日期工具：非线程安全SimpleDateFormat 和线程安全JodaTime（推荐）

1、SimpleDateFormat不是线程安全的对象

举个栗子：

@Slf4j
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 StringBuffer stringBuffer = new StringBuffer();

    public static void main(String[] args) throws InterruptedException {
        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 (InterruptedException e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

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

    }
    
}

输出：

at java.lang.Thread.run(Thread.java:748)

java.lang.NumberFormatException: multiple points

at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890)

at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)

at java.lang.Double.parseDouble(Double.java:538)

at java.text.DigitList.getDouble(DigitList.java:169)

at java.text.DecimalFormat.parse(DecimalFormat.java:2056)

at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1867)

at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)

at java.text.DateFormat.parse(DateFormat.java:364)

at com.example.concurrent.commonUnsafe.DateFormatExample1.update(DateFormatExample1.java:47)

at com.example.concurrent.commonUnsafe.DateFormatExample1.lambda$main$0(DateFormatExample1.java:33)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)

at java.lang.Thread.run(Thread.java:748)

2、改进上述SimpleDateFormat的用法（变为线程安全的）

改为局部变量（因为局部变量是堆栈封闭，线程安全的）每次声明一个新的变量来使用。

如下：

@Slf4j
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 StringBuffer stringBuffer = new StringBuffer();

    public static void main(String[] args) throws InterruptedException {
        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 (InterruptedException e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

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

    }
}

3、利用JodaTime（线程安全的） （ 推荐用这个，不关是因为线程安全的 ）

如下：

@Slf4j
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 InterruptedException {
        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 (InterruptedException e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
    }

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