udunits/src/main/java/ucar/units/UnitSystemManager.java
public static synchronized void setInstance(final UnitSystem instance)
			throws UnitSystemException {
		if (instance != null) {
			throw new UnitSystemException("Unit system already used");
		}
		UnitSystemManager.instance = instance;
	}
public static synchronized void setInstance(final UnitSystem instance)
			throws UnitSystemException {
		if (instance != null) {
			throw new UnitSystemException("Unit system already used");
		}
		UnitSystemManager.instance = 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.