struts2默认拦截器之prepare

http://blogwarning.iteye.com/blog/1390172

在struts2的struts-default.xml中定义了一个name为prepare拦截器，实现类是com.opensymphony.xwork2.interceptor.PrepareInterceptor，它的作用是为实现了com.opensymphony.xwork2.Preparable接口的action调用相关方法。该拦截器有两个参数：alwaysInvokePrepare，firstCallPrepareDo，两者的类型都是boolean,默认值分别是true，false。

该拦截器的核心代码如下：

public String doIntercept(ActionInvocation invocation) throws Exception {
    Object action = invocation.getAction();
    if (action instanceof Preparable) {
        try {
            String[] prefixes;
            if (firstCallPrepareDo) {
                prefixes = new String[] {ALT_PREPARE_PREFIX, PREPARE_PREFIX};
            } else {
                prefixes = new String[] {PREPARE_PREFIX, ALT_PREPARE_PREFIX};
            }
            PrefixMethodInvocationUtil.invokePrefixMethod(invocation, prefixes);
        }
        catch (InvocationTargetException e) {
            /*
             * The invoked method threw an exception and reflection wrapped it
             * in an InvocationTargetException.
             * If possible re-throw the original exception so that normal
             * exception handling will take place.
             */
            Throwable cause = e.getCause();
            if (cause instanceof Exception) {
                throw (Exception) cause;
            } else if(cause instanceof Error) {
                throw (Error) cause;
            } else {
                /*
                 * The cause is not an Exception or Error (must be Throwable) so
                 * just re-throw the wrapped exception.
                 */
                throw e;
            }
        }
        if (alwaysInvokePrepare) {
            ((Preparable) action).prepare();
        }
    }
    return invocation.invoke();
}

该代码的逻辑非常简单，如果action实现了com.opensymphony.xwork2.Preparable接口，则在调用setXXX和execute()方法之前调用系列方法。如果firstCallPrepareDo为true，则调用prepareDoXXX方法，否则调用prepareXXX方法(XXX为action对应的方法)。接下来查看alwaysInvokePrepare状态，如果其值为true则调用com.opensymphony.xwork2.Preparable接口的prepare方法。最后将action转交给下一个拦截器。