Why instance variable in Servlet is not thread-safe
Solution 1
The mistake you are making is here:
So, because all these threads create a new class instance for ActionServlet, so I don't see any problem here. because instances of these thread is separate of each other.
The container does not create a new instance of the Servlet class for each request. It reuses an existing one. This is why they are not thread safe.
The Stripes Action Framework DOES create a new instance for each request, so that's an okay assumption under that framework. However, for example, Struts 1 follows the Servlet model and does not create a new action per request.
That does not mean that the container is limited to a single instance, it in theory can create more than one, but it's not a specified behavior so can not be relied upon. Most of the popular ones do not.
Solution 2
because all these threads create a new class instance (action.java), so I don't see any problem
You are supposing that every thread create a class instance that will be used only by that thread, that's why you don't have any problem.
But try to imagine, with your specific sample, that the same instance is accessed from two threads. What's happend if both use in the same time your request
and response
members?
Maybe you will read data from unidentifiable request, and you will write an inconsistent response that mix two parts.
So in you case too, instance variable are not thread safe, because if two thread access the same instance they can disturb each other.
hqt
Those who don't speak math are doomed to speak nonsense. My brief profile: https://github.com/hqt/profile
Updated on February 14, 2020Comments
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hqt over 4 years
When I read
Head First Servlet and JSP
, they say that instance variable is non-thread safe.I don't understand this statement so much. For example: I have a servlet which name is
ActionServlet.java
. Each time, each user's request is sent to server, container will create a new thread and create newActionServlet
instance.ActionServlet
may be has a structure:public class ActionServlet extends HttpServlet { // example of instance variable Instance variable; public void processRequest(HttpServletRequest request, HttpServletResponse response) { // process something relating to instance variable } }
So, because all these threads create a new class instance for
ActionServlet
, so I don't see any problem here. because instances of these thread is separate of each other.Please figure out where problem when use instance variable in multithread environment.
Thanks :)
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JB Nizet about 12 yearsIn fact, the container MUST create only one instance. Here's what the specification says: "For a servlet not hosted in a distributed environment (the default), the servlet container must use only one instance per servlet declaration"
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JB Nizet about 12 yearsThere is no instance pool for servlet. There is only one servlet instance per servlet declaration.
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hqt about 12 yearsOh. Thanks for both @JBNizet and Will Hartung. this book doesn't show me that container just ONLY create one instance for one servlet.