ThreadLocal in Java

ThreadLocal in Java

ThreadLocal in Java is a special class that provides thread-local variables. Each thread that accesses a ThreadLocal variable has its own, independently initialized copy of the variable. This is extremely useful when you want to avoid sharing data between threads, such as in cases of per-thread counters, user sessions, or database connections.

Here’s a detailed breakdown:

1. Key Characteristics

  • Each thread sees its own version of the variable.
  • Variables stored in ThreadLocal are not shared between threads.
  • Helps avoid synchronization when threads need their own copy of data.
  • Often used in frameworks, like Spring, for things like request/session scoped data.

2. Basic Usage

public class ThreadLocalExample {
    // Create a ThreadLocal variable
    private static ThreadLocal<Integer> threadLocalCount = ThreadLocal.withInitial(() -> 0);

    public static void main(String[] args) {
        Runnable task = () -> {
            int count = threadLocalCount.get(); // get current thread's value
            count += 1;
            threadLocalCount.set(count); // set updated value
            System.out.println(Thread.currentThread().getName() + ": " + threadLocalCount.get());
        };

        Thread t1 = new Thread(task, "Thread-1");
        Thread t2 = new Thread(task, "Thread-2");

        t1.start();
        t2.start();
    }
}

Output (example):

Thread-1: 1
Thread-2: 1

Notice that each thread has its own copy of the variable, even though they are using the same ThreadLocal object.

3. Methods in ThreadLocal

  • T get(): Returns the value in the current thread.
  • void set(T value): Sets the value for the current thread.
  • void remove(): Removes the current thread’s value (important to avoid memory leaks, especially in thread pools).

4. ThreadLocal with Initial Value

You can initialize a ThreadLocal using:

ThreadLocal<Integer> threadLocalVar = ThreadLocal.withInitial(() -> 100);

5. Use Cases

  • Per-thread counters.
  • Database connections per thread.
  • User session or request context in web applications.
  • Avoiding synchronization for thread-specific data.

⚠️ Important Notes

  1. Always call remove() when done with thread pools to prevent memory leaks, because threads can be reused.
  2. ThreadLocal does not make objects thread-safe; it isolates variables per thread.

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