simpleDateFormat是我们比较常用的日期转换类，但是它是一个线程不安全的类。 举例证明

public class DateFormatExample1 {    //请求总数    private static int clientTotal = 1000;    //同时允许执行的线程总数    private static int threadTotal = 20;    private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");    public static void main(String[] args) throws InterruptedException {        final Semaphore semaphore = new Semaphore(threadTotal);        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);        ExecutorService executorService = Executors.newCachedThreadPool();        for (int i = 0; i < clientTotal; i++) {            executorService.execute(new Runnable() {                @Override                public void run() {                    try {                        semaphore.acquire();                        try {                            simpleDateFormat.parse("2018-09-26");                        } catch (ParseException e) {                            e.printStackTrace();                        }                        semaphore.release();                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                    countDownLatch.countDown();                }            });        }        countDownLatch.await();        executorService.shutdown();    }}

这段代码在运行的过程中，会报错。

Exception in thread "pool-1-thread-2" Exception in thread "pool-1-thread-1" Exception in thread "pool-1-thread-8" Exception in thread "pool-1-thread-9" java.lang.NumberFormatException: For input string: "E.199E1"	at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)	at sun.misc.FloatingDecimal.parseDouble(Unknown Source)	at java.lang.Double.parseDouble(Unknown Source)	at java.text.DigitList.getDouble(Unknown Source)	at java.text.DecimalFormat.parse(Unknown Source)	at java.text.SimpleDateFormat.subParse(Unknown Source)	at java.text.SimpleDateFormat.parse(Unknown Source)	at java.text.DateFormat.parse(Unknown Source)	at com.guoy.concurrency.commonUnsafe.DateFormatExample1$1.run(DateFormatExample1.java:31)	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)	at java.lang.Thread.run(Unknown Source)java.lang.NumberFormatException: multiple points	at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)	at sun.misc.FloatingDecimal.parseDouble(Unknown Source)	at java.lang.Double.parseDouble(Unknown Source)

定义一个静态的SimpleDateFormat实例，是大家在日期工具类中比较通常的写法，但是这种写法在多线程的情况下就会抛出异常。解决方案有很多种：

• 方法一：在每次使用的时候创建SimpleDateFormat的局部变量，缺点：会创建大量的SimpleDateFormat实例，代码不够简洁优雅。
• 方法二：其实还有另外一种写法：使用ThradLocal。 ThreadLocal是一个本地线程副本变量工具类。主要用于将私有线程和该线程存放的副本对象做一个映射，各个线程之间的变量互不干扰，在高并发场景下，可以实现无状态的调用，适合于各个线程依赖不同的变量值完成操作的场景。

举例：

package com.guoy.concurrency.commonUnsafe;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Semaphore;//thread not safepublic class DateFormatExample3 {    //请求总数    private static int clientTotal = 1000;    //同时允许执行的线程总数    private static int threadTotal = 20;    private static ThreadLocal
    
      simpleDateFormat = new ThreadLocal
     
      () {        @Override        protected SimpleDateFormat initialValue() {            return new SimpleDateFormat("yyyy-MM-dd");        }    };    public static void main(String[] args) throws InterruptedException {        final Semaphore semaphore = new Semaphore(threadTotal);        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);        ExecutorService executorService = Executors.newCachedThreadPool();        for (int i = 0; i < clientTotal; i++) {            executorService.execute(new Runnable() {                @Override                public void run() {                    try {                        semaphore.acquire();                        try {                            simpleDateFormat.get().parse("2018-09-26");                        } catch (ParseException e) {                            e.printStackTrace();                        }                        semaphore.release();                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                    countDownLatch.countDown();                }            });        }        countDownLatch.await();        executorService.shutdown();    }}
     
    

• 方法三：使用joda-time（推荐使用）

引入maven依赖

                
     
      joda-time
                 
     
      joda-time
                 
     
      2.9.9
             
    

import org.joda.time.DateTime;import org.joda.time.format.DateTimeFormat;import org.joda.time.format.DateTimeFormatter;public class DateFormatExample2 {    //请求总数    private static int clientTotal = 1000;    //同时允许执行的线程总数    private static int threadTotal = 20;    private static DateTimeFormatter dateTimeFormatter = DateTimeFormat.forPattern("yyyy-MM-dd");    public static void main(String[] args) throws InterruptedException {        final Semaphore semaphore = new Semaphore(threadTotal);        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);        ExecutorService executorService = Executors.newCachedThreadPool();        for (int i = 0; i < clientTotal; i++) {            executorService.execute(new Runnable() {                @Override                public void run() {                    try {                        semaphore.acquire();                        DateTime.parse("2018-12-04", dateTimeFormatter).toDate();                        semaphore.release();                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                    countDownLatch.countDown();                }            });        }        countDownLatch.await();        executorService.shutdown();    }