java中SimpleDateFormat 的多线程安全问题

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.CountDownLatch;

public class SimpleDateFormatThreadSafetyDemo {

    private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

    public static void main(String[] args) throws InterruptedException {
        int threadCount = 10;
        CountDownLatch latch = new CountDownLatch(threadCount);
        String dateString = "2023-10-25 15:30:00";

        for (int i = 0; i < threadCount; i++) {
            new Thread(() -> {
                try {
                    // 所有线程都尝试解析同一个字符串
                    Date date = sdf.parse(dateString);
                    // 打印解析结果和当前线程，如果线程不安全，结果会五花八门
                    System.out.println(Thread.currentThread().getName() + " - Parsed date: " + date);
                } catch (ParseException e) {
                    System.out.println(Thread.currentThread().getName() + " - Parse failed: " + e.getMessage());
                } catch (NumberFormatException e) {
                    // 多线程下可能抛出的其他异常
                    System.out.println(Thread.currentThread().getName() + " - NumberFormatException: " + e.getMessage());
                } finally {
                    latch.countDown();
                }
            }).start();
        }

        latch.await(); // 等待所有线程结束
        System.out.println("All threads finished.");
    }
}

Thread-2 - Parsed date: Wed Oct 25 15:30:00 CST 2023 // 正确
Thread-4 - Parsed date: Mon Nov 30 15:30:00 CST 2026 // 完全错误！
Thread-0 - Parse failed: For input string: ""
Thread-1 - NumberFormatException: multiple points
Thread-3 - Parsed date: Wed Oct 25 15:30:00 CST 2023 // 正确
...

public Date parseDate(String dateString) throws ParseException {
    // 每次调用都创建一个新的 SimpleDateFormat，线程私有，绝对安全
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    return sdf.parse(dateString);
}

public class DateUtils {
    private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

    public static Date parse(String dateString) throws ParseException {
        synchronized (sdf) { // 使用类对象或sdf对象作为锁
            return sdf.parse(dateString);
        }
    }

    public static String format(Date date) {
        synchronized (sdf) {
            return sdf.format(date);
        }
    }
}

public class ThreadSafeDateFormatter {

    private static final ThreadLocal<SimpleDateFormat> threadLocalDateFormat =
            ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));

    public static Date parse(String dateString) throws ParseException {
        // get() 方法会返回当前线程独有的 SimpleDateFormat 实例
        return threadLocalDateFormat.get().parse(dateString);
    }

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

    // 重要！如果使用线程池，在线程任务结束时最好清理 ThreadLocal，防止内存泄漏
    public static void remove() {
        threadLocalDateFormat.remove();
    }
}

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class Java8DateUtils {
    // DateTimeFormatter 是线程安全的，可以放心定义为常量
    private static final DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

    public static LocalDateTime parse(String dateString) {
        return LocalDateTime.parse(dateString, formatter); // 线程安全
    }

    public static String format(LocalDateTime dateTime) {
        return formatter.format(dateTime); // 线程安全
    }
}