上下文(context)ActionContext介绍（在Struts2中）

心已赠人 2022-07-28 09:00 206阅读 0赞

一种属性的有序序列，它们为驻留在环境内的对象定义环境。在对象的激活过程中创建上下文，对象被配置为要求某些自动服务，如同步、事务、实时激活、安全性等等。多个对象可以存留在一个上下文内。也有根据上下文理解意思的意思。

**ActionContext介绍（在Struts2中）**

在Web应用程序开发中,除了将请求参数自动设置到Action的字段中,我们往往也需要在Action里直接获取请求(Request)或会话 (Session)的一些信息, 甚至需要直接对JavaServlet Http的请求(HttpServletRequest),响应(HttpServletResponse)操作.

我们需要在Action中取得request请求参数"username"的值:

ActionContext context = ActionContext.getContext();

Map params = context.getParameters();

String username = (String) params.get("username");

ActionContext(com.opensymphony.xwork.ActionContext)是Action执行时的上下文,上下文可以看作是一个容器(其实我们这里的容器就是一个Map而已),它存放放的是Action在执行时需要用到的对象

一般情况,我们的ActionContext都是通过:ActionContext context = (ActionContext) actionContext.get();来获取的.我们再来看看这里的actionContext对象的创建:

static ThreadLocal actionContext = new ActionContextThreadLocal();,

ActionContextThreadLocal是实现ThreadLocal的一个内部类.ThreadLocal可以命名为"线程局部变量",它为每一个使用该变量的线程都提供一个变量值的副本,使每一个线程都可以独立地改变自己的副本, 而不会和其它线程的副本冲突.这样,我们ActionContext里的属性只会在对应的当前请求线程中可见,从而保证它是线程安全的.

下面我们看看怎么通过ActionContext取得我们的HttpSession:

Map session = ActionContext.getContext().getSession();

**ServletActionContext**(com.opensymphony.webwork. ServletActionContext),这个类直接继承了我们上面介绍的ActionContext,它提供了直接与JavaServlet相关对象访问的功能,它可以取得的对象有:

1, javax.servlet.http.HttpServletRequest:HTTPservlet请求对象

2, javax.servlet.http.HttpServletResponse;:HTTPservlet相应对象

3, javax.servlet.ServletContext:Servlet 上下文信息

4, javax.servlet.ServletConfig:Servlet配置对象

5, javax.servlet.jsp.PageContext:Http页面上下文

下面我们看看几个简单的例子,让我们了解如何从ServletActionContext里取得JavaServlet的相关对象:

1, 取得HttpServletRequest对象:

HttpServletRequest request = ServletActionContext. getRequest();

2, 取得HttpSession对象:

HttpSession session = ServletActionContext. getRequest().getSession();

ServletActionContext 和ActionContext有着一些重复的功能,在我们的Action中,该如何去抉择呢?我们遵循的原则是:如果ActionContext能够实现我们的功能,那最好就不要使用ServletActionContext,让我们的Action尽量不要直接去访问JavaServlet的相关对象.在使用ActionContext时有一点要注意:**不要在Action的构造函数里使用ActionContext.getContext(),因为这个时候ActionContext里的一些值也许没有设置,这时通过ActionContext取得的值也许是null.**

如果我要取得Servlet API中的一些对象,如request,response或session等,应该怎么做?在Strutx 2.0你可以有两种方式获得这些对象:非IoC(控制反转Inversion of Control)方式和IoC方式.

**A、非IoC方式**

要获得上述对象,关键Struts 2.0中com.opensymphony.xwork2.ActionContext类.我们可以***通过它的静态方法getContext()获取当前 Action的上下文对象***. 另外,org.apache.struts2.ServletActionContext作为辅助类(Helper Class),可以帮助您快捷地获得这几个对象.

HttpServletRequest request = ServletActionContext.getRequest();

HttpServletResponse response = ServletActionContext.getResponse();

HttpSession session = request.getSession();

**如果你只是想访问session的属性(Attribute),你也可以通过ActionContext.getContext().getSession()获取或添加session范围(Scoped)的对象.**

**B、IoC方式**

要使用IoC方式,我们首先要告诉IoC容器(Container)想取得某个对象的意愿,通过实现相应的接口做到这点.具体实现,请参考例6 IocServlet.java.

例6 classes/tutorial/NonIoCServlet.java

package tutorial;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpSession;

import org.apache.struts2.ServletActionContext;

import com.opensymphony.xwork2.ActionContext;

import com.opensymphony.xwork2.ActionSupport;

Public class** NonIoCServlet* extends ActionSupport*** \{

private String message;

public String getMessage() \{

return message;

HttpServletRequest request = ServletActionContext.getRequest();

HttpServletResponse response = ServletActionContext.getResponse();

HttpSession session = request.getSession();

@Override

public String execute() \{

***ActionContext.getContext().getSession().put("msg", "Hello World from Session!");***[\[A2\]][A2]

StringBuffer sb =new StringBuffer("Message from request: ");

sb.append(request.getParameter("msg"));

sb.append("<br>Response Buffer Size: ");

sb.append(response.getBufferSize());

sb.append("<br>Session ID: ");

sb.append(session.getId());

message = sb.toString();  //转换为字符串。

return SUCCESS;

