When I run :
Enumeration<String> enums = this.getServletConfig().getInitParameterNames();
I get the HTTP Status 500 error in my broswer.
HTTP Status 500 -
type Exception report
message
description The server encountered an internal error that prevented it from fulfilling this request.
exception
java.lang.NullPointerException com.ypd.config.ConfigDemo.doGet(ConfigDemo.java:32) javax.servlet.http.HttpServlet.service(HttpServlet.java:622) javax.servlet.http.HttpServlet.service(HttpServlet.java:729) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
note The full stack trace of the root cause is available in the Apache Tomcat/9.0.0.M17 logs. Apache Tomcat/9.0.0.M17
The snapshot:
My config code in web.xml:
<servlet>
<servlet-name>ConfigDemo</servlet-name>
<servlet-class>com.ypd.config.ConfigDemo</servlet-class>
<init-param>
<param-name>path</param-name>
<param-value>/Users/luowensheng/Downloads/图片1.png</param-value>
</init-param>
<init-param>
<param-name>aaa</param-name>
<param-value>aaa's value</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>ConfigDemo</servlet-name>
<url-pattern>/configDemo</url-pattern>
</servlet-mapping>
And in my ConfigDemo.java:
public class ConfigDemo 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 {
Enumeration<String> enums = this.getServletConfig().getInitParameterNames(); // this is 32th line, here I get the error.
while(enums.hasMoreElements()) {
String paramName = enums.nextElement();
String paramValue = this.getServletConfig().getInitParameter(paramName);
resp.getWriter().write(paramName + "=" + paramValue);
}
}
}
But in the end I found if I annotation the below code, there will not have the error：
private ServletConfig config;
@Override
public void init(ServletConfig config) throws ServletException {
this.config = config;
}
Why? why can not have this ServletConfig member variable here?
