Using wildcard when listing files in directory with java

Question

Why does a wildcard not work in java code below?
My request looks like http://localhost:8080/App/DataAccess?location=Dublin

rob@work:~$ ls /usr/local/CustomAppResults/Dublin/*/.history
/usr/local/CustomAppResults/Dublin/team1/.history
/usr/local/CustomAppResults/Dublin/team2/.history
/usr/local/CustomAppResults/Dublin/team3/.history

Servlet code (DataAccess.java).
(DataAccess.java:27) refers to the for loop ..

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        File[] files = finder("/usr/local/CustomAppResults/" + 
                            request.getParameter("location") + "/*/");

        for (int i = 0; i < files.length; i++){

                    System.out.println(files[i].getName());
        }
    }

    private File[] finder(String dirName) {

        File dir = new File(dirName);

        return dir.listFiles(new FilenameFilter() {
            public boolean accept(File dir, String filename) {
                return filename.endsWith(".history");
            }
        });
    }

Error:

The server encountered an internal error that prevented it
from fulfilling this request.
    java.lang.NullPointerException
    com.example.servlets.DataAccess.doGet(DataAccess.java:27)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)

Answer 1

The method public File[] listFiles(FilenameFilter filter)

Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

( http://docs.oracle.com/javase/7/docs/api/java/io/File.html )

So, why do you get this situation? You are trying to use a wildcard char ( * ) that is evaluated by your shell, but won't be evaluated in new File(path) . The new File(path) constructor only works for exact paths.

Things like DirectoryScanner (Apache Ant) or FileUtils (Apache commons-io) will solve your problem. See the comments above for further details on possible solutions, including the Java 7 NIO approach ( Files.newDirectoryStream( path, glob-pattern ) ).