Servlet

1. ServletConfig对象

ServletConfig是javax.servlet.包下的一个接口，ServletConfig它是Servlet的一个配置对象；

ServletConfig是由tomcat容器创建，通过init方法传入给Servlet；

作用：

• 获取Servlet的名称servlet-name标签内容，getServletName
• 获取Servlet的初始化参数，getInitParameter getInitParameterNames
• 获取ServletContext域对象

• getInitParameter(String parameterName)：根据参数名称获取指定的参数值
• getInitParameterNames():获取所有的参数名称

    <servlet>
        <servlet-name>TestServlet11</servlet-name>
        <servlet-class>com.kaka.test_01.servlet.TestServlet11</servlet-class>
        <!--Servlet的初始化参数-->
        <init-param>
            <param-name>username</param-name>
            <param-value>root</param-value>
        </init-param>
        <init-param>
            <param-name>password</param-name>
            <param-value>root123</param-value>
        </init-param>
    </servlet>
    <servlet-mapping>
        <servlet-name>TestServlet11</servlet-name>
        <url-pattern>/demo11</url-pattern>
    </servlet-mapping>

public class TestServlet11 extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        //1.获取ServletConfig对象
        ServletConfig servletConfig = getServletConfig();
        //2.获取Servlet的名称 , 获取到<servlet-name>标签内容
        String servletName = servletConfig.getServletName();
        System.out.println(servletName+"在运行------------");
        //3.获取Servlet中的初始化参数
        String username = servletConfig.getInitParameter("username");
        System.out.println(username);
        String password = servletConfig.getInitParameter("password");
        System.out.println(password);
        //动态获取初始化参数名称
        Enumeration<String> parameterNames = servletConfig.getInitParameterNames();
        while (parameterNames.hasMoreElements()) {
   
            String parameterName = parameterNames.nextElement();
            String parameterValue = servletConfig.getInitParameter(parameterName);
            System.out.println("name : " + parameterName + " , value : " + parameterValue);
        }
        //4.获取ServletContext对象(域对象)
        ServletContext servletContext = servletConfig.getServletContext();
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        doGet(req, resp);
    }
}

通过Servlet接口下的getServletConfig方法获取一个ServletConfig对象

2. ServletContext对象

ServletContext是javax.servlet包下的一个接口，又称上下文对象，ServletConfig对象中维护了ServletContext对象，我们可以通过ServletConfig对象来获取ServletContext对象；当服务器启动时，会为服务器中的每一个web应用程序创建一个ServletContext对象，一个ServletContext对象对应的就是一个web应用程序；在web应用中的servlet要想实现资源的共享，可以通过ServletContext来完成，ServletContext也叫做域对象；

2.1 用来存储数据

ServletContext是一个域对象，可以用来存储数据；

ServletContext域对象作用范围：在应用程序中的任何位置都能够访问；

常用方法：

• getAttribute(String parameterName) : 获取ServletContext域中指定名称的参数值；
• setAttribute(String paramterName,Object parameterValue):存储参数到ServletContext域中；
• removeAttribute(String parameterNam)：将ServletContext域中指定名称的参数移除；

2.1.1 存储参数到ServletContext域中

public class TestServlet12 extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        String msg = "hello";
        //将msg变量存储到ServletContext域中
        //获取ServletConfig对象
        ServletConfig servletConfig = getServletConfig();
        //获取ServletContext对象
        ServletContext servletContext = servletConfig.getServletContext();
        //往ServletContext域中，存储一个名称为msg的参数，值为"hello"
        servletContext.setAttribute("msg",msg);
    }

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

2.1.2 获取ServletContext域中指定名称的参数值

public class TestServlet13 extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        //从ServletContext域中取出msg的值
        ServletContext servletContext = getServletContext();
        Object msg = servletContext.getAttribute("msg");
        System.out.println(msg);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        doGet(req, resp);
    }
}

2.1.3 将ServletContext域中指定名称的参数移除

public class TestServlet14 extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        ServletContext servletContext = getServletContext();
        //将ServletContext域中的msg参数移除
        servletContext.removeAttribute("msg");
    }

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

2.2 获取全局初始化参数

会随着服务器的启动而初始化，SpringMVC框架中会有应用，在web.xml中配置的全局初始化参数，可以通过ServletContext对象获取；

• 全局初始化参数:整个应用程序，当服务器启动时，就可以有一个变量可以用，就用全局初始化参数 ；
• Servlet的初始化参数：Servlet初始化时所用携带的变量，当使用一个Servlet时，需要在Servlet初始化就有一个初始化参数可以使用；

