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/*
 * Copyright (c) 1998-2018 University Corporation for Atmospheric Research/Unidata
 * See LICENSE for license information.
 */
package ucar.units;

import java.io.Serializable;

/**
 * Provides support for managing a UnitSystem.
 * 
 * @author Steven R. Emmerson
 */
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 synchronized UnitSystem instance() throws UnitSystemException {
    if (instance == null) {
      instance = SI.instance();
    }
    return instance;
  }

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