content: /*
   * SystemFilter.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>SystemFilter</code> object is used to provide a filter
   * that will replace the specified values with system properties.
   * This can be given a delegate filter which can be used to resolve
   * replacements should the value requested not match a property. 
   * 
   * @author Niall Gallagher
   */
  public class SystemFilter implements Filter {
     
     /**
      * Filter delegated to if no system property can be resolved.
      */
     private Filter filter;        
          
     /**
      * Constructor for the <code>SystemFilter</code> object. This 
      * creates a filter that will resolve replacements using system
      * properties. Should the system properties not contain the
      * requested mapping this will return a null value.
      */
     public SystemFilter() {
        this(null);           
     }
          
     /**
      * Constructor for the <code>SystemFilter</code> object. This 
      * creates a filter that will resolve replacements using system
      * properties. Should the system properties not contain the
      * requested mapping this delegates to the specified filter.
      * 
      * @param filter the filter delegated to if resolution fails
      */   
     public SystemFilter(Filter filter) {
        this.filter = filter;           
     }
     
     /**
      * Replaces the text provided with the value resolved from the
      * system properties. If the system properties fails 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.getProperty(text);           
  
        if(value != null) {
           return value;                       
        }
        if(filter != null) {
           return filter.replace(text);              
        }      
        return null;
     }   
  }
  