HTTP Status 404-there is no action mapped for namespace [/] and action name [action! Method] Error Resolution

According to the STRUTS2 example in the book, an error will occur in the run:

HTTP Status 404-there is no action mapped for namespace [/] and action name [SAYHELLOACTION!SAYSTRUTS2] ...

It seems that the method of configuring Action in Struts.xml does not go wrong, but writes "action!" directly in the JSP. Method.action "will be an error.

Search for a long time no solution, and later found that the second writing called "DMI", will create a security risk, after a certain version of STRUTS2 is closed by default (I use Struts 2-2.3.20). Then add the following line to the <struts></struts> in Struts.xml:

<constant name="struts.enable.DynamicMethodInvocation" value="True  " />

HTTP Status 404-there is no action mapped for namespace [/] and action name [action! Method] Error Resolution