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HtmlCompressor is a small, fast and very easy to use Java library that minifies given HTML or XML source by removing extra whitespaces, comments and other unneeded characters without breaking the content structure. As a result pages become smaller in size and load faster. A command-line version of the compressor is also available.

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package com.googlecode.htmlcompressor.velocity;

/*
 * 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.
 */

import java.io.IOException;
import java.io.StringWriter;
import java.io.Writer;

import org.apache.velocity.context.InternalContextAdapter;
import org.apache.velocity.exception.MethodInvocationException;
import org.apache.velocity.exception.ParseErrorException;
import org.apache.velocity.exception.ResourceNotFoundException;
import org.apache.velocity.exception.TemplateInitException;
import org.apache.velocity.runtime.RuntimeServices;
import org.apache.velocity.runtime.directive.Directive;
import org.apache.velocity.runtime.log.Log;
import org.apache.velocity.runtime.parser.node.Node;

import com.googlecode.htmlcompressor.compressor.XmlCompressor;

/**
 * Velocity directive that compresses an XML content within #compressXml ... #end block.
 * Compression parameters are set by default.
 * 
 * @see XmlCompressor
 * 
 * @author Sergiy Kovalchuk
 */
public class XmlCompressorDirective extends Directive {
	
	private static final XmlCompressor xmlCompressor = new XmlCompressor();
	
	private Log log;

	public String getName() {
		return "compressXml";
	}

	public int getType() {
		return BLOCK;
	}
	
	@Override
	public void init(RuntimeServices rs, InternalContextAdapter context, Node node) throws TemplateInitException {
		super.init(rs, context, node);
		log = rs.getLog();
		
		//set compressor properties
		xmlCompressor.setEnabled(rs.getBoolean("userdirective.compressXml.enabled", true));
		xmlCompressor.setRemoveComments(rs.getBoolean("userdirective.compressXml.removeComments", true));
		xmlCompressor.setRemoveIntertagSpaces(rs.getBoolean("userdirective.compressXml.removeIntertagSpaces", true));
	}

    public boolean render(InternalContextAdapter context, Writer writer, Node node) 
    		throws IOException, ResourceNotFoundException, ParseErrorException, MethodInvocationException {
    	
    	//render content
    	StringWriter content = new StringWriter();
		node.jjtGetChild(0).render(context, content);
		
		//compress
		try {
			writer.write(xmlCompressor.compress(content.toString()));
		} catch (Exception e) {
			writer.write(content.toString());
			String msg = "Failed to compress content: "+content.toString();
            log.error(msg, e);
            throw new RuntimeException(msg, e);
            
		}
		return true;
    	
    }

}