Struts 2 简明教程

Struts 2 - Interceptors

从本质上讲，拦截器与 servlet 过滤器或 JDK 的 Proxy 类相同。拦截器允许跨模块功能与操作和框架分开发挥作用。您可以使用拦截器实现以下内容 −

Interceptors are conceptually the same as servlet filters or the JDKs Proxy class. Interceptors allow for crosscutting functionality to be implemented separately from the action as well as the framework. You can achieve the following using interceptors −

  1. Providing preprocessing logic before the action is called.

  2. Providing postprocessing logic after the action is called.

  3. Catching exceptions so that alternate processing can be performed.

Struts2 框架中提供的许多功能都是使用拦截器实现的；

Many of the features provided in the Struts2 framework are implemented using interceptors;

Examples 包括异常处理、文件上传、生命周期回调等。实际上，由于 Struts2 将其大部分功能强调在拦截器上，因此不太可能为每个操作分配 7 到 8 个拦截器。

Examples include exception handling, file uploading, lifecycle callbacks, etc. In fact, as Struts2 emphasizes much of its functionality on interceptors, it is not likely to have 7 or 8 interceptors assigned per action.

Struts2 Framework Interceptors

Struts2 框架提供了一个即用型开箱拦截器列表，这些拦截器已经过预配置并准备使用。下面列出了一些重要的拦截器 −

Struts 2 framework provides a good list of out-of-the-box interceptors that come preconfigured and ready to use. Few of the important interceptors are listed below −

Sr.No

Interceptor & Description

1

alias Allows parameters to have different name aliases across requests.

2

checkbox Assists in managing check boxes by adding a parameter value of false for check boxes that are not checked.

3

conversionError Places error information from converting strings to parameter types into the action’s field errors.

4

createSession Automatically creates an HTTP session if one does not already exist.

5

debugging Provides several different debugging screens to the developer.

6

execAndWait Sends the user to an intermediary waiting page while the action executes in the background.

7

exception Maps exceptions that are thrown from an action to a result, allowing automatic exception handling via redirection.

8

fileUpload Facilitates easy file uploading.

9

i18n Keeps track of the selected locale during a user’s session.

10

logger Provides simple logging by outputting the name of the action being executed.

11

params Sets the request parameters on the action.

12

prepare This is typically used to do pre-processing work, such as setup database connections.

13

profile Allows simple profiling information to be logged for actions.

14

scope Stores and retrieves the action’s state in the session or application scope.

15

ServletConfig Provides the action with access to various servlet-based information.

16

timer Provides simple profiling information in the form of how long the action takes to execute.

17

token Checks the action for a valid token to prevent duplicate formsubmission.

18

validation Provides validation support for actions

请在 Struts 2 文档中查找以上所述拦截器的完备详细信息。但我会向你展示如何在 Struts 应用程序中通常使用拦截器。

Please look into Struts 2 documentation for complete detail on the abovementioned interceptors. But I will show you how to use an interceptor in general in your Struts application.

How to Use Interceptors?

让我们了解如何在“Hello World”程序中使用已存在的拦截器。我们将使用 timer 拦截器，其目的是衡量执行操作方法所需的时间。同时，我正在使用 params 拦截器，其目的是将请求参数发送到操作。你可以尝试不使用此拦截器来验证你的示例，你会发现 name 属性没有被设置，这是因为参数无法到达操作。

Let us see how to use an already existing interceptor to our "Hello World" program. We will use the timer interceptor whose purpose is to measure how long it took to execute an action method. At the same time, I’m using params interceptor whose purpose is to send the request parameters to the action. You can try your example without using this interceptor and you will find that name property is not being set because parameter is not able to reach to the action.

我们将保留 HelloWorldAction.java、web.xml、HelloWorld.jsp 和 index.jsp 文件，因为它们已在 Examples 章节中创建，但让我们修改 struts.xml 文件以添加一个拦截器，如下所示：

We will keep HelloWorldAction.java, web.xml, HelloWorld.jsp and index.jsp files as they have been created in Examples chapter but let us modify the struts.xml file to add an interceptor as follows −

<?xml version = "1.0" Encoding = "UTF-8"?>
<!DOCTYPE struts PUBLIC
   "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
   "http://struts.apache.org/dtds/struts-2.0.dtd">
<struts>
   <constant name = "struts.devMode" value = "true" />

