Servlet简介

• Servlet是sun公司开发动态web资源的一门技术，它提供了一个接口类Servlet
• 如何使用Servlet
  1.编写一个类，实现servlet接口
  2.把开发好的Java类部署到web服务器中
• 总而言之：Servlet就是实现了Servlet接口的Java程序

继承关系：Servlet->Generic->HttpServlet，我们的类只需要继承HttpServlet即可

public interface Servlet {
    void init(ServletConfig var1) throws ServletException;

    ServletConfig getServletConfig();

    void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

    String getServletInfo();

    void destroy();
}

Servlet原理

• Servlet是由web服务器调用，web服务器在收到浏览器请求之后，会：
  

Mapping问题

1.一个Servlet映射一个路径

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.lxd.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

2.一个Servlet映射多个路径

• 访问hello和hello1都可以访问到HelloServlet

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.lxd.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello1</url-pattern>
    </servlet-mapping>

3.通用映射路径

• *为通配符，表示hello路径下的所有路径都能访问HelloServlet

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.lxd.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello/*</url-pattern>
    </servlet-mapping>

4.默认请求路径

• 启动服务器后会直接访问HelloServlet而不是index.jsp

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.lxd.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

5.指定后缀：*前面不能加映射的路径

• 路径只要以hello结尾就可以，前面怎么加都行

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.lxd.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>*.hello</url-pattern>
    </servlet-mapping>

• 确定的映射路径优先级最高，如果找不到就会走默认的处理请求/*

ServletContext对象：上下文对象

1.共享数据

• setAttribute()：设置属性和属性值
• getAttribute()：获取属性对应的属性值

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        //上下文对象
        ServletContext context = this.getServletContext();
        String username = "你好";
        context.setAttribute("username",username);
    }

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

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        resp.setContentType("text/html;charset=utf-8");
        String username = (String) context.getAttribute("username");
        resp.getWriter().print(username);
    }

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

2.获取初始化参数

• getInitParameter()：获取初始化参数

web.xml中
  <context-param>
    <param-name>url</param-name>
    <param-value>www.baidu.com</param-value>
  </context-param>

public class ServletDemo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = (String) context.getInitParameter("url");
        resp.getWriter().print(url);
    }

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

3.请求转发

• 转发：跳转到指定的Servlet，但是url路径不会改变
• 也就是说，A请求B，B跳转到了C，A是看不到C的

public class ServletDemo4 extends HelloServlet{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        //sd3为ServletDemo3的映射路径
        context.getRequestDispatcher("/sd3").forward(req,resp);
    }

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

4.读取资源文件

• Properties
  1.在java目录下新建properties
  2.在resources目录下新建properties
• 他们都会被打包到target/项目名(模块名)/WEB-INF/classes下
• 需要通过文件流来读取资源文件

db.properties文件中：
username=lxd
password=123456

public class PropertiesServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        InputStream is = this.getServletContext().getResourceAsStream("WEB-INF\\classes\\db.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);
    }
}