simpleDateFormat是我們比較常用的日期轉換類，但是它是一個執行緒不安全的類。 舉例證明

 public class DateFormatExample1 {

    //請求總數
    private static int clientTotal = 1000;
    //同時允許執行的執行緒總數
    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();
    }
}
 

這段程式碼在執行的過程中，會報錯。

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)
 

定義一個靜態的SimpleDateFormat例項，是大家在日期工具類中比較通常的寫法，但是這種寫法在多執行緒的情況下就會丟擲異常。解決方案有很多種：

• 方法一：在每次使用的時候建立SimpleDateFormat的區域性變數，缺點：會建立大量的SimpleDateFormat例項，程式碼不夠簡潔優雅。
• 方法二：其實還有另外一種寫法：使用ThradLocal。 ThreadLocal是一個本地執行緒副本變數工具類。主要用於將私有執行緒和該執行緒存放的副本物件做一個對映，各個執行緒之間的變數互不干擾，在高併發場景下，可以實現無狀態的呼叫，適合於各個執行緒依賴不同的變數值完成操作的場景。

舉例：

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 safe
public class DateFormatExample3 {

    //請求總數
    private static int clientTotal = 1000;
    //同時允許執行的執行緒總數
    private static int threadTotal = 20;

    private static ThreadLocal<SimpleDateFormat> simpleDateFormat = new ThreadLocal<SimpleDateFormat>() {
        @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（推薦使用）

引入maven依賴

        <dependency>
            <groupId>joda-time</groupId>
            <artifactId>joda-time</artifactId>
            <version>2.9.9</version>
        </dependency>

import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;
public class DateFormatExample2 {

    //請求總數
    private static int clientTotal = 1000;
    //同時允許執行的執行緒總數
    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();
    }