Getting list of directories within a directory - NULL

Question

I am getting a NULL POINTER EXCEPTION when trying to get a list of directories from a directory.
<%
main_dir = new File("test_dir");
dirs = main_dir.list();
if(dir==null){
%>
ERROR: dirs is null<p>
Main Dir: <%=index_dir%>
<% }
%>
It is returning that message that dirs is null.
The directory structure is
$TOMCAT_DIR/webapps/test_dir.jsp
$TOMCAT_DIR/webapps/test_dir
Within test_dir are two directories: test1, test2
I put the same code into a regular java code:
File f = new File ("test_dir");
String[] dirs = f.list();
if(dirs == null){
System.out.println("dirs is null");
It DOES NOT print out that message that dirs is null.
I'm just very confused. What am I doing wrong?

Answer 1

Pardon me but since I don't put the actual code on here sometiems there are typos.
I have a program where I open a directory. It's a search page where it displays about 10 results then has a button at the bottom says "next" for the next results.
Anyhow, one minute the webpage will be fine, open the directory with no problem. Then it will stop and say it can't open the directory when the directory is there. It's driving me nuts.
I'm using Java code within the JSP page, instead of beans so I dont' know if that causes some kind of error.
<%
index_dir = new File(master_index_dir);
indexes = index_dir.list();
if(indexes==null){
%>
ERROR: indexes is null<p>
Master Index Dir: <%=index_dir%>
<% }
else { 
// Other stuff
%>
Well just 5 minutes ago this worked fine, the index was open and the results shown. However just 2 minutes ago it stopped and returns the message that indexes is null.
Sometimes if I stop and start tomcat several times it starts to work again.