Servel init-param 返回 null答案

【问题标题】：Servel init-param returns nullServel init-param 返回 null
【发布时间】：2017-09-26 14:12:34
【问题描述】：

我正在尝试通过 web.xml 文件将一些 init-param 值加载到 serevlet 中，但它们一直显示为 null。我确实有两个可以正常工作的上下文参数。有谁知道我做错了什么？

这是我用于我的应用程序的 web.xml 文件。

<?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_3_1.xsd"
         version="3.1"
         >

        <display-name>Hello World Application</display-name> 
        <servlet>   
            <servlet-name>contextxParameterServlet</servlet-name>
            <servlet-class>com.wrox.HelloServlet</servlet-class>
            <init-param>
                <param-name>database</param-name>
                <param-value>CustomerSupport</param-value>
            </init-param>
            <init-param>
                <param-name>server</param-name>
                <param-value>10.0.12.5</param-value>
            </init-param>
            <load-on-startup>1</load-on-startup>
        </servlet>
        <servlet-mapping>
            <servlet-name>contextxParameterServlet</servlet-name>
            <url-pattern>/contextParameters</url-pattern>
        </servlet-mapping>
        <context-param>
            <param-name>settingOne</param-name>
            <param-value>foo</param-value>
        </context-param>
        <context-param>
            <param-name>settingTwo</param-name>
            <param-value>bar</param-value>
        </context-param>
        <servlet>
            <servlet-name>servletParameterServlet</servlet-name>
            <servlet-class>com.wrox.initParams</servlet-class>
            <init-param>
                <param-name>database</param-name>
                <param-value>CustomerSupport</param-value>
            </init-param>
            <init-param>
                <param-name>server</param-name>
                <param-value>10.0.12.5</param-value>
            </init-param>
        </servlet>
        <servlet-mapping>
            <servlet-name>servletParameterServlet</servlet-name>
            <url-pattern>/servletParameter</url-pattern>
        </servlet-mapping>
</web-app>

这是映射到 servletParameterServlet 的页面。

package com.wrox;

import javax.servlet.http.HttpServlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;
import java.util.List;

public class initParams extends HttpServlet{

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
    {
        ServletContext c = this.getServletContext();
        PrintWriter writer = response.getWriter();
        Enumeration<String> temp = c.getInitParameterNames();

        while(temp.hasMoreElements()) {
            writer.append(temp.nextElement());
        }



        writer.append("database: ").append(c.getInitParameter("database"))
              .append(", server: ").append(c.getInitParameter("server"));
    }

    @Override
    public void init(ServletConfig config) throws ServletException
    {
        super.init(config);
        System.out.println("Servlet " + this.getServletName() + " has started.");
    }

    @Override
    public void destroy() {
        System.out.println("Servlet " + this.getServletName() + " has stopped.");
    }
}

【问题讨论】：

标签： servlets jakarta-ee

【解决方案1】：

您初始化事物的方式，我认为您要查找的内容应该在

中可用

this.getServletConfig().getInitParameter("foo");

广义上讲，servlet 上下文由 JVM 内的 servlet 共享，但该对象存在于特定 servlet 的 servlet 配置中，而 <init-param> 则位于 ServletConfig 中。有关更多详细信息，我强烈建议您阅读文档。

【讨论】：