We have to understand the four scopes: application, session, request and page

First of all, I want to make a statement , So-called " Scope " Namely " Scope of information sharing ", In other words, how much information can be effective .

Web The basic unit of interaction is HTTP request . The process of each user from entering the website to leaving the website is called HTTP conversation , During the operation of a server, multiple users will access , That's more than one. HTTP conversation .

One 、application

Application  The scope of the server is to execute the service at the beginning of the server , Until the server is shut down Application  The most extensive 、 Stay the longest , Therefore, pay special attention when using, otherwise the server load may become heavier and heavier . Just store the data in application object , Range of data  (Scope)  for Application.

have application The object of the scope is bound to javax.servlet.ServletContext in . stay Web While the application is running , All pages can access objects in this range .

application The main method of object ：

1、 getAttribute(String name)     return Object     

2、 getAttributeNames()             return Enumeration

3、 getInitParameter(String name)

4、 getServletInfo()

5、 setAttribute(String name , Object object)

Two 、session  ！！！

HTTP conversation From the beginning to the end .Session  The scope of the function is a period of time that the user continues to connect with the server , But with service Breaker disconnection , This property is invalid .

Session  It's easier to judge the beginning of , It sends out the first HTTP The request is considered the beginning of the conversation . But the end is hard to judge , Because the server is not notified when the browser closes , Therefore, it can only be judged by the following method ： If the client does not respond within a certain time , The session is considered to be over .Tomcat The default value is 120 minute , But this value can also be passed through HttpSession Of setMaxInactiveInterval() Method to set .

have session The object of the scope is bound to javax.servlet.http.HttpSession In the object .

Session The main method of object ：

1、 getAttribute(String name) return Object

2、 getAttributeNames()return Enumeration

3、 getCreationTime()return long

4、 getId()return String

5、 getLastAccessedTime() return long

6、 getMaxInactiveInterval() return int

7、 removeAttribute(String name) void

8、 setAttribute(String name , java.lang.Object value) void 

3、 ... and 、requset ！！！

HTTP The time between the beginning and the end of the request .Request  The scope of refers to in a JSP  A web page sends a request to another JSP  Between pages , otherwise This property is invalidated . One HTTP The processing of the request may require multiple Servlet cooperation , And these Servlet Information can be transmitted in some way , But this information is invalid after the request is completed .
have request The object of the scope is bound to javax.servlet.ServletRequest In the object .

It should be noted that , Because the request object is different for each customer request , So for every new request , All objects in this range must be re created and deleted .

request The main method of object ：

1、 getParameter(String name) return String

2、 getParameterNames() return Enumeration

3、 getParameterValues(String name) return String[]

Four 、page

Scope of action ： The time between opening and closing the current page , It can only be valid on the same page .

have page The object of the scope is bound to javax.servlet.jsp.PageContext In the object .

5、 ... and 、 upgrade

request and page The life cycle of life is short , The difference between them ： One request It can contain more than one page page （include,forward And filter）

In order to avoid and Servlet API Couple together , convenient Action Class for unit testing ,Struts 2 Yes HttpServletRequest、HttpSession and ServletContext It was packaged , Three Map object To replace these three objects , stay Action in , Use it directly HttpServletRequest、HttpSession and ServletContext Corresponding Map Object to save and read data .

To get these three Map object , have access to com.opensymphony.xwork2.ActionContext class ,ActionContext yes action Execution context , stay ActionContext It's preserved in action A set of objects required for execution , Include parameters、request、session、application and locale etc. .

ActionContext Class defines the following methods , Used to get HttpServletRequest、HttpSession and ServletContext Corresponding Map object .

public Object get(Object key)

ActionContext Class does not provide anything like getRequest() Such a method to obtain the encapsulation HttpServletRequest Of Map object . To get a request Map object , You need to get() Method pass parameter “request”.

 

public Map getSession()

Get encapsulated HttpSession Of Map object .

 

public Map getApplication()

Get encapsulated ServletContext Of Map object .

 

example ： adopt ActionContext To get request、session and application Object's LoginAction1

package action;

ActionContext context =ActionContext.getContext();

Map request =(Map)context.get("request");

Map session = context.getSession();

Map application = context.getApplication();

request.put("greeting", " Welcome to the programmer's house ");

session.put("user", user);

On the success page , have access to JSP Built in expression language to access request、session.、

Let's use a few simple examples , Take a look at this 4 Application of a range object .

1． test page Range

test1.jsp

<%

pageContext.setAttribute("name","zhangsan");

out.println("test1.jsp: ");

out.println(pageContext.getAttribute("name"));

out.println("<p>");

pageContext.include("test2.jsp");

%>

test2.jsp

<%

out.println("test2.jsp: ");

out.println(pageContext.getAttribute("name"));

%>

visit test1.jsp, You will see the following output ：

test1.jsp: zhangsan

test2.jsp: null

The instructions are saved in pageContext The attribute in the object has page Range , Can only be accessed in the same page .

2． test request Range

modify test1.jsp and test2.jsp, As shown below .

test1.jsp

<%

request.setAttribute("name","zhangsan");

out.println("test1.jsp: ");

out.println(request.getAttribute("name"));

out.println("<p>");

pageContext.include("test2.jsp");

%>

test2.jsp

<%

out.println("test2.jsp: ");

out.println(request.getAttribute("name"));

%>

visit test1.jsp, You will see the following output ：

test1.jsp: zhangsan

test2.jsp: zhangsan

The instructions are saved in request The attribute in the object has request Range , During the lifetime of the request object , You can access objects in this range .

take pageContext.include("test2.jsp");  This sentence is annotated , First visit test1.jsp, Revisiting test2.jsp, You can see the following output ：

test2.jsp: null

This is because the client starts a new request .

3． test session Range

modify test1.jsp and test2.jsp, As shown below .

test1.jsp

<%

session.setAttribute("name","zhangsan");

%>

test2.jsp

<%

out.println("test2.jsp: ");

out.println(session.getAttribute("name"));

%>

First visit test1.jsp, And then in Same browser window Medium visit test2.jsp, You can see the following output ：

test2.jsp: zhangsan

The instructions are saved in session The attribute in the object has session Range , During the conversation , You can access objects in this range .

If we finish our visit test1.jsp after , Close the browser , Reopen the browser window , visit test2.jsp, You will see the following output ：

test2.jsp: null

This is because the client starts a new session with the server .

4． test application Range

modify test1.jsp and test2.jsp, As shown below .

test1.jsp

<%

application.setAttribute("name","zhangsan");

%>

test2.jsp

<%

out.println("test2.jsp: ");

out.println(application.getAttribute("name"));

%>

First visit test1.jsp, Then close the browser , Then open the browser window , visit test2.jsp, You can see the following output ：

test2.jsp: zhangsan

The instructions are saved in application The attribute in the object has application Range , stay Web While the application is running , Can access objects in this range .

Reprinted from ：http://blog.csdn.net/u013938465/article/details/48712137