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/*
 * ReadState.java April 2007
 *
 * Copyright (C) 2007, Niall Gallagher 
 *
 * 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 org.simpleframework.xml.strategy;

import org.simpleframework.xml.util.WeakCache;

/**
 * The ReadState object is used to store all graphs that
 * are currently been read with a given cycle strategy. The goal of
 * this object is to act as a temporary store for graphs such that 
 * when the persistence session has completed the read graph will be
 * garbage collected. This ensures that there are no lingering object
 * reference that could cause a memory leak. If a graph for the given
 * session key does not exist then a new one is created. 
 * 
 * @author Niall Gallagher
 * 
 * @see org.simpleframework.xml.util.WeakCache
 */
class ReadState extends WeakCache{
   
   /** 
    * This is the contract that specifies the attributes to use.
    */
   private Contract contract;
           
   /**
    * Constructor for the ReadState object. This is used
    * to create graphs that are used for reading objects from the XML
    * document. The specified strategy is used to acquire the names
    * of the special attributes used during the serialization.
    * 
    * @param contract this is name scheme used by the cycle strategy 
    */
   public ReadState(Contract contract) {
      this.contract = contract;
   }

   /**
    * This will acquire the graph using the specified session map. If
    * a graph does not already exist mapped to the given session then
    * one will be created and stored with the key provided. Once the
    * specified key is garbage collected then so is the graph object.
    * 
    * @param map this is typically the persistence session map used 
    * 
    * @return returns a graph used for reading the XML document
    */
   public ReadGraph find(Object map) throws Exception {
      ReadGraph read = fetch(map);
      
      if(read != null) {
         return read;
      }
      return create(map);
   }
   
   /**
    * This will acquire the graph using the specified session map. If
    * a graph does not already exist mapped to the given session then
    * one will be created and stored with the key provided. Once the
    * specified key is garbage collected then so is the graph object.
    * 
    * @param map this is typically the persistence session map used 
    * 
    * @return returns a graph used for reading the XML document
    */
   private ReadGraph create(Object map) throws Exception {
      ClassLoader loader = getClassLoader();
      
      if(loader == null) {
         loader = getCallerClassLoader();
      }
      return create(map, loader);
   }
   
   /**
    * This will acquire the graph using the specified session map. If
    * a graph does not already exist mapped to the given session then
    * one will be created and stored with the key provided. Once the
    * specified key is garbage collected then so is the graph object.
    * 
    * @param map this is typically the persistence session map used 
    * @param loader this is the class loader used by the read state
    * 
    * @return returns a graph used for reading the XML document
    */
   private ReadGraph create(Object map, ClassLoader loader) throws Exception {
      ReadGraph read = fetch(map);
      
      if(read == null) {
         read = new ReadGraph(contract, loader);
         cache(map, read);
      }
      return read;
   }   
   
   /**
    * This is used to acquire the caller class loader for this object.
    * Typically this is only used if the thread context class loader
    * is set to null. This ensures that there is at least some class
    * loader available to the strategy to load the class.
    * 
    * @return this returns the loader that loaded this class     
    */
   private ClassLoader getCallerClassLoader() {
      return getClass().getClassLoader();
   }
   
   /**
    * This is used to acquire the thread context class loader. This
    * is the default class loader used by the cycle strategy. When
    * using the thread context class loader the caller can switch the
    * class loader in use, which allows class loading customization.
    * 
    * @return this returns the loader used by the calling thread
    */
   private static ClassLoader getClassLoader() {
      return Thread.currentThread().getContextClassLoader();
   }
}