Servlet

標籤： Java與Web

HTTP協議

HTTP(hypertext transport protocol),即超文本傳輸協議.這個協議詳細規定了瀏覽器(Browser)和萬維網服務器(WebServer)之間互相通信的規則.其主要特點可簡單概括如下:
1) 簡單快速: 客戶端向服務器請求服務時,只需傳送請求方法和路徑, 因此使得HTTP服務器的程序規模小,通信速度快;
2) 靈活: HTTP允許傳輸任意類型的數據對象(傳輸類型由Content-Type控制);
3) 無連接: 無連接的含義是限制每次連接只處理一個請求;
4) 無狀態: 無狀態是指協議對於事務處理沒有記憶能力(如果後續處理需要前面的信息,則必須重傳.這樣可能導致每次連接傳送的數據量增大.但如果在服務器不需要先前信息時它的應答就會非常快快).

HTTP請求

一個HTTP請求通常包含三部分(中間已空行隔開):

請求行:        (方法 /統一資源標識符URI/協議/版本)
請求頭:        (Accept/Accept-Language等)
空行:     (CRLF)
請求體:        (攜帶的數據信息, GET請求沒有)

HTTP請求可以使用HTTP標準中定義的所有請求類型, HTTP1.1支持7種請求類型, 但在互聯網應用中最爲常用的只有GET與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

請求頭解析

請求頭	描述
User-Agent	瀏覽器與操作系統信息
Accept	當前瀏覽器可以接收的文檔類型
Accept-Language	當前瀏覽器支持的語言
Accept-Encoding	當前瀏覽器支持的壓縮格式:服務器會把數據壓縮後再發送到網絡中傳輸
Accept-Charset	當前瀏覽器支持的編碼
Connection	當前瀏覽器支持的連接方式(`keep-alive`即保持一段時間的連接,默認爲3000ms)
Cookie	如果不是第一次訪問該網址,可能會在請求中把上次服務器響應的Cookie數據一併發送過去

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

請求頭解析

請求頭	描述
Referer	表明請求來自哪個頁面
Content-Type	application/x-www-form-urlencoded：表單數據類型,說明會使用URL編碼來格式化數據
Content-Length	請求體長度
user_name=feiqing&user_password=pass	請求體: 請求攜帶的數據

HTTP響應

一個HTTP響應通常也包含三部分(中間已空行隔開):

響應行:        (協議/狀態碼/描述)
響應頭:        (Server/Content-Length/Set-Cookie等)
空行:     (CRLF)
響應體:        (攜帶的數據)

HTTP響應是由服務器發送給瀏覽器的數據,瀏覽器會根據HTTP響應來解析並顯示內容:

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

<html>
    ...
</html>

響應頭解析

響應頭	描述
Server	服務器信息
Content-Length	響應實體長度
Set-Cookie	響應給客戶端的Cookie
Expires: -1; / Cache-Control: no-cache; / Pragma: no-cache;	設置瀏覽器不要緩存數據
Refresh	自動刷新頁面

在HTML文件中可用<meta/>標籤來設置響應頭信息:

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

響應狀態碼

狀態碼說明了響應的真正含義:

狀態	描述
200	請求成功
404	請求資源沒找到
500	服務器內部錯誤
302	重定向: 表示服務器要求瀏覽器重新再發一個請求到服務器指定的一個`Location`
304	緩存未過期(服務器資源未曾修改), 詳細可參考理解HTTP/304響應

Tomcat

Tomcat是一個免費開源的Serlvet容器,它是Apache基金會的Jakarta項目中的一個核心項目,由Apache,Sun和其它一些公司及個人共同開發而成. 由於有了Sun的參與和支持, 因此最新的Servlet和Jsp規範總能在Tomcat中得到體現.主頁:http://tomcat.apache.org/.

Tomcat目錄結構

bin: 存放可執行腳本文件(如startup.bat/startup.sh等)
conf: 存放Tomcat相關配置文件:
- server.xml: 整個Tomcat運行環境配置(如端口號/虛擬主機等)
- web.xml: 部署描述符文件(定義了默認JSP/Servlet處理規則,是所有web項目中WEB-INF/web.xml的父文件)
- context.xml: 對所有應用的統一配置.
lib：Tomcat類庫, 該目錄中的jar包所有項目共享.
logs : Tomcat日誌目錄.
webapps：存放WEB應用,其每個子目錄都是一個項目;
work：運行時生成的文件.

