Servlet中声明的方法是线程安全的还是Servlet中的局部变量是线程安全的

Question

I have got to know that there is race around condition in my servlets as I have some globally declared variables which are used in it . So my question is if I declare those variables in some method apart from doGet or doPost method. Will they be not shared among different threads ?

If yes , how we can avoid race around condition . I want to avoid the use of ThreadLocal and usage of singelThreadModel .

Here is the example

public class UserServlet extends HttpServlet {


    private String userString1 = "";
    private String userString2 = "";


    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        someTask(request, response);
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        someTask(request, response);
    }

    public void someTask(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {
        userString1 = request.getParameter("someParameter");
        userString2 = request.getParameter("someParameter");

        }
}

NOTE : This is just an example , not the actual code .

Answer 1

Are methods declared in servlets are thread-safe

No.

or the local variables in servlets are thread safe

No.

I have got to know that there is race around condition in my servlets

There is a race condition.

as I have some globally declared variables which are used in it.

You have some instance variables.

So my question is if I declare those variables in some method apart from doGet() or doPost() method, will they be not shared among different threads?

Local variables are local to the method they are declared in.

If yes, how we can avoid race around condition.

I can't follow your double negative, but it's still a race condition. Don't misuse standard terminology. You need to avoid instance members of a Servlet unless they are common to and shared by all possible concurrent invocations of its methods. Per-request variables must be method-local and passed around between methods as parameters or return values.

I want to avoid the use of ThreadLocal and usage of SingleThreadModel .

You certainly do. There are no specified thread semantics in a Servlet that would make ThreadLocal work, and SingleThreadModel is deprecated and will be removed.

Answer 2

A Servlet is a Java class so its already thread - safe if it has no state ( ie if it doesn't have any state variables ) . No extra action is needed from your side.

Now , you introduce state variables like - userString1 & userString2 and your servlet can still be thread - safe since String is immutable but you are reassigning its reference for each request so it does not remain thread safe since assignments by one thread might change to some other value by some other thread ( by the time , thread one tries to use it ) . You can have an immutable as state & remain thread - safe , provided you don't reassign its reference the way you doing in someTask method.

So use those string variables provided its purpose is to pass some constant string to all request threads.

Also, in your code, you are putting lots of Strings on heap by that assignment expression in someTask method.

At last, you have to utilize java synchronization constructs to make your class thread safe if you have to use mutable state objects in those methods but again plain Java synchronization constructs are per JVM so you will face issues if deployment has to be distributed on a cluster. Some distributed locking mechanism will be needed in that case.

Answer 3

If you are declaring a global variable and assigning the values in doGet or doPost method, they will definitely be overriden by different threads calling the servlet. You should not assign values to global variables in servlet method calls.

In your code what is stopping you from declaring the userString1 & userString2 inside method? Why do you want to declare them globally? If you want these variables to be used in different methods of the servlet, then simply pass them as method arguments instead of declaring them as global variables.

eg

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String userString1 = request.getParameter("someParameter");
    String userString2 = request.getParameter("someParameter");
    someTask(request, response, userString1, userString2);
}

public void someTask(HttpServletRequest request, HttpServletResponse response, String userString1, String userString2) throws ServletException, IOException {
    //do whatever you want with these variables.
}

OR

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    someTask(request, response);
}

public void someTask(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String userString1 = request.getParameter("someParameter");
    String userString2 = request.getParameter("someParameter");

    //do whatever you want with these variables.
}