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Artifact for command (job) processing purposes according to the command pattern.
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// /////////////////////////////////////////////////////////////////////////////
// REFCODES.ORG
// =============================================================================
// This code is copyright (c) by Siegfried Steiner, Munich, Germany and licensed
// under the following (see "http://en.wikipedia.org/wiki/Multi-licensing")
// licenses:
// =============================================================================
// GNU General Public License, v3.0 ("http://www.gnu.org/licenses/gpl-3.0.html")
// together with the GPL linking exception applied; as being applied by the GNU
// Classpath ("http://www.gnu.org/software/classpath/license.html")
// =============================================================================
// Apache License, v2.0 ("http://www.apache.org/licenses/LICENSE-2.0")
// =============================================================================
// Please contact the copyright holding author(s) of the software artifacts in
// question for licensing issues not being covered by the above listed licenses,
// also regarding commercial licensing models or regarding the compatibility
// with other open source licenses.
// /////////////////////////////////////////////////////////////////////////////

package org.refcodes.command;

/**
 * A {@link Command} ({@link Command}) represents an (atomic) operation applied
 * to a context encapsulated in an object (as of object oriented programming).
 * Usually a {@link Command} also provides means to undo its operation applied
 * before. The {@link Command} is created by a client (e.g. the business logic)
 * and passed to the {@link Command}-bus for execution.
 *
 * @param  The context type to use, can by any {@link Component}, service
 *        or POJO.
 * @param  The return type of the {@link Command}'s proceedings.
 * @param  The exception type of the {@link Command}'s erroneous termination.
 */
public interface Command /* extends Resetable */ {

	/**
	 * The invoker executes a {@link Command} by providing it a context (being a
	 * service, a {@link Component} or a POJO). The method works synchronously
	 * and waits (blocks the caller#s thread) till a result is available.
	 * 
	 * @param aContext The target object (being a service, a {@link Component}
	 *        or a POJO) which is used by the {@link Command} to perform its
	 *        (atomic) operation.
	 * 
	 * @return The result of this {@link Command} after execution or Void in
	 *         case this {@link Command} does not process a result.
	 * 
	 * @throws Exception in case something went wrong.
	 */
	public RET execute( CTX aContext ) throws E;

	/**
	 * The invoker executes a {@link Command} by providing it a context (being a
	 * service, a {@link Component} or a POJO). The method works asynchronously;
	 * use
	 * 
	 * @param aContext The target object (being a service, a {@link Component}
	 *        or a POJO) which is used by the {@link Command} to perform its
	 *        (atomic) operation.
	 */
	// public void start( CTX aContext ); // throws E;

	/**
	 * After execution, the {@link Command} might provide a result. This method
	 * determines whether the {@link Command} can provide a result. In case we
	 * have a void {@link Command} then after execution it will not provide a
	 * result.
	 * 
	 * @return True in case the {@link Command} can provide a result, else
	 *         false.
	 */
	// boolean hasResult();

	/**
	 * In case the {@link Command} can provide a result after execution, it can
	 * be retrieved with this method. In case it is a {@link Void}
	 * {@link Command} without a result, then an instace of the {@link Void}
	 * class should be returned. Use the {@link #hasResult()} method to test
	 * before calling this method.
	 * 
	 * This method waits (halts the thread) until a result is available. In case
	 * you want to avoid synchronously waiting for the result, call the method
	 * {@link #hasResult()}.
	 * 
	 * @return The result of this {@link Command} after execution.
	 * 
	 * @throws UnsupportedOperationException in case the {@link Command} is a
	 *         void {@link Command} not returning any result.
	 * 
	 * @throws IllegalStateException Thrown in case the {@link Command} has not
	 *         been executed yet.
	 */
	// RET getResult() throws E;
}