Must Spring component classes be thread-safe

If you use Spring, must your component classes (@Controller, @Service, @Repository) be thread safe? Or does Spring use them in a thread-safe manner so you don't have to worry about thread safety?

That is, if I have a @RequestMapping method in my @Controller, could that method be called simultaneously for the same controller object by more than one thread?

(This has sort-of been asked before, but not answered as such).

Given

@Controller
public class MyController {
    @RequestMapping(value = "/index")
    public String respond() {
        return "index";
    }
}

Spring will create an instance of MyController. This is because Spring parses your configuration, <mvc:annotation-driven>, sees @Controller (which is like @Component) and instantiates the annotated class. Because it sees @RequestMapping as well, it generates a HandlerMapping for it, see the docs here.

Any HTTP requests the DispatcherServlet receives will be dispatched to this controller instance through the HandlerMapping registered before, calling respond() through java reflection on that instance.

If you have instance fields like

@Controller
public class MyController {
    private int count = 0;
    @RequestMapping(value = "/index")
    public String respond() {
        count++;
        return "index";
    }
}

count would be a hazard, because it might be modified by many threads and changes to it might be lost.

You need to understand how Servlet containers work. The container instantiates one instance of your Spring MVC DispatcherServlet. The container also manages a pool of Threads which it uses to respond to connections, ie. HTTP requests. When such a request arrives, the container picks a Thread from the pool and, within that Thread, executes the service() method on the DispatcherServlet which dispatches to the correct @Controller instance that Spring registered for you (from your configuration).

So YES, Spring MVC classes must be thread safe. You can do this by playing with different scopes for your class instance fields or just having local variables instead. Failing that, you'll need to add appropriate synchronization around critical sections in your code.

Must Spring component classes be thread-safe

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