content: /*
   * EnvironmentFilter.java May 2006
   *
   * Copyright (C) 2006, Niall Gallagher <niallg@users.sf.net>
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or 
   * implied. See the License for the specific language governing 
   * permissions and limitations under the License.
   */
  
  package com.aliyun.odps.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;
     }   
  }
  