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,整個Tomcat的配置信息
`<Service/>`	服務(在`<Server/>`中只能有一個`<Service/>`)
`<Connector/>`	連接
`<Engine/>`	引擎,是`<Service/>`組件核心
`<Host/>`	每個`<Host/>`元素表示一臺虛擬主機.每臺虛擬主機都有自己的主機名和項目目錄
`<Context/>`	每個`<Context/>`元素表示一個應用.如果應用在`<Host/>`的appBase指定的目錄下,那麼可以不配置`<Context/>`元素,如果是外部應用,那麼就必須配置`<Context/>`

Tomcat配置

1. 配置端口號

編輯%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. 配置外部應用

配置外部應用之後, 項目就可以不用拷貝到webapps目錄下,自定義項目存放位置,其配置方式有兩種:

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爲空(path=”“), 則默認訪問的項目就是/home/www/test, 而不再是webapps下的ROOT.

2: 編輯conf/catalana/localhost目錄:
新增test.xml文件

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

存放到%CATALANA_HOME%/conf/catalana/localhost目錄下, 文件名即爲應用名.

Servlet

Servlet技術核心就是Servlet接口,所有Servlet實現類都必須實現Servlet接口,Servlet容器(如Tomcat)會把Servlet類加載到內存並生成唯一實例,當有請求到來時調用其方法.

實現Servlet方式有三種:
1. 實現javax.servlet.Servlet接口
2. 繼承javax.servlet.GenericServlet類
3. 繼承javax.servlet.http.HttpServlet類

Servlet

Servlet接口定義如下

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	在第一次請求該Servlet(默認)或容器啓動時, Servlet容器就會調用`init()`, 且只調用一次
service	每次請求Servlet都會調用該方法
destroy	銷燬Servlet時(卸載應用/關閉容器時), 調用該方法

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/>用來指定Servlet的訪問路徑,必須以/開頭.

可以在<servlet-mapping/>配置多個<url-pattern/>, 此時一個Servlet實例就綁定多個URL.

可以在<url-pattern/>中使用通配符*,可以使一個Servlet綁定一組URL, 但*不能出現在中間位置,也不能只有*通配符, 另外, 通配符只是一種模糊匹配URL的方式,如果存在更具體的<url-pattern/>,那麼會優先選擇精確匹配.

配置在容器啓動時創建Servlet實例

<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/>元素可以讓容器在啓動時就創建該Servlet實例(調用init()方法),注意<load-on-startup/>元素的值必須是>=0的整數,它代表容器啓動時創建Servlet實例的順序.

GenericService

GenericService抽象類實現了Servlet接口並完成以下工作:
1. 將init()方法中的ServletConfig賦給一個實例變量, 使他可以通過getServletConfig()來獲取.
2. 爲Servlet接口的所有方法提供默認實現.
3. 提供方法來包裝ServletConfig.

Generic部分代碼

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的子類,它提供了對HTTP協議的支持.覆蓋了GenericServlet的service()方法,並新增了接受HttpServletRequest/HttpServletResponse參數的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()將請求/響應向下轉型爲HttpServletRequest/HttpServletResponse, 並調用新的service(). 由於HttpServlet在新的service()方法中已經做了很多工作, 因此在繼承HttpServlet實現自動以Servlet時, 則只需覆蓋doGet()/doPost()等即可, 而沒有必要覆蓋service()(極少數情況需要覆蓋doHead()等)

注意: Request/Response向下轉型總會成功:因爲在調用service()方法時,Servlet容器總會預計使用HTTP,從而直接創建並傳遞HttpServletRequest/HttpServletResponse實例.

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

對於每一個HTTP請求, Servlet容器會在調用service()方法時創建Request實例並傳遞給service形參, HttpServletRequest是Request在HTTP環境下的實例,其封裝了有關請求的信息:

封裝請求頭信息;
封裝請求正文數據(GET沒有正文);
提供請求轉發/包含功能;
作爲域對象, 可以傳遞數據.

獲取請求頭

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.

