Java ThreadLocal の使い方とメモリリーク防止策

import java.text.SimpleDateFormat;
import java.util.Date;

public class ThreadLocalDemo {

    // withInitial でスレッドごとに独立した SimpleDateFormat を保持
    private static final ThreadLocal<SimpleDateFormat> dateFormat =
        ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));

    // リクエストコンテキストの模擬
    private static final ThreadLocal<Integer> userId = new ThreadLocal<>();

    public static void setUserId(int id) { userId.set(id); }
    public static Integer getUserId() { return userId.get(); }
    public static void clearUserId() { userId.remove(); }

    public static String formatDate(Date date) {
        return dateFormat.get().format(date);
    }

    public static void main(String[] args) throws InterruptedException {
        System.out.println("=== ThreadLocal でスレッド固有データを保持 ===");

        Runnable task = () -> {
            var threadId = (int) (Thread.currentThread().getId() % 1000);
            setUserId(threadId);
            try {
                Thread.sleep(50);
                System.out.println(Thread.currentThread().getName()
                    + " -> userId=" + getUserId());
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            } finally {
                clearUserId(); // 必ず remove()
            }
        };

        var t1 = new Thread(task, "thread-A");
        var t2 = new Thread(task, "thread-B");
        var t3 = new Thread(task, "thread-C");
        t1.start(); t2.start(); t3.start();
        t1.join(); t2.join(); t3.join();

        System.out.println("\n=== SimpleDateFormat の ThreadLocal 解決策 ===");
        Runnable formatTask = () -> {
            var result = formatDate(new Date());
            System.out.println(Thread.currentThread().getName()
                + " -> " + result);
        };
        var f1 = new Thread(formatTask, "format-1");
        var f2 = new Thread(formatTask, "format-2");
        f1.start(); f2.start();
        f1.join(); f2.join();
    }
}