   <package name = "helloworld" extends = "struts-default">
      <action name = "hello"
         class = "com.tutorialspoint.struts2.HelloWorldAction"
         method = "execute">
         <interceptor-ref name = "params"/>
         <interceptor-ref name = "timer" />
         <result name = "success">/HelloWorld.jsp</result>
      </action>
   </package>
</struts>

右键单击项目名称并单击 Export > WAR File 以创建 War 文件。然后再将此 WAR 部署到 Tomcat 的 Web 应用目录中。最后，启动 Tomcat 服务器并尝试访问 URL http://localhost:8080/HelloWorldStruts2/index.jsp 。这将产生以下屏幕：

Right click on the project name and click Export > WAR File to create a War file. Then deploy this WAR in the Tomcat’s webapps directory. Finally, start Tomcat server and try to access URL http://localhost:8080/HelloWorldStruts2/index.jsp. This will produce the following screen −

helloworldstruts4

现在在给定的文本框中输入任意单词并单击马上问候按钮来执行已定义的操作。现在，如果你将检查生成的日志，你会发现以下文本：

Now enter any word in the given text box and click Say Hello button to execute the defined action. Now if you will check the log generated, you will find the following text −

INFO: Server startup in 3539 ms
27/08/2011 8:40:53 PM
com.opensymphony.xwork2.util.logging.commons.CommonsLogger info
INFO: Executed action [//hello!execute] took 109 ms.

此处底行是由 timer 拦截器生成的，其表明操作总共花费了 109 毫秒才得以执行。

Here bottom line is being generated because of timer interceptor which is telling that action took total 109ms to be executed.

Create Custom Interceptors

在应用程序中使用自定义拦截器是提供跨切割应用程序功能的优雅方式。创建自定义拦截器很容易；需要扩展的接口是以下 Interceptor 接口：

Using custom interceptors in your application is an elegant way to provide crosscutting application features. Creating a custom interceptor is easy; the interface that needs to be extended is the following Interceptor interface −

public interface Interceptor extends Serializable {
   void destroy();
   void init();
   String intercept(ActionInvocation invocation)
   throws Exception;
}

顾名思义，init() 方法提供一种方法来初始化拦截器，而 destroy() 方法则提供一种拦截器清理功能。与操作不同，拦截器将在所有请求中重复使用，且需要线程安全，尤其是 intercept() 方法。

As the names suggest, the init() method provides a way to initialize the interceptor, and the destroy() method provides a facility for interceptor cleanup. Unlike actions, interceptors are reused across requests and need to be threadsafe, especially the intercept() method.

ActionInvocation 对象提供对运行时环境的访问。它允许访问操作本身以及用于调用操作并确定是否已调用操作的方法。

The ActionInvocation object provides access to the runtime environment. It allows access to the action itself and methods to invoke the action and determine whether the action has already been invoked.

如果你不需要初始化或清理代码，则可以扩展 AbstractInterceptor 类。它提供了 init() 和 destroy() 方法的默认无效操作实现。

If you have no need for initialization or cleanup code, the AbstractInterceptor class can be extended. This provides a default nooperation implementation of the init() and destroy() methods.

Create Interceptor Class

让我们在 Java Resources > src 文件夹中创建以下 MyInterceptor.java：

Let us create the following MyInterceptor.java in Java Resources > src folder −

package com.tutorialspoint.struts2;

import java.util.*;
import com.opensymphony.xwork2.ActionInvocation;
import com.opensymphony.xwork2.interceptor.AbstractInterceptor;

public class MyInterceptor extends AbstractInterceptor {

   public String intercept(ActionInvocation invocation)throws Exception {

      /* let us do some pre-processing */
      String output = "Pre-Processing";
      System.out.println(output);

      /* let us call action or next interceptor */
      String result = invocation.invoke();

      /* let us do some post-processing */
      output = "Post-Processing";
      System.out.println(output);

      return result;
   }
}

如你所见，实际操作将通过调用 invocation.invoke() 由拦截器执行。因此，你可以根据你的要求进行某些预处理和后处理。

As you notice, actual action will be executed using the interceptor by *invocation.invoke()*call. So you can do some pre-processing and some postprocessing based on your requirement.

框架本身通过对 ActionInvocation 对象的 invoke() 进行首次调用来启动该进程。每次调用 invoke() 时，ActionInvocation 都会咨询其状态并执行随之而来的任何一个拦截器。当调用所有配置的拦截器后，invoke() 方法将导致执行操作本身。

The framework itself starts the process by making the first call to the ActionInvocation object’s invoke(). Each time invoke() is called, ActionInvocation consults its state and executes whichever interceptor comes next. When all of the configured interceptors have been invoked, the invoke() method will cause the action itself to be executed.