\} **  *//与LoginAction类似的方法。***

例6 classes/tutorial/IoCServlet.java

package tutorial;

import java.util.Map;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpSession;

import org.apache.struts2.interceptor.ServletRequestAware;

import org.apache.struts2.interceptor.ServletResponseAware;

import org.apache.struts2.interceptor.SessionAware;

import com.opensymphony.xwork2.ActionContext;

import com.opensymphony.xwork2.ActionSupport;

publicclass **IoCServlet extends ActionSupport** implements SessionAware,   ServletRequestAware, ServletResponseAware \{

private String message;

private Map att;

private HttpServletRequest request;

private HttpServletResponse response;

public String getMessage() \{

return message;

public void setSession(Map att) \{

this.att = att;

publicvoid setServletRequest(HttpServletRequest request) \{

this.request = request;

publicvoid setServletResponse(HttpServletResponse response) \{

this.response = response;

@Override

public String execute() \{

**att**.put("msg", "Hello World from Session!");

HttpSession session = request.getSession();

StringBuffer sb =new StringBuffer("Message from request: ");

sb.append(request.getParameter("msg"));

sb.append("<br>Response Buffer Size: ");

sb.append(response.getBufferSize());

sb.append("<br>Session ID: ");

sb.append(session.getId());

message = sb.toString();

return SUCCESS;

例6 Servlet.jsp

<%@ page contentType="text/html; charset=UTF-8" %>

<%@ taglib prefix="s" uri="/struts-tags"%>

<html>

<head>

<title>Hello World!</title>

</head>

<body>

<h2>

<s:property value="message" escape="false"/>

<br>Message from session: <s:property value="\#session.msg"/>

</h2>

</body>

</html>

例6 classes/struts.xml中NonIocServlet和IoCServlet Action的配置

<result>/Servlet.jsp</result>

</action>

<result>/Servlet.jsp</result>

</action>

运行Tomcat,在浏览器地址栏中键入http://localhost:8080/Struts2\_Action /NonIoCServlet.action?msg=Hello%20World!或http://localhost:8080/Struts2\_Action/IoCServlet.action?msg=Hello%20World!

在Servlet.jsp中,我用了两次property标志,第一次将escape设为false为了在JSP中输出<br>转行,第二次的value中的OGNL为"\#session.msg",它的作用与session.getAttribute("msg")等同.

** **

**附：ActionContext的常用方法（来自Struts2.0  API）**

**（一）get**

public [Object][] **get**([Object][] key)

Returns a value that is stored in the current ActionContext by doing a lookup using the value's key.

**Parameters:**

key- the key used to find the value.

**Returns:**

the value that was found using the key or null if the key was not found.

**（二）put**

public void **put**([Object][] key, [Object][] value)

Stores a value in the current ActionContext. The value can be looked up using the key.

**Parameters:**

key- the key of the value.

value- the value to be stored.

**（三）getContext**

public static [ActionContext][] **getContext**()

Returns the ActionContext specific to the current thread.

**Returns:**

the ActionContext for the current thread, is never null.

**（四）getSession**

public [Map][] **getSession**()

Gets the Map of HttpSession values when in a servlet environment or a generic session map otherwise.

**Returns:**

the Map of HttpSession values when in a servlet environment or a generic session map otherwise.

** (五)setSession**

public void **setSession**([Map][] session)

Sets a map of action session values.

**Parameters:**

session- the session values.

** **

**Struts2的action类继承**

**com.opensymphony.xwork2.ActionSupport类的常用方法**

** **

**addFieldError  //该方法主要用于验证的方法之中**

public void **addFieldError**([String][] fieldName,  [String][] errorMessage)

**Description copied from interface: [ValidationAware][]**

Add an error message for a given field.

**Specified by:**

[addFieldError][ValidationAware]in interface [ValidationAware][ValidationAware 1]

**Parameters:**

fieldName- name of field

errorMessage- the error message

**validate**

public void **validate**()

A default implementation that validates nothing. Subclasses should override this method to provide validations.

**Specified by:**

[validate][]in interface [Validateable][]

** **

--------------------

这一点比较的重要，例如：ActionContext.*getContext*().getSession().put("user", "value");

与右上角的是一个模式。

Java语法基础，使用stringBuffer。

对属性（实例）设置setter方法。

**方法的来源，见后面补充的常用方法：**

public void **setSession**([Map][] session)

Sets a map of action session values. 设置session值，

**Parameters:**

session- the session values.

为当前的session，所以可以调用put方法。详细信息见补充的知识。

常用于校验登陆程序的账号和密码是否为空，可以加入addFieldError方法。例如**public** **void** validate() \{

**if** (**null** == login.getUserID() || "".equals(login.getUserID())) \{          **this**.addFieldError("login.userID", "学号不能为空");       \}

**if** (**null** == login.getUserPassword()              || "".equals(login.getUserPassword())) \{         **this**.addFieldError("login.userPassword", "密码不能为空");    \}

### [上下文(context)ActionContext介绍（在Struts2中）][context_ActionContext_Struts2] ###

[A2]: http://271072008-qq-com.iteye.com/admin/blogs/#_msocom_2
[Object]: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Object.html
[ActionContext]: http://271072008-qq-com.iteye.com/com/opensymphony/xwork2/ActionContext.html
[Map]: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html
[String]: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html
[ValidationAware]: http://271072008-qq-com.iteye.com/com/opensymphony/xwork2/ValidationAware.html#addFieldError%28java.lang.String,%20java.lang.String%29
[ValidationAware 1]: http://271072008-qq-com.iteye.com/com/opensymphony/xwork2/ValidationAware.html
[validate]: http://271072008-qq-com.iteye.com/com/opensymphony/xwork2/Validateable.html#validate%28%29
[Validateable]: http://271072008-qq-com.iteye.com/com/opensymphony/xwork2/Validateable.html
[context_ActionContext_Struts2]: http://271072008-qq-com.iteye.com/blog/1165663