上下文(context)ActionContext介绍（在Struts2中）
????? 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）
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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();
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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.
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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;
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public String getMessage() {
return message;
}
HttpServletRequest request = ServletActionContext.getRequest();
HttpServletResponse response = ServletActionContext.getResponse();
HttpSession session = request.getSession();
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@Override
public String execute() {
ActionContext.getContext().getSession().put("msg", "Hello World from Session!");[A2]?
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StringBuffer sb =new StringBuffer("Message from request: ");
sb.append(request.getParameter("msg"));
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sb.append("<br>Response Buffer Size: ");
sb.append(response.getBufferSize());
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sb.append("<br>Session ID: ");
sb.append(session.getId());
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message = sb.toString();? //转换为字符串。
return SUCCESS;
}
} ??//与LoginAction类似的方法。
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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;
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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;
}
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@Override
public String execute() {
att.put("msg", "Hello World from Session!");
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HttpSession session = request.getSession();
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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());
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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
<action name="NonIoCServlet" style="margin: 0cm 0cm 0pt 21pt; text-indent: 21pt;"><result>/Servlet.jsp</result>
</action>
<action name="IoCServlet" style="margin: 0cm 0cm 0pt 21pt; text-indent: 21pt;"><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.
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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.
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（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.
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Struts2的action类继承
com.opensymphony.xwork2.ActionSupport类的常用方法
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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:
addFieldErrorin interface ValidationAware
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:
validatein interface Validateable
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public void setSession(Map session)
Sets a map of action session values. 设置session值，
Parameters:
session- the session values.
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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");??? }
??? }