下图通过请求流程显示了相同概念 -

The following diagram shows the same concept through a request flow −

actioninvocation

Create Action Class

让我们在 Java Resources > src 中使用包名 com.tutorialspoint.struts2 创建一个 java 文件 HelloWorldAction.java ，内容如下。

Let us create a java file HelloWorldAction.java under Java Resources > src with a package name com.tutorialspoint.struts2 with the contents given below.

package com.tutorialspoint.struts2;

import com.opensymphony.xwork2.ActionSupport;

public class HelloWorldAction extends ActionSupport {
   private String name;

   public String execute() throws Exception {
      System.out.println("Inside action....");
      return "success";
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }
}

这是一个我们在之前的示例中见过的类。我们为“名称”属性有标准的 getter 和 setter 方法，还有一个返回字符串“成功”的执行方法。

This is a same class which we have seen in previous examples. We have standard getters and setter methods for the "name" property and an execute method that returns the string "success".

Create a View

让我们在 Eclipse 项目的 WebContent 文件夹中创建以下 jsp 文件 HelloWorld.jsp 。

Let us create the below jsp file HelloWorld.jsp in the WebContent folder in your eclipse project.

<%@ page contentType = "text/html; charset = UTF-8" %>
<%@ taglib prefix = "s" uri = "/struts-tags" %>

<html>
   <head>
      <title>Hello World</title>
   </head>

   <body>
      Hello World, <s:property value = "name"/>
   </body>
</html>

Create Main Page

我们还需要在 WebContent 文件夹中创建 index.jsp 。这个文件将作为初始操作 URL，用户可以单击它来告诉 Struts 2 框架调用 HelloWorldAction 类的定义方法，并呈现 HelloWorld.jsp 视图。

We also need to create index.jsp in the WebContent folder. This file will serve as the initial action URL where a user can click to tell the Struts 2 framework to call the a defined method of the HelloWorldAction class and render the HelloWorld.jsp view.

<%@ page language = "java" contentType = "text/html; charset = ISO-8859-1"
   pageEncoding = "ISO-8859-1"%>
<%@ taglib prefix = "s" uri = "/struts-tags"%>
   <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
   "http://www.w3.org/TR/html4/loose.dtd">

<html>
   <head>
      <title>Hello World</title>
   </head>

   <body>
      <h1>Hello World From Struts2</h1>
      <form action = "hello">
         <label for = "name">Please enter your name</label><br/>
         <input type = "text" name = "name"/>
         <input type = "submit" value = "Say Hello"/>
      </form>
   </body>
</html>

在以上视图文件中定义的操作 hello 将使用 struts.xml 文件映射到 HelloWorldAction 类及其执行方法。

The hello action defined in the above view file will be mapped to the HelloWorldAction class and its execute method using struts.xml file.

Configuration Files

现在，我们需要注册我们的拦截器，然后调用它，就像我们在前一个示例中调用缺省拦截器一样。要注册一个新定义的拦截器，需要将 <interceptors>…​</interceptors> 标签直接放在 struts.xml 文件的 <package> 标签下面。你可以跳过为缺省拦截器执行这一步骤，就像我们在前一个示例中所做的那样。但此处让我们注册并使用它，如下所示 -

Now, we need to register our interceptor and then call it as we had called default interceptor in previous example. To register a newly defined interceptor, the <interceptors>…​</interceptors> tags are placed directly under the <package> tag ins*struts.xml* file. You can skip this step for a default interceptors as we did in our previous example. But here let us register and use it as follows −

<?xml version = "1.0" Encoding = "UTF-8"?>
<!DOCTYPE struts PUBLIC
   "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
   "http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>
   <constant name = "struts.devMode" value = "true" />
   <package name = "helloworld" extends = "struts-default">

      <interceptors>
         <interceptor name = "myinterceptor"
            class = "com.tutorialspoint.struts2.MyInterceptor" />
      </interceptors>

      <action name = "hello"
         class = "com.tutorialspoint.struts2.HelloWorldAction"
         method = "execute">
         <interceptor-ref name = "params"/>
         <interceptor-ref name = "myinterceptor" />
         <result name = "success">/HelloWorld.jsp</result>
      </action>

   </package>
</struts>

需要注意的是，你可以在 <package> 标签内注册多个拦截器，同时你可以在 <action> 标签内调用多个拦截器。你可以使用不同的操作调用同一个拦截器。

It should be noted that you can register more than one interceptors inside <package> tag and same time you can call more than one interceptors inside the <action> tag. You can call same interceptor with the different actions.

