SimpleDateFormat高并发下的进化之路

SimpleDateFormat 类不都是线程安全的，在多线程环境下调用format()和parse()方法都会引发多线程安全问题。究其原因，是因为每个线程都更改了Calendar值

@Slf4j
@NotThreadSafe
public class DateFormatExample1 {

    private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update() {
        try {
            simpleDateFormat.parse("20180208");
        } catch (Exception e) {
            log.error("parse exception", e);
        }
    }
}

高并发下会报错

采用堆栈封闭方式安全使用SimpleDateFormat

@Slf4j
@ThreadSafe
public class DateFormatExample2 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update() {
        try {
            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
            simpleDateFormat.parse("20180208");
        } catch (Exception e) {
            log.error("parse exception", e);
        }
    }
}

使用优秀的第三方库，例如Joda-Time

Joda-Time内部是通过ConcurrentHashMap实现来线程安全的，与我们自己手写ThreadLocal来保证线程安全类似。

@Slf4j
@ThreadSafe
public class DateFormatExample3 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyyMMdd");

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal; i++) {
            final int count = i;
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update(count);
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update(int i) {
        log.info("{}, {}", i, DateTime.parse("20180208", dateTimeFormatter).toDate());
    }
}

引入Maven依赖

<!-- https://mvnrepository.com/artifact/joda-time/joda-time -->
<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.10.10</version>
</dependency>

使用ThreadLocal变量

@Slf4j
@ThreadSafe
public class DateFormatExample4 {
    public static ThreadLocal<SimpleDateFormat> dateFormatThreadLocal = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyyMMdd"));
    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    // private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyyMMdd");

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal; i++) {

            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    update();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

    private static void update() {
        try {
            SimpleDateFormat simpleDateFormat = dateFormatThreadLocal.get();
            simpleDateFormat.parse("20180208");
        } catch (Exception e) {
            log.error("parse exception", e);
        }
    }
}

推荐使用优秀的第三方库，例如Joda-Time；或者使用DateTimeFormatter代替SimpleDateFormat；来避免SimpleDateFormat在多线程环境下的时间异常；