The short answer is:
👉 Yes, you absolutely should worry about thread safety when working with servlets.

Let’s break this down very clearly:

🧠 Why?

In a servlet container (like Tomcat, Jetty, etc.),
only one instance of your servlet class is created by default.
Many threads (one per incoming request) are sent to the same servlet instance to handle requests at the same time.
This means multiple threads can execute your doGet(), doPost(), etc. simultaneously!

⚡ If you have shared mutable state in your servlet (like instance variables),
you risk:

Race conditions
Inconsistent data
Deadlocks
Security issues

🎯 Very Important:

Servlet Object	Shared between Threads?	Thread Safe?
`Servlet instance`	✅ Yes (one instance for all threads)	❌ No (you must protect shared data)
`HttpServletRequest`	❌ No (each thread gets its own request)	✅ Yes (no need to protect)
`HttpServletResponse`	❌ No (each thread gets its own response)	✅ Yes (no need to protect)

✅ Request and Response objects are thread-safe (separate for each request).
❌ Servlet instance fields are shared (dangerous if modified).

🛠️ Bad Example (NOT thread-safe!)

public class UnsafeServlet extends HttpServlet {
    private int counter = 0; // SHARED mutable field!

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        counter++;  // NOT thread-safe!
        resp.getWriter().println("Counter: " + counter);
    }
}

Problem:

Two users might increase counter at the same time, causing wrong values (lost updates).

✅ Correct Approaches

1. Avoid using instance variables

Simply use only local variables inside doGet(), doPost():

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    int counter = 0; // local to this request
    counter++;
    resp.getWriter().println("Counter: " + counter);
}

2. Use thread-safe objects (if you must share state)

Use AtomicInteger, ConcurrentHashMap, etc.

private AtomicInteger counter = new AtomicInteger(0);

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    int currentCount = counter.incrementAndGet();
    resp.getWriter().println("Counter: " + currentCount);
}

✅ AtomicInteger safely increments across threads without race conditions.

3. Synchronize carefully (but try to avoid it)

You can synchronize manually:

private int counter = 0;

@Override
protected synchronized void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    counter++;
    resp.getWriter().println("Counter: " + counter);
}

⚡ But beware:

Synchronization can cause performance bottlenecks under heavy load.
Better to design without shared mutable state whenever possible.

🛡️ Practical Rule of Thumb:

Question	Best Practice
Do you need to store data per request?	✅ Use local variables.
Do you need to share data across users?	✅ Use thread-safe collections / atomic variables / application scope (carefully).
Do you need locks?	❌ Only if absolutely necessary, otherwise redesign.

🔥 Real-Life Examples of Servlet Thread Safety Issues:

Incorrect login counters
Random failures in shopping carts
Inconsistent session management
Mixed-up responses between users

Almost always caused by improperly handled shared variables inside servlets.

🛠️ Quick Recap

Aspect	Servlet Behavior
Servlet instance per app?	✅ Yes, only one
Multiple threads per servlet?	✅ Yes
Safe to use instance fields?	❌ No, unless they are thread-safe
Local variables inside `doGet()`?	✅ Yes, perfectly safe
Use `Atomic`, `Concurrent` types for shared data?	✅ Yes, best practice

Java.Servlet.Should I worry about multithreaded safety when working with servlets?