JavaWeb學習筆記8 —— ServletContext 常用方法
參考教程B站狂神https://www.bilibili.com/video/BV12J411M7Sj
web容器在啓動的時候,它會爲每個web程序都創建一個對應的ServletContext對象,它代表了當前的web應用;
主要方法:
ServletContext context = this.getServletContext();
注意下面的這些之後會被別的方法替換掉,不怎麼用!
1、共享數據
用到的主要方法:
//設置
context.setAttribute("參數名",參數值(可以是變量名));
//獲取
context.getAttribute("參數名");
在這個Servlet中保存的數據,可以在另外一個servlet中拿到;
在HelloServlet 裏設置上下文數據
package com.ylw.servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class HelloServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//this.getInitParameter() 初始化參數
//this.getServletConfig() Servlet配置
//this.getServletContext() Servlet上下文
ServletContext context = this.getServletContext();
String username = "影龍武"; //數據
context.setAttribute("username",username); //將一個數據保存在了ServletContext,數據的名字爲username,值爲變量username的值
System.out.println("Hello,進入doGet");
}
}
在GetServlet 裏獲取上下文
package com.ylw.servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class GetServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext context = this.getServletContext(); //這裏獲取的上下文就是HellServlet裏設置的
String username = (String) context.getAttribute("username"); //獲取上下文中名爲username的值
resp.setContentType("text/html"); //設置文本類型
resp.setCharacterEncoding("utf-8"); //設置編碼
resp.getWriter().print("用戶名:"+username); //寫在網頁上
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp); //注意這裏改成doGet(req, resp),形成一個規範
}
}
web.xml裏的註冊
<servlet>
<servlet-name>hello</servlet-name>
<servlet-class>com.ylw.servlet.HelloServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>hello</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
<servlet>
<servlet-name>getContext</servlet-name>
<servlet-class>com.ylw.servlet.GetServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>getContext</servlet-name>
<url-pattern>/getContext</url-pattern>
</servlet-mapping>
重啓Tomcat。
然後打開瀏覽器,先訪問/hello進行設置,再訪問/getContext進行獲取
2、獲取初始化參數
用到的主要方法:
context.getInitParameter("參數名")
現在web.xml文件裏配置初始參數
<!--配置一些web應用的初始化參數-->
<context-param>
<param-name>url</param-name>
<param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
</context-param>
寫一個類獲取
package com.ylw.servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class ServletDemo03 extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext context = this.getServletContext(); //這裏獲取的上下文就是HellServlet裏設置的
String url = context.getInitParameter("url"); //獲取上web.xml文件中設置的參數<context-param>
resp.getWriter().print(url); //寫在網頁上
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);//注意這裏改成doGet(req, resp),形成一個規範
}
}
在web.xml裏註冊這個類
<servlet>
<servlet-name>gp</servlet-name>
<servlet-class>com.ylw.servlet.ServletDemo03</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>gp</servlet-name>
<url-pattern>/gp</url-pattern>
</servlet-mapping>
重啓Tomcat。
然後打開瀏覽器,訪問/gp進行獲取
3、請求轉發
用到的主要方法:
context.getRequestDispatcher("轉發路徑").forward(req,resp);
轉發代碼
package com.ylw.servlet;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class ServletDemo04 extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ServletContext context = this.getServletContext(); //這裏獲取的上下文就是HellServlet裏設置的
System.out.println("進入了Demo04");
// RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp"); //設置轉發路徑
// requestDispatcher.forward(req,resp); //調用forward實現請求轉發
//合併上面兩句
context.getRequestDispatcher("/gp").forward(req,resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);//注意這裏改成doGet(req, resp),形成一個規範
}
}
在web.xml裏註冊類
<servlet>
<servlet-name>sd4</servlet-name>
<servlet-class>com.ylw.servlet.ServletDemo04</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>sd4</servlet-name>
<url-pattern>/sd4</url-pattern>
</servlet-mapping>
重啓Tomcat,
打開瀏覽器,進入/sd4,發現路徑沒有變,但是顯示的是/gp的頁面,這就是轉發的作用(重定向纔會改變路徑)
由於我們設置了輸出語句,在控制檯裏可以看到,確實進入了demo04,只不過顯示的是轉發頁面
4、讀取資源文件 getResourceAsStream()
用到的主要方法:
//獲取文件,變成流
this.getServletContext().getResourceAsStream("文件路徑")
//加載流
Properties prop = new Properties();
prop.load(inputStream); //加載上面文件轉化成的流
prop.getProperty("屬性名"); //獲取文件中的一個屬性
Properties
比較分析:
- 在java目錄下新建properties
- 在resources目錄下新建properties
發現:都被打包到了同一個路徑下:classes,我們俗稱這個路徑爲classpath:
在java目錄下的properties,發現導出的target中沒有該文件,就需要在porm.xml中添加配置,詳見博客https://blog.csdn.net/qq_43594119/article/details/106199248中的解決資源導出失敗問題
讀取資源文件:
1在resources目錄下新建一個properties文件作爲資源文件
2.寫一個類用來讀取這個資源文件,下面是截圖和代碼
package com.ylw.servlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;
public class ServletDemo05 extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//獲得資源,變成一個流
InputStream inputStream = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
Properties prop = new Properties();
prop.load(inputStream); //加載上面文件轉化成的流
String user = prop.getProperty("username"); //獲取文件中的一個屬性
String pwd = prop.getProperty("password"); //獲取文件中的一個屬性
//在網頁上顯示出來
resp.getWriter().print("user: " + user +'\n' + "pwd: " + pwd);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp); //注意這裏改成doGet(req, resp),形成一個規範
}
}
注意這裏的路徑是 /WEB-INF/classes/db.properties
第一個 / 代表當前項目
3.在web.xml中註冊該類
<servlet>
<servlet-name>sd5</servlet-name>
<servlet-class>com.ylw.servlet.ServletDemo05</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>sd5</servlet-name>
<url-pattern>/sd5</url-pattern>
</servlet-mapping>
4.重啓tomcat,訪問/sd5