-name>admin</param-name>             <param-value>com.fq</param-value>         </init-param>         <init-param>             <param-name>e-mail</param-name>             <param-value>zhujifang666@163.com</param-value>         </init-param>     </servlet>

ServletContext

ServletConfig 中提供了获取 ServletContext 的方法 getServletContext() , ServletContext 代表Servlet应用程序,且每个应用程序仅有一个 ServletContext 实例,其在容器启动时创建, 在容器关闭时销毁, 因此可以利用其在多个Servlet中传递数据.

所有域对象都有存取数据的功能,因为域对象内部有一个Map,用来存储数据,下面是ServletContext对象用来操作数据的方法：

方法	描述
`void setAttribute(String name, Object object)`	Binds an object to a given attribute name in this ServletContext.
`Object getAttribute(String name)`	Returns the servlet container attribute with the given name, or null if there is no attribute by that name.
`Enumeration<String> getAttributeNames()`	Returns an Enumeration containing the attribute names available within this ServletContext.
`void removeAttribute(String name)`	Removes the attribute with the given name from this ServletContext.

应用初始化参数

前面看到 ServletConfig 可以获取针对本Servlet的初始化参数,而利用 ServletContext 可以获取针对本应用程序的公共初始化参数:

方法	描述
`String getInitParameter(String name)`	Returns a String containing the value of the named context-wide initialization parameter, or null if the parameter does not exist.
`Enumeration<String> getInitParameterNames()`	Returns the names of the context’s initialization parameters as an Enumeration of String objects, or an empty Enumeration if the context has no initialization parameters.

web.xml

<context-param>     <param-name>admin</param-name>     <param-value>feiqing</param-value> </context-param> <context-param>     <param-name>e-mail</param-name>     <param-value>zhujifang666@163.com</param-value> </context-param>

java

@Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {     ServletContext context = getServletContext();     String admin = context.getInitParameter("admin");     String email = context.getInitParameter("e-mail");     System.out.printf("admin: %s%n", admin);     System.out.printf("e-mail: %s%n", email); }

获取资源

可以使用 ServletContext 来获取Web应用下的资源路径/资源流等内容:

方法	描述
`String getRealPath(String path)`	Gets the real path corresponding to the given virtual path.
`URL getResource(String path)`	Returns a URL to the resource that is mapped to the given path.
InputStream getResourceAsStream(String path)`	Returns the resource located at the named path as an InputStream object.
`Set<String> getResourcePaths(String path)`	Returns a directory-like listing of all the paths to resources within the web application whose longest sub-path matches the supplied path argument.