UnitSystemManager.java

// $Id: UnitSystemManager.java 64 2006-07-12 22:30:50Z edavis $
/*
 * Copyright 1998-2009 University Corporation for Atmospheric Research/Unidata
 *
 * Portions of this software were developed by the Unidata Program at the
 * University Corporation for Atmospheric Research.
 *
 * Access and use of this software shall impose the following obligations
 * and understandings on the user. The user is granted the right, without
 * any fee or cost, to use, copy, modify, alter, enhance and distribute
 * this software, and any derivative works thereof, and its supporting
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 * notice appears in all copies of the software, derivative works and
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 */
package ucar.units;

import java.io.Serializable;

/**
 * Provides support for managing a UnitSystem.
 * 
 * @author Steven R. Emmerson
 * @version $Id: UnitSystemManager.java 64 2006-07-12 22:30:50Z edavis $
 */
public final class UnitSystemManager implements Serializable {
	private static final long	serialVersionUID	= 1L;
	/**
	 * The singleton instance of the system of units.
	 * 
	 * @serial
	 */
	private static UnitSystem	instance;

	/**
	 * Returns an instance of the system of units.
	 * 
	 * @return An instance of the system of units.
	 */
	public static final UnitSystem instance() throws UnitSystemException {
		synchronized (UnitSystemManager.class) {
			if (instance == null) {
				instance = SI.instance();
			}
		}
		return instance;
	}

	/**
	 * Sets the system of units. This must be called before any call to
	 * <code>instance()</code>.
	 * 
	 * @param instance
	 *            The system of units.
	 * @throws UnitSystemException
	 *             <code>instance()</code> was called earlier.
	 */
	public static final synchronized void setInstance(final UnitSystem instance)
			throws UnitSystemException {
		if (instance != null) {
			throw new UnitSystemException("Unit system already used");
		}
		UnitSystemManager.instance = instance;
	}
}