Java : ThreadLocal with Examples

class SharedTask implements Runnable {
    private final ThreadLocal<Integer> local = ThreadLocal.withInitial(() -> 0);

    @Override
    public void run() {
        try {
            final var count = local.get() + 1;
            final var id = Thread.currentThread().threadId();

            System.out.printf("thread id = %d : count = %d%n", id, count);

            TimeUnit.MILLISECONDS.sleep(100);
            local.set(count);

        } catch (InterruptedException e) {
            System.out.println("InterruptedException!");
        }
    }
}

try (final var executor = Executors.newFixedThreadPool(3)) {

    final var sharedTask = new SharedTask();
    for (int i = 0; i < 9; i++) {
        executor.submit(sharedTask);
    }
}

// Result
// ↓
//thread id = 32 : count = 1
//thread id = 33 : count = 1
//thread id = 34 : count = 1
//thread id = 32 : count = 2
//thread id = 33 : count = 2
//thread id = 34 : count = 2
//thread id = 32 : count = 3
//thread id = 33 : count = 3
//thread id = 34 : count = 3

ThreadLocal ()

Creates a thread local variable.

final var local = new ThreadLocal<String>();
System.out.println(local.get()); // null

local.set("abc");
System.out.println(local.get()); // abc

final var local = new ThreadLocal<Integer>();
System.out.println(local.get()); // null

local.set(123);
System.out.println(local.get()); // 123

T get ()

Returns the value in the current thread's copy of this thread-local variable.

class SharedTask implements Runnable {
    private final ThreadLocal<Integer> local = ThreadLocal.withInitial(() -> 0);

    @Override
    public void run() {
        try {
            final var count = local.get() + 1;
            final var id = Thread.currentThread().threadId();

            System.out.printf("thread id = %d : count = %d%n", id, count);

            TimeUnit.MILLISECONDS.sleep(100);
            local.set(count);

        } catch (InterruptedException e) {
            System.out.println("InterruptedException!");
        }
    }
}

try (final var executor = Executors.newFixedThreadPool(3)) {

    final var sharedTask = new SharedTask();
    for (int i = 0; i < 9; i++) {
        executor.submit(sharedTask);
    }
}

// Result
// ↓
//thread id = 32 : count = 1
//thread id = 33 : count = 1
//thread id = 34 : count = 1
//thread id = 32 : count = 2
//thread id = 33 : count = 2
//thread id = 34 : count = 2
//thread id = 32 : count = 3
//thread id = 33 : count = 3
//thread id = 34 : count = 3

protected T initialValue ()

Returns the current thread's "initial value" for this thread-local variable.

Please see also : withInitial(Supplier<? extends S> supplier)

final var local = new ThreadLocal<String>() {
    @Override
    protected String initialValue() {
        return "abc";
    }
};
System.out.println(local.get()); // abc

local.set("XYZ");
System.out.println(local.get()); // XYZ

void remove ()

Removes the current thread's value for this thread-local variable.

final var local = ThreadLocal.withInitial(() -> "abc");
System.out.println(local.get()); // abc

local.set("XYZ");
System.out.println(local.get()); // XYZ

local.remove();
System.out.println(local.get()); // abc

final var local = new ThreadLocal<Integer>();
System.out.println(local.get()); // null

local.set(123);
System.out.println(local.get()); // 123

local.remove();
System.out.println(local.get()); // null

void set (T value)

Sets the current thread's copy of this thread-local variable to the specified value.

Please see get().

static <S> ThreadLocal<S> withInitial (Supplier<? extends S> supplier)

Creates a thread local variable.

final var local = ThreadLocal.withInitial(() -> "abc");
System.out.println(local.get()); // abc

local.set("XYZ");
System.out.println(local.get()); // XYZ