需要在 WebContent 下面的 WEB-INF 文件夹下创建 web.xml 文件，如下所示 -

The web.xml file needs to be created under the WEB-INF folder under WebContent as follows −

<?xml version = "1.0" Encoding = "UTF-8"?>
<web-app xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
   xmlns = "http://java.sun.com/xml/ns/javaee"
   xmlns:web = "http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
   xsi:schemaLocation = "http://java.sun.com/xml/ns/javaee
   http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
   id = "WebApp_ID" version = "3.0">

   <display-name>Struts 2</display-name>

   <welcome-file-list>
      <welcome-file>index.jsp</welcome-file>
   </welcome-file-list>

   <filter>
      <filter-name>struts2</filter-name>
      <filter-class>
         org.apache.struts2.dispatcher.FilterDispatcher
      </filter-class>
   </filter>

   <filter-mapping>
      <filter-name>struts2</filter-name>
      <url-pattern>/*</url-pattern>
   </filter-mapping>
</web-app>

右键单击项目名称并单击 Export > WAR File 以创建 War 文件。然后再将此 WAR 部署到 Tomcat 的 Web 应用目录中。最后，启动 Tomcat 服务器并尝试访问 URL http://localhost:8080/HelloWorldStruts2/index.jsp 。这将产生以下屏幕：

Right click on the project name and click Export > WAR File to create a War file. Then deploy this WAR in the Tomcat’s webapps directory. Finally, start Tomcat server and try to access URL http://localhost:8080/HelloWorldStruts2/index.jsp. This will produce the following screen −

helloworldstruts4

现在，在给定文本框中输入任意单词，然后单击“Say Hello”按钮以执行定义的操作。现在，如果你检查生成的日志，你将会在底部找到以下文本 -

Now enter any word in the given text box and click Say Hello button to execute the defined action. Now if you will check the log generated, you will find the following text at the bottom −

Pre-Processing
Inside action....
Post-Processing

Stacking Multiple Interceptors

如你所想象的，必须为每个操作配置多个拦截器会很快变得极度难以管理。因此，拦截器使用拦截器栈进行管理。以下就是一个直接来自 strutsdefault.xml 文件的示例 -

As you can imagine, having to configure multiple interceptor for each action would quickly become extremely unmanageable. For this reason, interceptors are managed with interceptor stacks. Here is an example, directly from the strutsdefault.xml file −

<interceptor-stack name = "basicStack">
   <interceptor-ref name = "exception"/>
   <interceptor-ref name = "servlet-config"/>
   <interceptor-ref name = "prepare"/>
   <interceptor-ref name = "checkbox"/>
   <interceptor-ref name = "params"/>
   <interceptor-ref name = "conversionError"/>
</interceptor-stack>

上述栈称为 basicStack ，且你可以在配置中使用它，如下所示。此配置节点放在 <package …​/> 节点下面。每个 <interceptor-ref …​/> 标签都引用在此当前拦截器栈之前配置的拦截器或拦截器栈。因此，在配置初始拦截器和拦截器栈时，确保所有拦截器和拦截器栈配置中的名称是唯一的非常重要。

The above stake is called basicStack and can be used in your configuration as shown below. This configuration node is placed under the <package …​/> node. Each <interceptor-ref …​/> tag references either an interceptor or an interceptor stack that has been configured before the current interceptor stack. It is therefore very important to ensure that the name is unique across all interceptor and interceptor stack configurations when configuring the initial interceptors and interceptor stacks.

我们已经了解了如何对操作应用拦截器，对拦截器栈应用拦截器与此没有什么不同。事实上，我们使用完全相同的标签 -

We have already seen how to apply interceptor to the action, applying interceptor stacks is no different. In fact, we use exactly the same tag −

<action name = "hello" class = "com.tutorialspoint.struts2.MyAction">
   <interceptor-ref name = "basicStack"/>
   <result>view.jsp</result>
</action

对“basicStack”的上述注册将完全登记在 hello 操作中六个拦截器的全部赌注。需要注意的是，拦截器按照其配置的顺序执行。例如，在上述情况下，exception 将首先执行，其次是 servlet-config，依此类推。

The above registration of "basicStack" will register complete stake of all the six interceptors with hello action. This should be noted that interceptors are executed in the order, in which they have been configured. For example, in the above case, exception will be executed first, second would be servlet-config and so on.