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I need to know why the servlets are not thread safe ? And whats the reason that in Struts 2.0 framework controller servlet is thread safe ?
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A Java servlet container / web server is typically multithreaded. That means, that multiple requests to the same servlet may be executed at the same time. Therefore, you need to take concurrency into consideration when you implement your servlet.
We can deduce from the Java Servlet Specification, that sharing attributes between servlets via ServletContext#setAttribute and ServletContext#getAttribute is indeed thread-safe.
So in you case too, instance variable are not thread safe, because if two thread access the same instance they can disturb each other.
It is because, Servlets create only one instance and multiple threads access it. So in that case Instance Variables are not thread safe.
I need to know why the servlets are not thread safe ?
Servlet instances are inherently not thread safe because of the multi threaded nature of the Java programming language in general. The Java Virtual Machine supports executing the same code by multiple threads. This is a great performance benefit on machines which have multiple processors. This also allows the same code to be executed by multiple concurrent users without blocking each other.
Imagine a server with 4 processors wherein a normal servlet can handle 1000 requests per second. If that servlet were threadsafe, then the web application would act like as if it runs on a server with 1 processor wherein the servlet can handle only 250 requests per second (okay, it's not exactly like that, but you got the idea).
If you encounter threadsafety issues when using servlets, then it is your fault, not Java's nor Servlet's fault. You'd need to fix the servlet code as such that request or session scoped data is never assigned as an instance variable of the servlet. For an in-depth explanation, see also How do servlets work? Instantiation, sessions, shared variables and multithreading.
And whats the reason that in Struts 2.0 framework controller servlet is thread safe ?
It is not thread safe. You're confusing the Struts dispatcher servlet filter with Struts actions. The struts actions are re-created on every single request. So every single request has its own instance of the request scoped Struts action. The Struts dispatcher servlet filter does not store them as its own instance variable. Instead, it stores it as an attribute of the HttpServletRequest.
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