/*

  * EnvironmentFilter.java May 2006

  *

  * Copyright (C) 2006, Niall Gallagher <niallg@users.sf.net>

  *

  * This library is free software; you can redistribute it and/or

  * modify it under the terms of the GNU Lesser General Public

  * License as published by the Free Software Foundation.

  *

  * This library is distributed in the hope that it will be useful,

  * but WITHOUT ANY WARRANTY; without even the implied warranty of

  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

  * GNU Lesser General Public License for more details.

  *

  * You should have received a copy of the GNU Lesser General

  * Public License along with this library; if not, write to the

  * Free Software Foundation, Inc., 59 Temple Place, Suite 330,

  * Boston, MA  02111-1307  USA

  */

 package org.simpleframework.xml.filter;

 /**

  * The <code>EnvironmentFilter</code> object is used to provide a 

  * filter that will replace the specified values with an environment

  * variable from the OS. This can be given a delegate filter which 

  * can be used to resolve replacements should the value requested 

  * not match an environment variable from the OS. 

  * 

  * @author Niall Gallagher

  */

 public class EnvironmentFilter implements Filter {

    /**

     * Filter delegated to if no environment variable is resolved.

     */

    private Filter filter;        

    /**

     * Constructor for the <code>EnvironmentFilter</code> object. This 

     * creates a filter that resolves replacements using environment

     * variables. Should the environment variables not contain the

     * requested mapping this will return a null value.

     */

    public EnvironmentFilter() {

       this(null);           

    }

    /**

     * Constructor for the <code>EnvironmentFilter</code> object. This 

     * creates a filter that resolves replacements using environment

     * variables. Should the environment variables not contain the

     * requested mapping this will delegate to the specified filter.

     * 

     * @param filter the filter delegated to should resolution fail

     */       

    public EnvironmentFilter(Filter filter) {

       this.filter = filter;           

    }

    /**

     * Replaces the text provided with the value resolved from the

     * environment variables. If the environment variables fail this 

     * will delegate to the specified <code>Filter</code> if it is 

     * not a null object. If no match is found a null is returned.

     * 

     * @param text this is the text value to be replaced

     * 

     * @return this will return the replacement text resolved

     */   

    public String replace(String text) {

       String value = System.getenv(text);           

       if(value != null) {

          return value;                       

       }

       if(filter != null) {

          return filter.replace(text);              

       }      

       return null;

    }   

 }