AdapterStateManager.java

/**************************************************************************************
 * Copyright (C) 2008 EsperTech, Inc. All rights reserved.                            *
 * http://esper.codehaus.org                                                          *
 * http://www.espertech.com                                                           *
 * ---------------------------------------------------------------------------------- *
 * The software in this package is published under the terms of the GPL license       *
 * a copy of which has been included with this distribution in the license.txt file.  *
 **************************************************************************************/
package com.espertech.esper.adapter;

/**
 * A utility to manage the state transitions for an InputAdapter.
 */
public class AdapterStateManager
{
	private AdapterState state = AdapterState.OPENED;
	private boolean stateTransitionsAllowed = true;

	/**
	 * @return the state
	 */
	public AdapterState getState()
	{
		return state;
	}

	/**
	 * Transition into the STARTED state (from the OPENED state).
	 * @throws IllegalStateTransitionException if the transition is not allowed
	 */
	public void start() throws IllegalStateTransitionException
	{
		assertStateTransitionsAllowed();
		if(state != AdapterState.OPENED)
		{
			throw new IllegalStateTransitionException("Cannot start from the " + state + " state");
		}
		state = AdapterState.STARTED;
	}

	/**
	 * Transition into the OPENED state.
	 * @throws IllegalStateTransitionException if the transition isn't allowed
	 */
	public void stop() throws IllegalStateTransitionException
	{
		assertStateTransitionsAllowed();
		if(state != AdapterState.STARTED && state != AdapterState.PAUSED)
		{
			throw new IllegalStateTransitionException("Cannot stop from the " + state + " state");
		}
		state = AdapterState.OPENED;
	}

	/**
	 * Transition into the PAUSED state.
	 * @throws IllegalStateTransitionException if the transition isn't allowed
	 */
	public void pause() throws IllegalStateTransitionException
	{
		assertStateTransitionsAllowed();
		if(state != AdapterState.STARTED)
		{
			throw new IllegalStateTransitionException("Cannot pause from the " + state + " state");
		}
		state = AdapterState.PAUSED;
	}

	/**
	 * Transition into the STARTED state (from the PAUSED state).
	 * @throws IllegalStateTransitionException if the state transition is not allowed
	 */
	public void resume() throws IllegalStateTransitionException
	{
		assertStateTransitionsAllowed();
		if(state != AdapterState.PAUSED)
		{
			throw new IllegalStateTransitionException("Cannot resume from the " + state + " state");
		}
		state = AdapterState.STARTED;
	}

	/**
	 * Transition into the DESTROYED state.
	 * @throws IllegalStateTransitionException if the transition isn't allowed
	 */
	public void destroy() throws IllegalStateTransitionException
	{
		if(state == AdapterState.DESTROYED)
		{
			throw new IllegalStateTransitionException("Cannot destroy from the " + state + " state");
		}
		state = AdapterState.DESTROYED;
	}

	/**
	 * Disallow future state changes, and throw an IllegalStateTransitionException if they
	 * are attempted.
	 */
	public void disallowStateTransitions()
	{
		stateTransitionsAllowed = false;
	}

	private void assertStateTransitionsAllowed()
	{
		if(!stateTransitionsAllowed)
		{
			throw new IllegalStateTransitionException("State transitions have been disallowed");
		}
	}
}