獲取請求來源

@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>來源地址: " + referer + "</h1>");
    } else {
        writer.print("<h1>來自瀏覽器地址欄</h1>");
    }

    writer.print("<hr>");
    writer.print("<h1>來源信息: " + userAgent + "</h1>");
}

獲取請求參數

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.

獲取微信請求消息

@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();
    }

    // ...
}

請求轉發/包含

Request提供了getRequestDispatcher()來獲取一個RequestDispatcher, 並由其提供請求轉發/請求包含功能.

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.

請求轉發/請求包含都是由多個Servlet協作完成一個請求, 因此需要從一個Servlet中跳到另一個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.

請求轉發: 原Servlet只會保留設置的響應頭信息.
請求包含: 原Servlet既會保留響應頭, 還會保留響應體內容.

注意: 請求轉發時, 可能會因爲原Servlet設置了過多的響應體內容導致拋出異常java.lang.IllegalStateException: Cannot forward after response has been committed

域對象傳遞數據

由於請求轉發/請求包含都只是一次請求, 因此在多個Servlet之間都是共用一個Reqeust, 因此可以利用Request的在多個Servlet之間共享數據:

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容器會在每次調用service()方法時創建Response實例並傳遞給service()形參, HttpServletResponse是Response綁定在HTTP環境下的實例, 其隱藏了將響應發送給瀏覽器的複雜性:

設置響應狀態碼;
設置響應頭信息;
設置響應正文;

設置響應狀態碼

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.

關於狀態碼的描述, 詳見HTTP協議部分介紹, 在此就不再贅述.

響應404

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

設置響應頭信息

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.

關於HTTP響應頭的描述, 詳見HTTP協議部分介紹, 在此就不再贅述.

設置禁用瀏覽器緩存(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);
}

設置重定向(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還提供了另外一種重定向的方式, 直接使用sendRedirect()方法, 避免了以上的步驟:

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

設置響應正文

Response提供瞭如下兩個方法來獲取輸出流對象以響應HTTP正文內容

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傳輸二進制數據流(字節數據), 常用作文件下載; Writer傳輸字符數據, 常用作響應HTTP正文內容(如HTML/XML等).

注意: 在一個請求中,不能同時使用這兩個流, 否則會拋出IllegalStateException.

字符響應流

@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>");
}

緩衝區
PrintWriter的默認緩衝區大小爲8K, 因此當響應數據大小＜8K時, 數據存放在緩衝區, 而不會立刻發送到瀏覽器, 直到Servlet執行結束,因此如果希望馬上發送給瀏覽器, 需要調用Response的flushBuffer()方法手動刷新緩衝區.

ServletConfig

在容器初始化Servlet時, 會將一個ServletConfig實例傳給init()方法,其封裝了@WebServlet/部署描述符傳遞給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>[email protected]</param-value>
        </init-param>
    </servlet>

ServletContext

ServletConfig中提供了獲取ServletContext的方法getServletContext(), ServletContext代表Servlet應用程序,且每個應用程序僅有一個ServletContext實例,其在容器啓動時創建, 在容器關閉時銷燬, 因此可以利用其在多個Servlet中傳遞數據.
所有域對象都有存取數據的功能,因爲域對象內部有一個Map,用來存儲數據,下面是ServletContext對象用來操作數據的方法：

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.

應用初始化參數

前面看到ServletConfig可以獲取針對本Servlet的初始化參數,而利用ServletContext可以獲取針對本應用程序的公共初始化參數:

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);
}

獲取資源

可以使用ServletContext來獲取Web應用下的資源路徑/資源流等內容:

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.

Servlet - 基礎

Servlet

HTTP協議

HTTP請求

HTTP-GET

HTTP-POST

HTTP響應

響應狀態碼

Tomcat

Tomcat目錄結構

server.xml

Tomcat配置

1. 配置端口號

2. 配置外部應用

Servlet

Servlet

GenericService

HttpServlet

HttpServletRequest

獲取請求頭

獲取請求參數

請求轉發/包含

域對象傳遞數據

HttpServletResponse

設置響應狀態碼

設置響應頭信息

設置響應正文

字符響應流

ServletConfig

ServletContext

應用初始化參數

獲取資源