java 多线程：线程安全问题，示例DateFormat多线程执行冲突解决方案ThreadLocal、方法内变量

SimpleDateFormat多线程中执行报错

• java.lang.NumberFormatException: For input string: ""  

  import java.text.DateFormat;
  import java.text.ParseException;
  import java.text.SimpleDateFormat;
  import java.util.*;
  
  /**
   * @ClassName VarNotSafeDateFormat
   * @projectName: object1
   * @author: Zhangmingda
   * @description： XXX
   * date: 2021/4/26.
   */
  public class VarNotSafeDateFormat {
      private static final DateFormat DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd"); //使用共享的共享对象可能报错
      private static class DateFormatTransfer extends Thread{
          private String dateStr;
  
          public DateFormatTransfer(String dateStr) {
              this.dateStr = dateStr;
          }
  
          @Override
          public void run() {
  //            DateFormat DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd");  方法内变量安全，每个线程执行方法的时候独立创建实例
              try {
                  Date date = DATE_FORMAT.parse(dateStr);
                  String stringDateAfterTrans = DATE_FORMAT.format(date);
                  System.out.println(dateStr + "转换后：" + stringDateAfterTrans);
              } catch (ParseException e) {
                  e.printStackTrace();
              }
          }
      }
  
      public static void main(String[] args) {
          List<String> dates = Arrays.asList("2021-04-21","2021-04-22","2021-04-23","2021-04-24","2021-04-25",
                  "2021-04-26","2021-04-27","2021-04-28","2021-04-29","2021-04-30");
          Set<Thread> threads = new HashSet<>();
          dates.forEach(dateStr ->{
  //            new DateFormatTransfer(dateStr).start();
              threads.add(new DateFormatTransfer(dateStr));
          });
          threads.forEach(thread -> thread.start());
      }
  }

  

ThreadLocal 变量 解决方案：

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.*;

/**
 * @ClassName VarNotSafeDateFormat
 * @projectName: object1
 * @author: Zhangmingda
 * @description： XXX
 * date: 2021/4/26.
 */
public class VarNotSafeDateFormat {

    /**
     * ThreadLocal 线程安全变量，和方法变量异曲同工之妙，实际都创建了不同的子实例用
     */
    private static final ThreadLocal<DateFormat> DATE_FORMAT_THREAD_LOCAL = new ThreadLocal<>(){
        @Override
        protected DateFormat initialValue() {
            return new SimpleDateFormat("yyyy-MM-dd");
        }
    };

    private static class DateFormatTransfer extends Thread{
        private String dateStr;

        public DateFormatTransfer(String dateStr) {
            this.dateStr = dateStr;
        }

        @Override
        public void run() {
//            DateFormat DATE_FORMAT = new SimpleDateFormat("yyyy-MM-dd");  方法内变量安全，每个线程执行方法的时候独立创建实例
            try {
                Date date = DATE_FORMAT_THREAD_LOCAL.get().parse(dateStr);
                String stringDateAfterTrans = DATE_FORMAT_THREAD_LOCAL.get().format(date);
                System.out.println(dateStr + "转换后：" + stringDateAfterTrans);
            } catch (ParseException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        List<String> dates = Arrays.asList("2021-04-21","2021-04-22","2021-04-23","2021-04-24","2021-04-25",
                "2021-04-26","2021-04-27","2021-04-28","2021-04-29","2021-04-30");
        Set<Thread> threads = new HashSet<>();
        dates.forEach(dateStr ->{
            threads.add(new DateFormatTransfer(dateStr));
        });
        threads.forEach(thread -> thread.start());
    }
}