Struts2(3):Action接口和ActionSupport基类

为了让用户开发的Action类更规范，Struts2提供了一个Action接口，这个接口定义了Struts2的Action处理类应该实现的规范。下面是Action接口的源码：

public interface Action {     public static final String ERROR="error";     public static final String INPUT="input";     public static final String LOGIN="login";     public static final String NONE="none";     public static final String SUCCESS="success";     public String execute( ) throws Exception;}

上面的Action接口里只定义了一个execute方法，该接口的规范规定了Action类应该包含一个execute方法，该方法返回一个字符串。除此之外，该接口还定义了5个字符串常量，它们的作用是统一execute方法的返回值。例如当Action类处理用户请求成功后，有人喜欢返回welcome字符串，有人喜欢返回success字符串......这样不利于项目的统一管理。Action接口定义了如上5个字符串常量，分别代表了特定的含义。当然如果开发者依然希望使用特定的字符串作为逻辑视图名，依然可以返回自己的视图名。

另外，Struts2为Action接口提供了一个实现类：ActionSupport：

public class ActionSupport implements Action, Validateable, ValidationAware,     TextProvider, LocaleProvider, Serializable {    protected static Logger LOG = LoggerFactory.getLogger(ActionSupport.class);    private final transient TextProvider textProvider =             new TextProviderFactory().createInstance(getClass(), this);    private final ValidationAwareSupport validationAware = new ValidationAwareSupport();    public void setActionErrors(Collection<String> errorMessages) {        validationAware.setActionErrors(errorMessages);    }    public Collection<String> getActionErrors() {        return validationAware.getActionErrors();    }    public void setActionMessages(Collection<String> messages) {        validationAware.setActionMessages(messages);    }    public Collection<String> getActionMessages() {        return validationAware.getActionMessages();    }    /**     * @deprecated Use {@link #getActionErrors()}.     */    @Deprecated public Collection<String> getErrorMessages() {        return getActionErrors();    }    /**     * @deprecated Use {@link #getFieldErrors()}.     */    @Deprecated public Map<String, List<String>> getErrors() {        return getFieldErrors();    }    public void setFieldErrors(Map<String, List<String>> errorMap) {        validationAware.setFieldErrors(errorMap);    }    public Map<String, List<String>> getFieldErrors() {        return validationAware.getFieldErrors();    }    public Locale getLocale() {        ActionContext ctx = ActionContext.getContext();        if (ctx != null) {            return ctx.getLocale();        } else {            LOG.debug("Action context not initialized");            return null;        }    }    public boolean hasKey(String key) {        return textProvider.hasKey(key);    }    public String getText(String aTextName) {        return textProvider.getText(aTextName);    }    public String getText(String aTextName, String defaultValue) {        return textProvider.getText(aTextName, defaultValue);    }    public String getText(String aTextName, String defaultValue, String obj) {        return textProvider.getText(aTextName, defaultValue, obj);    }    public String getText(String aTextName, List<Object> args) {        return textProvider.getText(aTextName, args);    }    public String getText(String key, String[] args) {        return textProvider.getText(key, args);    }    public String getText(String aTextName, String defaultValue, List<Object> args) {        return textProvider.getText(aTextName, defaultValue, args);    }    public String getText(String key, String defaultValue, String[] args) {        return textProvider.getText(key, defaultValue, args);    }    public String getText(String key, String defaultValue, List<Object> args,                          ValueStack stack) {        return textProvider.getText(key, defaultValue, args, stack);    }    public String getText(String key, String defaultValue, String[] args,                          ValueStack stack) {        return textProvider.getText(key, defaultValue, args, stack);    }    public ResourceBundle getTexts() {        return textProvider.getTexts();    }    public ResourceBundle getTexts(String aBundleName) {        return textProvider.getTexts(aBundleName);    }    public void addActionError(String anErrorMessage) {        validationAware.addActionError(anErrorMessage);    }    public void addActionMessage(String aMessage) {        validationAware.addActionMessage(aMessage);    }    public void addFieldError(String fieldName, String errorMessage) {        validationAware.addFieldError(fieldName, errorMessage);    }    public String input() throws Exception {        return INPUT;    }    public String doDefault() throws Exception {        return SUCCESS;    }    /**     * A default implementation that does nothing an returns "success".     * <p/>     * Subclasses should override this method to provide their business logic.     * <p/>     * See also {@link com.opensymphony.xwork2.Action#execute()}.     *     * @return returns {@link #SUCCESS}     * @throws Exception can be thrown by subclasses.     */    public String execute() throws Exception {        return SUCCESS;    }    public boolean hasActionErrors() {        return validationAware.hasActionErrors();    }    public boolean hasActionMessages() {        return validationAware.hasActionMessages();    }    public boolean hasErrors() {        return validationAware.hasErrors();    }    public boolean hasFieldErrors() {        return validationAware.hasFieldErrors();    }    /**     * Clears field errors. Useful for Continuations and other situations     * where you might want to clear parts of the state on the same action.     */    public void clearFieldErrors() {        validationAware.clearFieldErrors();    }    /**     * Clears action errors. Useful for Continuations and other situations     * where you might want to clear parts of the state on the same action.     */    public void clearActionErrors() {        validationAware.clearActionErrors();    }    /**     * Clears messages. Useful for Continuations and other situations     * where you might want to clear parts of the state on the same action.     */    public void clearMessages() {        validationAware.clearMessages();    }    /**     * Clears all errors. Useful for Continuations and other situations     * where you might want to clear parts of the state on the same action.     */    public void clearErrors() {        validationAware.clearErrors();    }    /**     * Clears all errors and messages. Useful for Continuations and other situations     * where you might want to clear parts of the state on the same action.     */    public void clearErrorsAndMessages() {        validationAware.clearErrorsAndMessages();    }    /**     * A default implementation that validates nothing.     * Subclasses should override this method to provide validations.     */    public void validate() {    }    @Override public Object clone() throws CloneNotSupportedException {        return super.clone();    }    /**     * <!-- START SNIPPET: pause-method -->     * Stops the action invocation immediately (by throwing a PauseException)      * and causes the action invocation to return     * the specified result, such as {@link #SUCCESS}, {@link #INPUT}, etc.     * <p/>     * <p/>     * The next time this action is invoked (and using the same continuation ID),     * the method will resume immediately     * after where this method was called, with the entire call stack      * in the execute method restored.     * <p/>     * <p/>     * Note: this method can <b>only</b> be called      * within the {@link #execute()} method.     * <!-- END SNIPPET: pause-method -->     *     * @param result the result to return - the same type of      * return value in the {@link #execute()} method.     */    public void pause(String result) {    }}

ActionSupport是一个默认的Action实现类，该类里已经提供了许多默认方法，这些默认方法包括获取国际化信息的方法，数据校验的方法，默认的处理用户请求的方法等。实际上，ActionSupport类是Struts2默认的Action处理类，如果让开发者的Action类继承ActionSupport，会大大简化Action的开发。所以实际开发中Action一般都选择继承ActionSupport基类。