The ValueStack in Struts 2

In this tutorial we will discuss about the ValueStack in the Struts2 how its work behind the scene. Fist lets see the high level picture of flow request data to Struts2 framework.

Step 1. Client send the request to server.
Step 2. Struts configuration file map to specific action class.
Step 3. Action class has business service class to operate business logic and return result data to the action class and action class set this data to self member variables.
Step 4. After setting result to member variable it send to the JSP to view.
Step 5. JSP using the some TAGS to display result.
Step 6. JSP prepare the response as HTML and return to client.

Now we will see the How Servlet work. High Level flow of request to Servlet.

Step-
1.  We are given objects to use, but we will need to do the "push" and "pull" of data ourseleves.
2. Servlet places the business data into the session and the JSP accesses it.
3 This has to be done for every servlet-to-JSP flow that data to be communicated.

Now we move to Struts2 framework how it is work internally.
Step 1. New Action objects are created for each request.

In the above every requests have own Action instances with member variables. Now here there are some complicated to display member variable to JSP. it may be override to each other. So overcome this problem Struts 2 introduce to ValueStack.

ValueStack-
A valueStack is simply a stack that contains application specific objects such as action objects and other model object. At the execution time, action is placed on the top of the stack. We can put objects in the valuestack, query it and delete it.

1. It is not really a stack in the traditional sense.
2. It does stack up objects.
3. Behave like a virtual object.
4. Makes it easy to refer to member variables.

The value stack is a set of several objects which keeps the following objects in the provided order:

SN	Objects & Description
1	Temporary Objects There are various temporary objects which are created during execution of a page. For example the current iteration value for a collection being looped over in a JSP tag.
2	The Model Object If you are using model objects in your struts application, the current model object is placed before the action on the value stack
3	The Action Object This will be the current action object which is being executed.
4	Named Objects These objects include #application, #session, #request, #attr and #parameters and refer to the corresponding servlet scopes

The value stack can be accessed via the tags provided for JSP, Velocity or Freemarker. There are various tags which we will study in separate chapters, are used to get and set struts 2.0 value stack. You can get valueStack object inside your action as follows:

ActionContext.getContext().getValueStack()

Once you have a ValueStack object, you can use following methods to manipulate that object:

SN	ValueStack Methods & Description
1	Object findValue(String expr) Find a value by evaluating the given expression against the stack in the default search order.
2	CompoundRoot getRoot() Get the CompoundRoot which holds the objects pushed onto the stack.
3	Object peek() Get the object on the top of the stack without changing the stack.
4	Object pop() Get the object on the top of the stack and remove it from the stack.
5	void push(Object o) Put this object onto the top of the stack.
6	void set(String key, Object o) Sets an object on the stack with the given key so it is retrievable by findValue(key,...)
7	void setDefaultType(Class defaultType) Sets the default type to convert to if no type is provided when getting a value.
8	void setValue(String expr, Object value) Attempts to set a property on a bean in the stack with the given expression using the default search order.
9	int size() Get the number of objects in the stack.

The OGNL:

The Object-Graph Navigation Language (OGNL) is a powerful expression language that is used to reference and manipulate data on the ValueStack. OGNL also helps in data transfer and type conversion.

The OGNL is very similar to the JSP Expression Language. OGNL is based on the idea of having a root or default object within the context. The properties of the default or root object can be referenced using the markup notation, which is the pound symbol.

As mentioned earlier, OGNL is based on a context and Struts builds an ActionContext map for use with OGNL. The ActionContext map consists of the following:

• application - application scoped variables
• session - session scoped variables
• root / value stack - all your action variables are stored here
• request - request scoped variables
• parameters - request parameters
• atributes - the attributes stored in page, request, session and application scope

It is important to understand that the Action object is always available in the value stack. So, therefore if your Action object has properties x and y there are readily available for you to use.

Objects in the ActionContext are referred using the pound symbol, however, the objects in the value stack can be directly referenced, for example if employee is a property of an action class then it can get referenced as follows:

<s:property value="name"/>

instead of

<s:property value="#name"/>

If you have an attribute in session called "login" you can retrieve it as follows:

<s:property value="#session.login"/>

OGNL also supports dealing with collections - namely Map, List and Set. For example to display a dropdown list of colors, you could do:

<s:property name="color" list="{'red','yellow','green'}"/>

ValueStack/OGNL Example:-

CREATE ACTION:
Let us consider following action class where we are accessing valueStack and then setting few keys which we will access using OGNL in our view ie. JSP page.

package com.dineshonjava.struts2.action;

import java.util.HashMap;
import java.util.Map;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;
import com.opensymphony.xwork2.util.ValueStack;

/**
 * @author Dinesh Rajput
 *
 */
public class HelloWorldAction extends ActionSupport {
 private static final long serialVersionUID = 4956157388836635122L;
 private String name;

    public String execute() throws Exception {
       ValueStack stack = ActionContext.getContext().getValueStack();
       Map<String, Object> context = new HashMap<String, Object>();

       context.put("key1", new String("This is key1")); 
       context.put("key2", new String("This is key2"));
       stack.push(context);

       System.out.println("Size of the valueStack: " + stack.size());
       return "success";
    }  

    public String getName() {
       return name;
    }

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