Summary of issues with the target is null for SetProperty

This problem has occurred, the error is

Com.opensymphony.xwork2.ognl.OgnlValueStack warn-error setting expression ' costrecord.yearmonth ' with value ' [ Ljava.lang.String, @d2877a '
Ognl. Ognlexception:target is null for setProperty (NULL, "Yearmonth", [Ljava.lang.String; @d2877a) search on the internet for a long time, did not find a solution, Online summary There are 5 kinds of possibilities, but have been checked, are not resolved.

1. The corresponding property does not have a Get Set method
2. There are more name attributes in the submission form and the corresponding action or model does not have this method (it is important to include a picture button that is often difficult to find) as shown below:
<input type= "image" Name= "submit" value= "submitted" onclick= "return Checkpwd ();" src= "Images/a_tj.gif"/> "
Error message: Target is null for setProperty (NULL, "X", [Ljava.lang.String; @b0c40e)
3. In the absence of spring help injection, the model in our action must be manually new and add an empty construction method inside the model.
4.Action inside the model instance object name is inconsistent with the name in our form
5. Also encountered is the mistake between our class name is very long time metaphor:
Pivate ABDEFGHJIJKLIJEJFJDKEIFJ ABEFGHJIJKLIJEJFJDKEIFJ Then if we use MyEclipse to automatically add the Get set method, we will find its getab (); And we're actually going to be getab ();

my mistake was that the requested URL was written incorrectly and asked for another action.

despise me.