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XWork is an command-pattern framework that is used to power WebWork as well as other applications. XWork provides an Inversion of Control container, a powerful expression language, data type conversion, validation, and pluggable configuration.

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/*
 * Copyright (c) 2002-2006 by OpenSymphony
 * All rights reserved.
 */
package com.opensymphony.xwork2;

/**
 * All actions may implement this interface, which exposes
 * the execute() method. However, as of XWork 1.1, this is
 * not required and is only here to assist users. You are
 * free to create POJOs that honor the same contract
 * defined by this interface without actually implementing
 * the interface.
 */
public interface Action {
    /**
     * The action execution was successful. Show result
     * view to the end user.
     */
    public static final String SUCCESS = "success";

    /**
     * The action execution was successful but do not
     * show a view. This is useful for actions that are
     * handling the view in another fashion like redirect.
     */
    public static final String NONE = "none";

    /**
     * The action execution was a failure.
     * Show an error view, possibly asking the
     * user to retry entering data.
     */
    public static final String ERROR = "error";

    /**
     * The action execution require more input
     * in order to succeed.
     * This result is typically used if a form
     * handling action has been executed so as
     * to provide defaults for a form. The
     * form associated with the handler should be
     * shown to the end user.
     * 
     * This result is also used if the given input
     * params are invalid, meaning the user
     * should try providing input again.
     */
    public static final String INPUT = "input";

    /**
     * The action could not execute, since the
     * user most was not logged in. The login view
     * should be shown.
     */
    public static final String LOGIN = "login";


    /**
     * Where the logic of the action is executed.
     *
     * @return a string representing the logical result of the execution.
     *         See constants in this interface for a list of standard result values.
     * @throws Exception thrown if a system level exception occurs.
     *                   Application level exceptions should be handled by returning
     *                   an error value, such as Action.ERROR.
     */
    public String execute() throws Exception;
}