NullTool.java

package org.opennaas.extensions.router.junos.commandsets.velocity;

/*
 * Copyright 2003-2004 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.
 */

import org.apache.velocity.context.Context;

/**
 * Tool for working with <code>null</code> in Velocity templates. It provides a method to set a VTL reference back to <code>null</code>. Also provides
 * methods to check if a VTL reference is <code>null</code> or not.
 * <p>
 * NOTE: These examples assume you have placed an instance of the current context within itself as 'ctx'. And, of course, the NullTool is assumed to
 * be available as 'null'.
 * </p>
 * <p>
 * 
 * <pre>
 * Example uses:
 *  $foo                              -> bar
 *  $null.isNull($foo)                -> false
 *  $null.isNotNull($foo)             -> true
 * 
 *  $null.setNull($ctx, "foo")
 *  $foo                              -> $foo (null)
 *  $null.isNull($foo)                -> true
 *  $null.isNotNull($foo)             -> false
 * 
 *  $null.set($ctx, $foo, "hoge")
 *  $foo                              -> hoge
 *  $null.set($ctx, $foo, $null.null)
 *  $foo                              -> $foo (null)
 * </pre>
 * 
 * </p>
 * 
 * <p>
 * This tool is entirely threadsafe, and has no instance members. It may be used in any scope (request, session, or application).
 * </p>
 * 
 * @author <a href="mailto:shinobu@ieee.org">Shinobu Kawai</a>
 * @version $Id: $
 */
public class NullTool
{

	/**
	 * Default constructor.
	 */
	public NullTool()
	{
	}

	/**
	 * Sets the given VTL reference back to <code>null</code>.
	 * 
	 * @param context
	 *            the current Context
	 * @param key
	 *            the VTL reference to set back to <code>null</code>.
	 */
	public void setNull(Context context, String key)
	{
		if (this.isNull(context))
		{
			return;
		}
		context.remove(key);
	}

	/**
	 * Sets the given VTL reference to the given value. If the value is <code>null</code>, the VTL reference is set to <code>null</code>.
	 * 
	 * @param context
	 *            the current Context
	 * @param key
	 *            the VTL reference to set.
	 * @param value
	 *            the value to set the VTL reference to.
	 */
	public void set(Context context, String key, Object value)
	{
		if (this.isNull(context))
		{
			return;
		}
		if (this.isNull(value))
		{
			this.setNull(context, key);
			return;
		}
		context.put(key, value);
	}

	/**
	 * Checks if a VTL reference is <code>null</code>.
	 * 
	 * @param object
	 *            the VTL reference to check.
	 * @return <code>true</code> if the VTL reference is <code>null</code>, <code>false</code> if otherwise.
	 */
	public boolean isNull(Object object)
	{
		return object == null;
	}

	/**
	 * Checks if a VTL reference is not <code>null</code>.
	 * 
	 * @param object
	 *            the VTL reference to check.
	 * @return <code>true</code> if the VTL reference is not <code>null</code>, <code>false</code> if otherwise.
	 */
	public boolean isNotNull(Object object)
	{
		return !this.isNull(object);
	}

	/**
	 * A convinient method which returns <code>null</code>. Actually, this tool will work the same without this method, because Velocity treats
	 * non-existing methods as null. :)
	 * 
	 * @return <code>null</code>
	 */
	public Object getNull()
	{
		return null;
	}

}