simpleDateFormat线程不安全

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 safe
public 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> 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（推荐使用）

引入maven依赖

        <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 {

    //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();
    }

来源：oschina

链接：https://my.oschina.net/u/3316877/blog/2967466