Struts2知识积累(2)_核心概念：拦截器

前言：Struts2的三大组件：动作、拦截器、数据转移。动作组件可能是我们日常中经常接触的。但是在后台默默无闻工作的拦截器，却是真正的核心。在Struts2中没有一个动作被单独调用，总是包含了一系列的拦截器在动作执行之前或之后执行。通过创建一个ActionInvocation的对象，封装了一系列被配置在动作之前或之后触发的拦截器。Struts2提供了一组强大的智能默认值，如defaultStack提供了一个常用的拦截器组合。做web应用时，整个系统的权限设置，日志控制，依靠拦截器可以很方便的的实现。

1.拦截器的原理：
1.1 总指挥ActionInvocation接口，是理解拦截器的关键。它控制着整个动作的执行，以及与之相关的拦截器栈的执行顺序。当struts2框架接收到一个request-->决定url映射到哪个动作-->这个动作的一个实例会被加入到一二新创建的ActionInvocation实例中-->通过xml配置发现那些触发器按么顺序触发.

public interface ActionInvocation
extends Serializable {
Object getAction();
boolean isExecuted();
ActionContext getInvocationContext();
ActionProxy getProxy();
Result getResult() throws Exception;
String getResultCode();
void setResultCode(String resultCode);
ValueStack getStack();
void addPreResultListener(PreResultListener listener);
/**
* Invokes the next step in processing this ActionInvocation.
* <p/>
* If there are more Interceptors, this will call the next one. If Interceptors choose not to short-circuit
* ActionInvocation processing and return their own return code, they will call invoke() to allow the next Interceptor
* to execute. If there are no more Interceptors to be applied, the Action is executed.
* If the {@link ActionProxy#getExecuteResult()} method returns <tt>true</tt>, the Result is also executed.
*
* @throws Exception can be thrown.
* @return the return code.
*/
String invoke() throws Exception;
String invokeActionOnly() throws Exception;
void setActionEventListener(ActionEventListener listener);
void init(ActionProxy proxy) ;
}

public interface ActionInvocation extends Serializable {
    Object getAction();
    boolean isExecuted();
    ActionContext getInvocationContext();
    ActionProxy getProxy();
    Result getResult() throws Exception;
    String getResultCode();
    void setResultCode(String resultCode);
    ValueStack getStack();
    void addPreResultListener(PreResultListener listener);
    /**
     * Invokes the next step in processing this ActionInvocation.
     * <p/>
     * If there are more Interceptors, this will call the next one. If Interceptors choose not to short-circuit
     * ActionInvocation processing and return their own return code, they will call invoke() to allow the next Interceptor
     * to execute. If there are no more Interceptors to be applied, the Action is executed.
     * If the {@link ActionProxy#getExecuteResult()} method returns <tt>true</tt>, the Result is also executed.
     *
     * @throws Exception can be thrown.
     * @return the return code.
     */
    String invoke() throws Exception;
    String invokeActionOnly() throws Exception;
    void setActionEventListener(ActionEventListener listener);
    void init(ActionProxy proxy) ;
}

框架通过调用ActionInvocation的invoke方法开始动作的执行,但并不总是映射到第一个拦截器，ActinInvocation负责跟踪执行的状态，并且把控制交给合适的拦截器。通过调用拦截器的intercept（）方法将控制交给拦截器。.实际应用中编写的拦截器要实现Interceptor接口，通过intercept（）方法返回的控制字符串决定页面的跳转，实现登录，权限控制。这就是拦截器的原理。

public interface Interceptor
extends Serializable {
void destroy();
void init();
/**
* Allows the Interceptor to do some processing on the request before and/or after the rest of the processing of the
* request by the {@link ActionInvocation} or to short-circuit the processing and just return a String return code.
*
* @param invocation the action invocation
* @return the return code, either returned from {@link ActionInvocation#invoke()}, or from the interceptor itself.
* @throws Exception any system-level error, as defined in {@link com.opensymphony.xwork2.Action#execute()}.
*/
String intercept(ActionInvocation invocation) throws Exception;

}