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Tip of the Day
Language: Java
Expertise: Advanced
Mar 22, 2005

Storing Request-specific Data Without Using ServletRequest

There are times when you need to store request-specific attributes (pieces of data that must be available to one specific http-request during the lifetime of that http-request) in order to retrieve them later.

This is easy—if you have access to the ServletRequest instance. You simply use its methods, getAttribute() and setAttribute(), for attribute storage.

But sometimes your code does not provide access to the current request's ServletRequest instance. One way around this is to modify the parameter-list of methods to pass the ServletRequest instance all the way down to the method that needs the request-specific attributes. This would use the ServletRequest's getAttribute() method. However, this refactoring is not always possible.

Another solution is to use the thread's local memory, because each ServletRequest is executed in one thread only (it does not switch threads).

First, define a ThreadLocal class and a class holding your data:


class RequestCache extends ThreadLocal
{
    protected Object initialValue()
    {
        return new RequestCacheEntry();
    }
}

class RequestCacheEntry
{
    HttpServletReqeust mRequest;
    // ... or any other member you'd like to store in this class.
}
Then, modify your servlet to set the request-specific data and a public method to retrieve it:

public class MyServlet extends HttpServlet 
{
    ...
    private static final RequestCache smReqCache = new RequestCache();

    public static RequestCacheEntry getCurrentRequestData()
    {
        return (RequestCacheEntry)smReqCache.get();
    }
    ...
}
Next, add these pieces of code in your servlet's doGet and/or doPost methods:

public void doGet(HttpServletRequest pRequest, HttpServletResponse pResponse)
{
    RequestCacheEntry reqCacheEntry = (RequestCacheEntry)smReqCache.get();
    reqCacheEntry.mRequest = pRequest;
    
    ..
    .. do all your servlet stuff :)

    reqCacheEntry.mRequest = null;
}
You can call MyServlet.getCurrentRequestData() to obtain your request-specific data anywhere in your servlet-code where you don't have access to the current HttpServletRequest instance.

Anton Spaans
 
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