前言

0、創建module

請參考Day05創建module

一、ServletConfig

ServletConfig是為每一個Servlet專屬的設定檔，可以透過web.xml設定檔，再由web container初始化Servlet時透過GenericServlet init方法交給我們ServletConfig物件，由於GenericServlet也實作了ServletConfig interface我們可以透過定義好的方法取得container傳遞給我們的設定檔內容

• getInitParameterNames()取得參數名稱enumeration物件
• getServletContext()
• getInitParameter(String name)透過key取出value值

(1) 創建DemoServletConfig

public class DemoServletConfig extends HttpServlet {

    public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
        PrintWriter out = res.getWriter();
        out.println("keyA"+"=="+getInitParameter("keyA"));
        out.println("===========hello servletConfig==========");

        //取得所有初始化參數名稱
        Enumeration<String> initParameterNames = getServletConfig().getInitParameterNames();
        while (initParameterNames.hasMoreElements()) {
            String name = initParameterNames.nextElement();
            out.println(name + "==" + getServletConfig().getInitParameter(name));
        }
        out.println("===========hello servletConfig==========");
    }
}

修改web.xml設定檔，透過tag init-param設定Servlet初始參數

<web-app>
  <servlet>
    <servlet-name>DemoServletConfig</servlet-name>
    <servlet-class>com.swj.DemoServletConfig</servlet-class>
    <init-param>
      <param-name>keyA</param-name>
      <param-value>paramA</param-value>
    </init-param>
    <init-param>
      <param-name>keyB</param-name>
      <param-value>paramB</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>DemoServletConfig</servlet-name>
    <url-pattern>/DemoServletConfig</url-pattern>
  </servlet-mapping>
</web-app>

Demo

啟動server，訪問路徑http://localhost:8080/MyWebApp/DemoServletConfig

(2) 使用Annotation

首先先註解掉web.xml中的相關設定，再對DemoServletConfig進行修改

@WebServlet(value = "/DemoServletConfig",
            initParams = {@WebInitParam(name = "keyA",value = "annotationValueA"),
                    @WebInitParam(name = "keyB",value = "annotationValueB")})
public class DemoServletConfig extends HttpServlet {
  //略
}

Annotation Demo

二、ServletContext

ServletContext api doc提及此介面提供的方法是Servlet用來與Servlet Container溝通用的，既是溝通所以帶我們可以取得Container相關資訊，亦可對Container設定資訊，所以每個Servlet使用的是同一個ServletContext。我們可以透過下面3個API取得ServletContext

• getServletContext()
• getServletConfig().getServletContext()
• ServletRequest.getServletContext()

創建DemoContextServlet

<web-app>
    <context-param>
        <param-name>encoding</param-name>
        <param-value>UTF-8</param-value>
    </context-param>
    <context-param>
        <param-name>contextParamA</param-name>
        <param-value>contextValueA</param-value>
    </context-param>
    <servlet>
        <servlet-name>HelloContextServlet</servlet-name>
        <servlet-class>com.swj.DemoContextServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloContextServlet</servlet-name>
        <url-pattern>/DemoContextServlet</url-pattern>
    </servlet-mapping>
</web-app>

public class DemoContextServlet extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
        PrintWriter out = res.getWriter();
        out.println("===========DemoContextServlet==========");
        //get ServletContext method
        ServletContext context1 = getServletContext();
        ServletContext context2 = getServletConfig().getServletContext();
        ServletContext context3 = req.getServletContext();
        out.println(context1 == context2);
        out.println(context2 == context3);
        out.println(context1);
        out.println("getEncoding use getInitParameter(\"encoding\")"+"=="+context1.getInitParameter("encoding"));
        Enumeration<String> initParameterNames = context1.getInitParameterNames();
        while (initParameterNames.hasMoreElements()) {
            String name = initParameterNames.nextElement();
            out.println(name + "==" + getServletConfig().getInitParameter(name));
        }
        out.println("===========DemoContextServlet==========");
    }
}

修改DemoServletConfig來確認是否兩個Servlet取得同一個ServletContext物件

@WebServlet(value = "/DemoServletConfig",
            initParams = {@WebInitParam(name = "keyA",value = "annotationValueA"),
                    @WebInitParam(name = "keyB",value = "annotationValueB")})
public class DemoServletConfig extends HttpServlet {

    public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
      //略
      out.println("ServletContext:"+getServletContext());
      out.println("===========hello servletConfig==========");   
    }
}

Demo

demo取得context中的初始參數

DemoServletConfig與DemoContextServlet取得同一個ServletContext物件

小結

web.xml是用來跟web container溝通的地方，他為每一個Servlet提供專屬的設定檔配置物件ServletConfig，也為所有的Servlet提供共用設定配置物件ServletContext。

Reference

• Servlet Tutorial - Javatpoint
• Context in programming - ACM Digital Library
• The term "Context" in programming? - stackoverflow Brian Kelly
• ServletConfig - jakarta api doc
• ServletContext - jakarta api doc