struts in action中少的一个类

package app;import java.io.IOException;import java.io.InputStream;import java.io.FileOutputStream;import java.util.Enumeration;import java.util.Properties;import java.net.URL;public class UserDirectory {  private static final String UserDirectoryFile =    "resources/users.properties";   private static final String UserDirectoryHeader =    "${user}=${password}";  private static UserDirectory userDirectory = null;  private static Properties p;  private UserDirectory() throws UserDirectoryException {        java.io.InputStream i = null;        p = null;        i = this.getClass().getClassLoader().          getResourceAsStream(UserDirectoryFile);        if (null==i) {          throw new UserDirectoryException();        }        else {          try {            p = new Properties();            p.load(i);            i.close();          }          catch (java.io.IOException e) {            p = null;            System.out.println(e.getMessage());            throw new UserDirectoryException();          }          finally {            i = null;          }        } // end else  }  public static UserDirectory getInstance() throws  UserDirectoryException {if (null==userDirectory) {  userDirectory = new UserDirectory();}return userDirectory;}/**Transform id so that it will match any conventions used by userdirectory. The default implementation forces the id touppercase. Does not expect the userId to be null andwill throw a NPE if it is.@exception Throws Null Pointer Exception if userId is null.*/public String fixId(String userId) {return userId.toUpperCase();}public boolean isValidPassword(String userId, String password) {  // no null passwordsif (null==password) return false;  // conform userId to uppercaseString _userId = fixId(userId);  // no passwords for non-usersif (!isUserExist(_userId)) return false;  // does password match user's passwordreturn (password.equals(getPassword(_userId)));}public boolean isUserExist(String userId) {  // no null usersif (null==userId) return false;  // if not null, it's a userreturn !(null==p.getProperty(userId));}public String getPassword(String userId) {return p.getProperty(userId);}public Enumeration getUserIds() {return p.propertyNames();}public void setUser(String userId, String password) throws  UserDirectoryException {// no nullsif ((null==userId) || (null==password)) {  throw new UserDirectoryException();}try {  // conform userId to uppercase when stored  p.put(fixId(userId), password);  String o = this.getClass().getClassLoader().getResource(UserDirectoryFile).getFile();  p.store(new FileOutputStream(o), UserDirectoryHeader);}catch (IOException e) {  throw new UserDirectoryException();}}}       // end UserDirectory

package app;public class UserDirectoryException extends Exception{}