/**
* Copyright (C) 2009-2010 Wilfred Springer
*
* This file is part of Preon.
*
* Preon is free software; you can redistribute it and/or modify it under the
* terms of the GNU General Public License as published by the Free Software
* Foundation; either version 2, or (at your option) any later version.
*
* Preon is distributed in the hope that it will be useful, but WITHOUT ANY
* WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
* A PARTICULAR PURPOSE. See the GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along with
* Preon; see the file COPYING. If not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Linking this library statically or dynamically with other modules is making a
* combined work based on this library. Thus, the terms and conditions of the
* GNU General Public License cover the whole combination.
*
* As a special exception, the copyright holders of this library give you
* permission to link this library with independent modules to produce an
* executable, regardless of the license terms of these independent modules, and
* to copy and distribute the resulting executable under terms of your choice,
* provided that you also meet, for each linked independent module, the terms
* and conditions of the license of that module. An independent module is a
* module which is not derived from or based on this library. If you modify this
* library, you may extend this exception to your version of the library, but
* you are not obligated to do so. If you do not wish to do so, delete this
* exception statement from your version.
*/
package org.codehaus.preon.codec;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
import org.codehaus.preon.el.Expression;
import org.codehaus.preon.*;
import org.codehaus.preon.annotation.LazyLoading;
import org.codehaus.preon.buffer.BitBuffer;
import org.codehaus.preon.channel.BitChannel;
import org.codehaus.preon.descriptor.PassThroughCodecDescriptor2;
import java.io.IOException;
import java.lang.reflect.AnnotatedElement;
import java.lang.reflect.Method;
/**
* An attempt to create a general purpose {@link CodecFactory} whose {@link Codec Codecs} will only load their data once
* operations are invoked upon them. <p/> <p> Note that there are some concurrency issues with this approach. The {@link
* Codec} will create a proxy. When an operation is invoked on that proxy, the proxy will check if it already obtained
* the actual value. This is where it's getting nasty. Multiple threads might come in simultaneously. </p>
*
* @author Wilfred Springer
*/
public class LazyLoadingCodecDecorator implements CodecDecorator {
public <T> Codec<T> decorate(Codec<T> decorated, AnnotatedElement metadata,
Class<T> type, ResolverContext context) {
if (metadata != null && metadata.isAnnotationPresent(LazyLoading.class)) {
return new LazyLoadingCodec<T>(decorated, type);
} else {
return decorated;
}
}
/**
* A {@link Codec} that will only start loading the data when one of the
* methods of that object are invoked.
*
* @author Wilfred Springer (wis)
*
* @param <T>
*/
public static class LazyLoadingCodec<T> implements Codec<T> {
/**
* The {@link Codec} to use.
*/
private Codec<T> wrapped;
/**
* The type of object created.
*/
private Class<T> type;
/**
* Constructs a new instance.
*
* @param wrapped
* The {@link Codec} to use when loading the data.
* @param type
* The type of object that will be returned.
*/
public LazyLoadingCodec(Codec<T> wrapped, Class<T> type) {
this.wrapped = wrapped;
this.type = type;
}
/*
* (non-Javadoc)
*
* @see org.codehaus.preon.Codec#decode(org.codehaus.preon.buffer.BitBuffer,
* org.codehaus.preon.Resolver, org.codehaus.preon.Builder)
*/
@SuppressWarnings("unchecked")
public T decode(final BitBuffer buffer, final Resolver resolver,
final Builder builder) throws DecodingException {
final int size = wrapped.getSize().eval(resolver);
final long pos = buffer.getBitPos();
ClassLoader loader = this.getClass().getClassLoader();
Enhancer enhancer = new Enhancer();
enhancer.setClassLoader(loader);
enhancer.setSuperclass(type);
enhancer.setCallback(new MethodInterceptor() {
private Object actual;
public Object intercept(Object target, Method method,
Object[] args, MethodProxy proxy) throws Throwable {
if (actual == null) {
buffer.setBitPos(pos);
actual = wrapped.decode(buffer, resolver, builder);
}
return proxy.invoke(actual, args);
}
});
buffer.setBitPos(pos + size);
return (T) enhancer.create();
}
public void encode(T value, BitChannel channel, Resolver resolver) throws IOException {
wrapped.encode(value, channel, resolver);
}
/*
* (non-Javadoc)
*
* @see org.codehaus.preon.Codec#getTypes()
*/
public Class<?>[] getTypes() {
return wrapped.getTypes();
}
/*
* (non-Javadoc)
*
* @see org.codehaus.preon.Codec#getSize()
*/
public Expression<Integer, Resolver> getSize() {
return wrapped.getSize();
}
/*
* (non-Javadoc)
*
* @see org.codehaus.preon.Codec#getType()
*/
public Class<?> getType() {
return type;
}
public CodecDescriptor getCodecDescriptor() {
return new PassThroughCodecDescriptor2(wrapped
.getCodecDescriptor(), false);
}
}
}