Servlet - 基础

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转载最新推荐文章于 2026-05-18 06:27:32 发布 328 阅读 AI 写同款· GEO 优化 ›

Servlet基础之实现第一个servlet程序实现第一个Servlet程序1. 创建servlet文件在某个盘下创建一个新的文件夹，（文件夹名字自己开心就好，也可以是原来命名好的文件夹）在此目录下创建一个XXXX.java文件，内容如下packagejava_web;//这是包名，也就是你创建的文件夹的名字，从大到小，我是直接在D盘下创建的java_web的文件夹，所以包名就直接是java_webimportjava.io.*;imp... 阅读详情

本文转自：http://blog.youkuaiyun.com/zjf280441589/article/details/51247469

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">
 1
 1

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

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

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 &quot;%r&quot; %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 &quot;%r&quot; %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"/>
 1
 1

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>

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.

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.