用ThreadLocal解决SimpleDateFormat线程不安全问题

import java.text.*;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class DataUtils {

    private DataUtils() {

    }

    /**
     * key为pattern value为存有ThreadLocal
     */
    private static Map<String, ThreadLocal<SimpleDateFormat>> sdfMap = new HashMap<>();

    /**
     * 初始化sdfMap
     *
     * @param pattern
     * @return
     */
    private static ThreadLocal<SimpleDateFormat> initSdfMap(String pattern) {
        //如果sdfMap有pattern的value就直接返回，如果没有就创建一个，然后返回
        ThreadLocal<SimpleDateFormat> t1 = sdfMap.computeIfAbsent(pattern, p ->
                //用ThreadLocal作为容器，当每个线程访问这个 threadLocal 变量时，ThreadLocal会为每个线程提供一份变量，各个线程互不影响。
                ThreadLocal.withInitial(() -> new SimpleDateFormat(pattern))
        );

        return t1;
    }

    public static String getDataStr(Date date, String pattern) {
        SimpleDateFormat simpleDateFormat = initSdfMap(pattern).get();
        return simpleDateFormat.format(date);
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(4);


        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/01/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/02/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/03/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/04/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/05/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/06/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/07/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/08/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/09/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/10/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/11/17"), "yyyy-MM-dd")));
        executorService.execute(() -> System.out.println(getDataStr(new Date("2021/12/17"), "yyyy-MM-dd")));
        executorService.shutdown();
    }


}

线程不安全的本质，多个线程操作共享变量