xml中的参数获取

由于在servlet创建时没有构造函数，当我们需要参数时可以使用初始化参数来初始化一些参数

在web.xml中添加<init-param>

<param-name>param1</param-name>
<param-value>param1</param-value>
</init-param>但是这些参数的作用域仅限于这个sevlet 不可以在多个servlet间共享

我们有两种方法获取servlet初始化参数

1 使用servlet的getInitParameter(String )方法 如果有，则返回他的值 没有返回null

2 使用servletConfig对象的getInitParameter(String ）方法 返回值同上

    <servlet>
        <servlet-name>FristServlet</servlet-name>
        <servlet-class>qdm.test.FirstServlet</servlet-class>
        <init-param>
        <param-name>param1</param-name>
        <param-value>param1</param-value>
        </init-param>
        <init-param>
            <param-name>param2</param-name>
            <param-value>param2</param-value>
        </init-param>
        <init-param>
            <param-name>param3</param-name>
            <param-value>param3</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>FristServlet</servlet-name>
        <url-pattern>/servlet/FirstServlet</url-pattern>
    </servlet-mapping>

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("第一种方法获取servlet初始化参数");
        String param1=getInitParameter("param1");
        String param2=getInitParameter("param2");
        String param3=getInitParameter("param3");
        System.out.println("param1"+param1);
        System.out.println("param2"+param2);
        System.out.println("param3"+param3);
        System.out.println("第二种方法获取servlet初始化参数");
        ServletConfig config=getServletConfig();
        String p1=config.getInitParameter("param1");
        String p2=config.getInitParameter("param2");
        String p3=config.getInitParameter("param3");
        System.out.println("param1"+p1);
        System.out.println("param2"+p2);
        System.out.println("param3"+p3);
    }

运行结果如下

如果想在多个servlet间共享一些参数 则需要在上下文（context）设置初始化参数

<context-param>    <param-name>context-name1</param-name>   <param-value>context-value1</param-value></context-param>我们可以通过getServletConfig().getServletContext()来获取Context对象然后通过调用getInitParameter(String )来获取指定的参数  getIniteParameterNames()来获取全部参数名称

    <context-param>
        <param-name>context-name1</param-name>
        <param-value>context-value1</param-value>
    </context-param>
    <context-param>
        <param-name>context-name2</param-name>
        <param-value>context-value2</param-value>

 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("获取context参数");
        ServletContext context=getServletContext();

        System.out.println("获取全部参数");
        Enumeration names=context.getInitParameterNames();
        while(names.hasMoreElements())
        {
            String paraname=(String)names.nextElement();
            String paravalue=context.getInitParameter(paraname);
            System.out.println("参数名："+paraname+"-----值："+paravalue);
        }
    }

运行效果如图