ResultObjectFactoryUtil.java

/*
 *  Copyright 2006 The Apache Software Foundation
 *
 *  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.ibatis.sqlmap.engine.mapping.result;

import com.ibatis.common.resources.*;

import java.util.*;

/**
 * This class is used to create instances of result objects.  It will
 * use the configured ResultObjectFactory if there is one, otherwise
 * it will use iBATIS' normal methods.
 * 
 * Note that this class is somewhat tightly coupled with
 * SqlExecuter - SqlExecute must call the setStatementId() and
 * setResultObjectFactory() methods before executing a statement.
 * This is a result of using a ThreadLocal to hold the current
 * configuration for the statement under execution.  Using a
 * ThreadLocal is a solution for IBATIS-366.  Without a ThreadLocal,
 * the current factory and statement id would have to be added to 
 * many method signatures - often in inappropriate places.
 * 
 * @author Jeff Butler
 */
public class ResultObjectFactoryUtil {
  
  /**
   * Use a ThreadLocal to hold the current statementId and
   * factory.  This is much easier than passing these
   * items all over the place, and it has no measurable impact on
   * performance (I did a test with 100000 result rows and found
   * no impact - Jeff Butler).
   */
  private static ThreadLocal factorySettings = new ThreadLocal();
  
  /**
   * Utility class - no instances
   */
  private ResultObjectFactoryUtil() {
    super();
  }

  /**
   * Algorithm:
   * 
   * <ul>
   *   <li>If factory is null, then create object internally()</li>
   *   <li>Otherwise try to create object through factory</li>
   *   <li>If null returned from factory, then create object internally</li>
   * </ul>
   * 
   * This allows the factory to selectively create objects, also allows for
   * the common possibility that a factory is not configured.
   * 
   * @param factory
   *          the factory to use. May be null!
   * @param statementId
   *          the ID of the statement that generated the call to this method
   * @param clazz
   *          the type of object to create
   * @return a new instance of the specified class.  The instance must
   *   be castable to the specified class.
   * @throws InstantiationException
   *           if the instance cannot be created. If you throw this Exception,
   *           iBATIS will throw a runtime exception in response and will end.
   * @throws IllegalAccessException
   *           if the constructor cannot be accessed. If you throw this
   *           Exception, iBATIS will throw a runtime exception in response and
   *           will end.
   */
  public static Object createObjectThroughFactory(Class clazz) throws InstantiationException,
      IllegalAccessException {
    
    FactorySettings fs = getFactorySettings();
    
    Object obj;
    if (fs.getResultObjectFactory() == null) {
      obj = createObjectInternally(clazz);
    } else {
      obj = fs.getResultObjectFactory().createInstance(fs.getStatementId(), clazz);
      if (obj == null) {
        obj = createObjectInternally(clazz);
      }
    }
    
    return obj;
  }

  /**
   * This method creates object using iBATIS' normal mechanism.  We
   * translate List and Collection to ArrayList, and Set to HashSet
   * because these interfaces may be requested in nested resultMaps
   * and we want to supply default implementations.
   * 
   * @param clazz
   * @return
   * @throws InstantiationException
   * @throws IllegalAccessException
   */
  private static Object createObjectInternally(Class clazz) throws InstantiationException, IllegalAccessException {
    Class classToCreate;
    if (clazz == List.class || clazz == Collection.class) {
      classToCreate = ArrayList.class;
    } else if (clazz == Set.class) {
      classToCreate = HashSet.class;
    } else {
      classToCreate = clazz;
    }
    
    Object obj = Resources.instantiate(classToCreate);
    return obj;
  }
  
  public static void setResultObjectFactory(ResultObjectFactory resultObjectFactory) {
    getFactorySettings().setResultObjectFactory(resultObjectFactory);
  }
  
  public static void setStatementId(String statementId) {
    getFactorySettings().setStatementId(statementId);
  }
  
  private static FactorySettings getFactorySettings() {
    FactorySettings fs = (FactorySettings) factorySettings.get();
    if (fs == null) {
      fs = new FactorySettings();
      factorySettings.set(fs);
    }
    
    return fs;
  }
  
  private static class FactorySettings {
    private ResultObjectFactory resultObjectFactory;
    private String statementId;
    
    public ResultObjectFactory getResultObjectFactory() {
      return resultObjectFactory;
    }
    
    public void setResultObjectFactory(ResultObjectFactory resultObjectFactory) {
      this.resultObjectFactory = resultObjectFactory;
    }
    
    public String getStatementId() {
      return statementId;
    }
    
    public void setStatementId(String statementId) {
      this.statementId = statementId;
    }
  }
}