常用方法：

• getInitParameter()：获取指定参数名称的全局参数值
• getInitParameterNames()：获取所有的全局初始化参数名称

 <context-param>
        <param-name>username</param-name>
        <param-value>root456</param-value>
    </context-param>
    <servlet>
        <servlet-name>TestServlet15</servlet-name>
        <servlet-class>com.kaka.test_01.servlet.TestServlet15</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>TestServlet15</servlet-name>
        <url-pattern>/demo15</url-pattern>
    </servlet-mapping>

public class TestServlet15 extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        //获取全局初始化参数
        ServletContext servletContext = getServletContext();
        //动态获取参数名称
        Enumeration<String> parameterNames = servletContext.getInitParameterNames();
        while (parameterNames.hasMoreElements()) {
   
            String parameterName = parameterNames.nextElement();
            String parameterValue = servletContext.getInitParameter(parameterName);
            System.out.println("name : " + parameterName + " , value : " + parameterValue);
        }
    }

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

2.3 获取服务器真实磁盘路径

getRealPath：ServletContext类，依据当前项目去生成真实磁盘路径；

• 如：servletContext.getRealPath("upload")："当前项目的服务器磁盘路径/upload"
• 如：servletContext.getRealPath("upload/img")："当前项目的服务器磁盘路径/upload/img"

public class TestServlet16 extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        //获取upload的服务器真实路径
        ServletContext servletContext = getServletContext();
        String upload = servletContext.getRealPath("upload");
        System.out.println(upload);
        //获取img文件的服务器真实路径
        String imgPath = servletContext.getRealPath("upload/img");
        System.out.println(Path);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        doGet(req, resp);
    }
}

9. 站点访问次数案例（ServletContext对象）

• 使用ServletContext域对象

步骤：

1. 判断是否是第一次请求
2. 如果是第一次请求

• 定义一个变量count=1
• 打印count变量
• 将count变量存储ServletContext域中

1. 如果不是第一次请求

• 将count自加
• 打印count变量
• 将count变量再存储ServletContext域中

/**
 * 统计站点访问次数
 */
public class TestServlet17 extends HttpServlet {
   
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        //获取count变量
        ServletContext servletContext = getServletContext();
        //在域中拿count
        Integer count = (Integer)servletContext.getAttribute("count");
        if (null == count) {
   
            //第一次请求,存储一个count=1变量到ServletContext中
            count = 1;
            System.out.println("站点访问次数 : "+count);
            //将count变量存储到ServletContext域中
            servletContext.setAttribute("count",count);
        } else {
   
            //当不是第一次请求
            count++;
            System.out.println("站点访问次数 : "+count);
            //再将count变量存储到ServletContext域中
            servletContext.setAttribute("count",count);
        }
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        doGet(req,resp);
    }
}

10. Servlet 3.0

• 概念

  Servlet2.5，不支持注解开发；

  Servlet3.0，支持注解开发；（由注解配置来代替web.xml配置）

• 常用注解

  @Test：单元测试

  @Override：方法重写

  @WebServlet：声明配置Servlet，取代了web.xml配置

  @WebServlet(name = "TestServlet18",urlPatterns = "/demo18")
  public class Demo18Servlet extends HttpServlet {
       
   @Override
      protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       
          System.out.println("Demo18Servlet处理请求");
      }
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       
          doGet(req, resp);
      }
  }
  

  以上配置代码，相当于web.xml配置如下：

   <servlet>
          <servlet-name>TestServlet18</servlet-name>
          <servlet-class>com.kaka.test_01.servlet.TestServlet18</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>TestServlet18</servlet-name>
          <url-pattern>/demo18</url-pattern>
      </servlet-mapping>
  

常用属性：

name:String

• 设置Servlet名称

urlPatterns:String[]

• 设置Servlet的访问路径

loadOnStartup:int

• 设置Servlet的load-on-startup属性（随着服务器的启动而启动）

initParams:WebInitParam[]

• 设置Servlet的初始化参数

@WebServlet(name = "TestServlet01", urlPatterns = {
   "/demo01","/mydemo01"},loadOnStartup = 1,
        initParams = {
   @WebInitParam(name = "username",value = "root"),@WebInitParam(name = "password",value = "root")})
public class TestServlet01 extends HttpServlet {
   
    @Override
    //用来监听
    public void init(ServletConfig config) throws ServletException {
   
        System.out.println("TestServlet01 init");
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
   
        System.out.println("TestServlet01");
    }

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