servlet中init-param与context-param的区别「建议收藏」

<servlet>
    <servlet-name>demo01</servlet-name>
    <servlet-class>com.lanou3g.Demo01</servlet-class>
    <init-param>
        <param-name>username</param-name>
        <param-value>张飞</param-value>
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>demo01</servlet-name>
    <url-pattern>/demo01</url-pattern>
</servlet-mapping>

init-param配置在servlet标签中,用来初始化当前的Servlet的,属性存放在servletConfig中
因此可以通过获取servletConfig对象来获取servlet中init-param里配置的属性,作用域
限制在当前的Servlet中

1.声明一个ServletConfig当做成员变量
2.重写初始化方法init
  通过该方法的参数 自动获取ServletConfig对象
  ServletConfig对象中保存的是Servlet中的配置信息

public class Demo01 extends HttpServlet{ 
   
    private ServletConfig config;
    @Override
    public void init(ServletConfig config) throws ServletException {    
        this.config = config;
    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        传入参数 配置时的 username(相当于key)
        用key获取对应的value
        String value = this.config.getInitParameter("username");
        System.out.println(value);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    }
}

public class Demo01 extends HttpServlet { 
   
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String value = this.getServletConfig().getInitParameter("username");
        System.out.println(value);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

<context-param>
    <param-name>username</param-name>
    <param-value>关羽</param-value>
</context-param>
<servlet>
    <servlet-name>demo01</servlet-name>
    <servlet-class>com.lanou3g.Demo01</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>demo01</servlet-name>
    <url-pattern>/demo01</url-pattern>
</servlet-mapping>

public class Demo01 extends HttpServlet { 
   
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String string = context.getInitParameter("username");
        System.out.println(string);
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

在context-param中不存在这种获取context-param属性的方法
public class Demo01 extends HttpServlet { 
   
    ServletContext config;
    public void init(ServletContext config) throws ServletException {
        // TODO Auto-generated method stub
        config = config;
    }
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println(config.getInitParameter("username"));
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}
这种获取的方式是错误的,因为系统的内部实现没有init(ServletContext config)这种方法
然而有init(ServletConfig config)这种方法,所有init-param有两种获取方式,而context-param
只有一种获取方式

init-param写在servlet中,web.xml中可以写多个servlet,而每个servlet中都可以设置一个
init-param,即init-param作用域仅对自己的servlet起作用
context-param写在servlet之外,web.xml中只能有一个context-param,作用域属于整个程序的
而不限制于某一个servlet,context-param更多用来交互比如获取form表单中的内容