chainedbeancreatorfactory.java

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/*
 * Copyright 2001-2004 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 org.apache.commons.betwixt.io.read;

import org.apache.commons.betwixt.ElementDescriptor;
import org.apache.commons.logging.Log;

/**  
  * Group of factory methods for <code>ChainedBeanCreator</code>'s.
  * The standard implementations used by Betwixt are present here.
  *
  * @author Robert Burrell Donkin
  * @since 0.5
  */
public class ChainedBeanCreatorFactory {
    
    /** Singleton instance for creating derived beans */
    private static final ChainedBeanCreator derivedBeanCreator 
        = new ChainedBeanCreator() {
            public Object create(
                                ElementMapping elementMapping, 
                                ReadContext context, 
                                BeanCreationChain chain) {
                                
                String className 
                    = elementMapping
                        .getAttributes().getValue( context.getClassNameAttribute() );
                if ( className != null ) {
                    try {
                        // load the class we should instantiate
                        ClassLoader classLoader = context.getClassLoader();
                        if ( classLoader == null ) {
                            context.getLog().warn( 
            "Could not create derived instance: read context classloader not set." );
                        }
                        Class clazz = classLoader.loadClass( className );
                        return clazz.newInstance();
                                        
                    } catch (Exception e) {
                        // it would be nice to have a pluggable strategy for exception management
                        context.getLog().warn( "Could not create instance of type: " + className );
                        context.getLog().debug( "Create new instance failed: ", e );
                        return null;
                    }
                    
                } else {
                    // pass responsibility down the chain
                    return chain.create( elementMapping, context );
                }
            }
        };
    
    /**
      * Creates a <code>ChainedBeanCreator</code> that constructs derived beans.
      * These have their classname set by an xml attribute.
      * @return <code>ChainedBeanCreator</code> that implements Derived beans logic, not null
      */
    public static final ChainedBeanCreator createDerivedBeanCreator() {
        return derivedBeanCreator;
    }
    
    /** Singleton instance that creates beans based on type */
    private static final ChainedBeanCreator elementTypeBeanCreator 
        = new ChainedBeanCreator() {
            public Object create(
                                ElementMapping element, 
                                ReadContext context, 
                                BeanCreationChain chain) {
                
                Log log = context.getLog();
                Class theClass = null;
                
                ElementDescriptor descriptor = element.getDescriptor();
                if ( descriptor != null ) {
                    // created based on implementation class
                    theClass = descriptor.getImplementationClass();
                }
                
                if ( theClass == null ) {
                    // create based on type
                    theClass = element.getType();
                }
                
                if ( log.isTraceEnabled() ) {
                    log.trace(
                        "Creating instance of class " + theClass.getName() 
                        + " for element " + element.getName());
                }
                
                try {

                    return theClass.newInstance();
                    
                } catch (Exception e) {
                    // it would be nice to have a pluggable strategy for exception management
                    context.getLog().warn( 
                        "Could not create instance of type: " + theClass.getName() );
                    context.getLog().debug( "Create new instance failed: ", e );
                    return null;
                }
            }
        }; 
    
    /**
      * Creates a <code>ChainedBeanCreator</code> that constructs beans based on element type.
      * @return <code>ChainedBeanCreator</code> that implements load by type beans logic, not null
      */
    public static final ChainedBeanCreator createElementTypeBeanCreator() {
        return elementTypeBeanCreator;
    }
    
    /** Singleton instance that creates beans based on IDREF */
    private static final ChainedBeanCreator idRefBeanCreator 
        = new ChainedBeanCreator() {
            public Object create(
                                ElementMapping elementMapping, 
                                ReadContext context, 
                                BeanCreationChain chain) {
                if ( context.getMapIDs() ) {
                    String idref = elementMapping.getAttributes().getValue( "idref" );
                    if ( idref != null ) {
                        // XXX need to check up about ordering
                        // XXX this is a very simple system that assumes that 
                        // XXX id occurs before idrefs
                        // XXX would need some thought about how to implement a fuller system
                        context.getLog().trace( "Found IDREF" );
                        Object bean = context.getBean( idref );
                        if ( bean != null ) {
                            if ( context.getLog().isTraceEnabled() ) {
                                context.getLog().trace( "Matched bean " + bean );
                            }
                            return bean;
                        }
                        context.getLog().trace( "No match found" );
                    }
                }
                return chain.create( elementMapping, context );
            }
        }; 
    
    /**
      * Creates a <code>ChainedBeanCreator</code> that finds existing beans based on their IDREF.
      * @return <code>ChainedBeanCreator</code> that implements IDREF beans logic, not null
      */
    public static final ChainedBeanCreator createIDREFBeanCreator() {
        return idRefBeanCreator;
    }
}