Servlet(三)ServletContext

ServletContextd对象：

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

共享数据

我在一个Servlet中保存的数据，可以在另一个Servlet中拿到

实际开发时，少用servletContext，用session

//放置数据的类
public class HelloServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {


//        this.getInitParameter(); 初始化参数
//        this.getServletConfig(); Servlet配置
//        this.getServletContext(); Servlet上下文
        ServletContext servletContext = this.getServletContext();
        String username = "困了就睡";
        servletContext.setAttribute("username",username);//将一个数据保存到ServletContext中，名字：username,值：username,看成键值对的形式
    }
}

//读取数据的类
public class GetServlet extends HttpServlet{

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        String username = (String) servletContext.getAttribute("username");

        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html");
        resp.getWriter().print(username);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

//配置xml
<servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.godwin.servlet.HelloServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    
    <servlet>
        <servlet-name>get</servlet-name>
        <servlet-class>com.godwin.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>get</servlet-name>
        <url-pattern>/get</url-pattern>
    </servlet-mapping>

测试访问时，

注意：读取数据的类，记得解决乱码问题

resp.setCharacterEncoding("utf-8");
resp.setContentType("text/html");

第一次访问http://localhost:8080/s2/get，页面显示为null，

接着访问http://localhost:8080/s2/hello，页面为空,但是拿了数据

在访问http://localhost:8080/s2/get，拿到数据，并且页面显示“困了就睡”,即设置好的数据

ServletContextd应用

获取初始化参数

实际开发中，几乎不用这么做

web.xml

<context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

<servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>com.godwin.servlet.ServletDemo01</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>

public class ServletDemo01 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);

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

请求转发

实际开发中，我们用Request去做

请求转发时，路径不会变

web.xml

<servlet>
        <servlet-name>tp</servlet-name>
        <servlet-class>com.godwin.servlet.ServletDemo02</servlet-class>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>tp</servlet-name>
        <url-pattern>/tp</url-pattern>
    </servlet-mapping>

public class ServletDemo02 extends HttpServlet{

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
//        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//转发的请求路径
//        requestDispatcher.forward(req,resp);//调用forward实现请求转发
        context.getRequestDispatcher("/gp").forward(req,resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

读取资源文件

实际开发中，用类加载或者反射等去做，几乎不用这个方式去做

Properties:

在java目录下新建properties
在resources目录下新建properties

发现都被打包到同一路径下：classes,俗称：类路径。

思路：需要一个文件流

解决资源导出问题：

<!--在build中配置resources,来防止我们资源导出失败的问题-->
    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>true</filtering>
            </resource>
        </resources>
    </build>

public class ServletDemo03 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/dp.properties");
        Properties prop = new Properties();
        prop.load(is);
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        resp.getWriter().print(username + ":" + password);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

<servlet>
        <servlet-name>sd3</servlet-name>
        <servlet-class>com.godwin.servlet.ServletDemo03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>sd3</servlet-name>
        <url-pattern>/sd3</url-pattern>
    </servlet-mapping>

username=root
password=root

复习笔记资料参考B站UP主狂神说