将对象添加到会话线程安全

Question

Is the adding object to session threadsafe?

in my jsp file, i am checking if session doesn't contain my object then i fetch if from the DB and put it in session.

Is this operation thread safe(in case multiple requests get to same line of code at the same time)?

Answer 1

The access to HttpSession attributes is not thread safe. Multiple servlets executing request threads may have active access to the same session object at the same time.

The Developer has the responsibility for threadsafe access to the attribute objects themselves. You can synchronize on the session object to get/set attributes to it:

HttpSession session=request.getSession();
synchronized(session)
{
//get/set attributes to session object
} 

Answer 2

Even though there is a single session (for a user or connectivity) based on the sessionId, any number of concurrent requests can be initiated from that user within a given session. For example, a user may be loading this same JSP file that you have written code to add an object from DB to session.

Following is an example code to denote what you might be writing.

object = session.getAttribute("key");
if (object == null) {
   object = getObjectFromDB();
   session.setAttribute("key", object);
}

Since thread switching can happen at any line of the above code segment, there is a possibility that multiple threads noticing the object as null and trying to put the object into session. Since each invocation of session.setAttribute(key,value) with the same key overrides the existing value, there will not be an error thrown.

However you will have to consider your business requirement and see whether invoking getObjectFromDB() multiple times unnecessarily is causing issues to the business expectations.