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9.2 The Session Tracking API

203

HttpSession session = request.getSession(true);

ShoppingCart cart =

(ShoppingCart)session.getValue("shoppingCart");

if (cart == null) { // No cart already in session

cart = new ShoppingCart();

session.putValue("shoppingCart", cart);

}

doSomethingWith(cart);

In most cases, you have a specific attribute name in mind and want to find

the value (if any) already associated with that name. However, you can also

discover all the attribute names in a given session by calling 

getValueNames

,

which returns an array of strings. This method is your only option for finding

attribute names in version 2.1, but in servlet engines supporting version 2.2

of the servlet specification, you can use 

getAttributeNames

. That method is

more consistent in that it returns an 

Enumeration

, just like the 

getHeader 

Names

 and 

getParameterNames

 methods of 

HttpServletRequest

. 

Although the data that was explicitly associated with a session is the part

you care most about, there are some other pieces of information that are

sometimes useful as well. Here is a summary of the methods available in the

HttpSession

 class.

public Object getValue(String name)

public Object getAttribute(String name)

These methods extract a previously stored value from a session object. 

They return 

null

 if there is no value associated with the given name. Use 

getValue

 in version 2.1 of the servlet API. Version 2.2 supports both 

methods, but 

getAttribute

 is preferred and 

getValue

 is deprecated.

public void putValue(String name, Object value)

public void setAttribute(String name, Object value)

These methods associate a value with a name. Use 

putValue

 with 

version 2.1 servlets and either 

setAttribute

 (preferred) or 

putValue

(deprecated) with version 2.2 servlets. If the object supplied to 

putValue

 or 

setAttribute

 implements the 

HttpSessionBinding 

Listener

 interface, the object's 

valueBound

 method is called after it is 

stored in the session. Similarly, if the previous value implements 

HttpSessionBindingListener

, its 

valueUnbound

 method is called.

public void removeValue(String name)

public void removeAttribute(String name)

These methods remove any values associated with the designated name. 

If the value being removed implements 

HttpSessionBindingLis 

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