java - ServletContext

ServletContext:

web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用；

作用：共享数据---->我们在这个servlet保存的数据可以在另外一个servlet中拿到：

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public class Helloservlet extends HttpServlet {     @Override     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         System.out.println("Hello,servlet");         //this.getInitParameter()初始化参数         //this.getServletConfig()servlet配置         //this.getServletContext()servlet上下文         ServletContext context=this.getServletContext();         String username="java_小白";//数据         context.setAttribute("username",username);//将一个数据保存在了servletContext中，名字为：username = 值为 username       }       @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {     } }

　　我们在helloservlet中创建了一个ServletContext对象，通过ServletContext的setAttribute方法，把一个数据（username）放入了这个对象之中；用于被公共取用

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public class Getservlet extends HttpServlet {     @Override     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         ServletContext context1=this.getServletContext();         String username =(String) context1.getAttribute("username");         resp.setContentType("text/hrml");         resp.setCharacterEncoding("utf-8");     resp.getWriter().print("名字:"+username);     }       @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {       } }

　　我们继续在Getservlet中创建了一个ServletContext对象，不同的是我们使用了ServletContext的getAttribute方法；用于拿到username中的值，并且将它输出来

      值得注意的是，这个时候的返回值是object类，所以在我们知道返回值是String类型的时候我们可以将它强制转换为String，才不发生错误

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="3.1"
         metadata-complete="true">
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>top.lostyou.servlet.Helloservlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>getc</servlet-name>
        <servlet-class>top.lostyou.servlet.Getservlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getc</servlet-name>
        <url-pattern>/getc</url-pattern>
    </servlet-mapping>
</web-app>

我们在编辑好了servlet后，每编辑好一个我们都要取对应的XML中注册

分别是   servlet   和   servlet-mapping   两个

resp.setContentType("text/hrml");

resp.setCharacterEncoding("utf-8");

以上两个代码是用于处理servlet的乱码问题的，比如：中文输出为？？？？

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1	<em id="__mceDel"><em id="__mceDel"><em id="__mceDel"><br>二.获得初始化参数<br></em></em></em>

<!--web应用配置的一些初始化参数放在了context对象当中,可以通过servlet中的getInitParameter方法拿到这些初始化参数-->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:myseq://localhost:3306/mybatis</param-value>
    </context-param>
    <servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>top.lostyou.servlet.ServletDemo03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>

我们将初始化的信息放在<context-param>下，就可以通过ServletContext对象中的方法拿到这个初始化的信息

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public class ServletDemo03 extends HttpServlet {     @Override     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         ServletContext context = this.getServletContext();         String url=context.getInitParameter("url");         resp.getWriter().print("url="+url);     }       @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         super.doPost(req, resp);     } }

　这里我们使用的市另外的一个对象方法getServletContext，用于去拿我们在尾部容器下的初始化信息

三.请求转发

    <servlet-name>sd4</servlet-name>
        <servlet-class>top.lostyou.servlet.ServletDemo04</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd4</servlet-name>
        <url-pattern>/sd4</url-pattern>
    </servlet-mapping>

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public class ServletDemo04 extends HttpServlet {     @Override     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         ServletContext context = this.getServletContext();         //RequestDispatcher dispatcher=context.getRequestDispatcher("/gp");//将要转发到那个路径上         //dispatcher.forward(req,resp);//调用forward实现请求转发         context.getRequestDispatcher("/gp").forward(req,resp);//简写调用方法       }       @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         super.doPost(req, resp);     } }

　　这次使用的方法是ServletContext对象的getRequestDispatcher方法，这是请求转发的方法，在这个方法下写出路径后，程序就会在XML中寻找这个路径，然后执行

四.读取资源文件

不管是在Java目录下还是在resources目录下新建properties，都被打包在了一个目录下：classes，我们俗称这个路径为：classpath

读取资源文件我们所需要用到的思路：需要一个文件流

 

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public class SerletDemo05 extends HttpServlet {     @Override     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         java.io.InputStream is = this.getServletContext().getResourceAsStream("/WEB-IF/classes/db.propertis");         Properties prop =new Properties();         prop.load(is);         String user =prop.getProperty("username");         String ped =prop.getProperty("password");         resp.getWriter().print(user+":"+ped);     }       @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         super.doPost(req, resp);     } }

　　通过ServletContext对象中的getResourceAsStream的方法，我们需要传入获得路径的地址：相对地址，然后再返回一个io流

在通过Properties对象加载这个流，然后通过properties对象的getProperty方法取出我们的资源

<build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

如果Java目录下我们导出包不成功的话，我们就需要在pom.xml中新增这个依赖就可以了