config.getInitParameter always return null

问题

Why does config.getInitParameter(String) always return null in the following code example?

public void init(ServletConfig config) throws ServletException
{
    super.init(config);
    filename = config.getInitParameter("addressfile");

This is web.xml file

<servlet>
<servlet-name>ListManagerServlet</servlet-name>
<servlet-class>savva.listmanagerservlet.ListManagerServlet</servlet-class>
<init-param>
    <param-name>addressfile</param-name>
    <param-value>d:\temp\demo.txt</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>ListManagerServlet</servlet-name>
<url-pattern>/ListManagerServlet</url-pattern>
</servlet-mapping>

UPD: Eclipse EE Indigo, Java 1.6, Tomcat 7.0

回答1:

The canonical way is to just use the inherited GenericServlet#getInitParameter() in the argumentless init() method (and remove any init(config) method).

@Override
public void init() throws ServletException {
    filename = getInitParameter("addressfile");
}

If that still doesn't work, then your web.xml is not properly been deployed, or you have a typo in the parameter name, or you actually accessed a different instance variable than filename to use/test it.

回答2:

Ensure your servlet is calling super.init(config) on its init method, else it won't work.

回答3:

Make sure you have really deployed the proper web.xml. Also check with config.getInitParameterNames() what parameters have been found.

回答4:

It's never a good idea to override the init(config) method. Instead use the provided init() convenience method and do a getServletConfig() to get the configuration parameters:

http://docs.oracle.com/javaee/1.2.1/api/javax/servlet/GenericServlet.html#init() http://docs.oracle.com/javaee/1.2.1/api/javax/servlet/GenericServlet.html#getServletConfig()

回答5:

If use the IDE STS4, checkout if annotation on the class name exists, use BOTH "annotation" and "web.xml" may cause the value null.

来源：https://stackoverflow.com/questions/8401851/config-getinitparameter-always-return-null

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java

servlets