JAVA – SimpleDateFormat 的线程不安全性-一一网

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线程不安全验证

SimpleDateFormat线程不安全验证实验:

package cn.xxxx.concurrentdate;

import java.text.SimpleDateFormat;
import java.util.Date;

public class ConcurrentDateTest {

	private static final SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd");

	private static String date1String = "2010-03-04";
	//private static String date2String  = "2013-04-05";

	public static void main(String[] args) {


		for (int i = 0; i < 10; i++) {
			new Thread(new Runnable() {

				@Override
				public void run() {
					try {
						Date date1 = simpleDateFormat.parse(date1String);
						//String date1S = simpleDateFormat.format(date1);
						System.out.println(Thread.currentThread().getName() + ":" +date1);
					} catch (Exception e) {
						System.out.println("Thread error");
						 //throw new RuntimeException("parse failed", e);
					}
				}
			}).start();
		}

	}

}
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输出结果：

Thread error
Thread error
Thread-1:Sat Mar 04 00:00:00 CST 2220
Thread-9:Thu Mar 04 00:00:00 CST 2010
Thread-6:Thu Mar 04 00:00:00 CST 2010
Thread-4:Thu Mar 04 00:00:00 CST 2010
Thread-3:Thu Mar 04 00:00:00 CST 2010
Thread-7:Thu Mar 04 00:00:00 CST 2010
Thread-8:Thu Mar 04 00:00:00 CST 2010
Thread-5:Thu Mar 04 00:00:00 CST 2010
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执行结果表明，日期要么是错的，要么发生了异常，SimpleDateFormat存在线程安全问题。

解决办法：

1. 加锁

线程内：synchronized(simpleDateFormat) { ..do parse and format. }:

这种方式会存在性能问题。比如Web程序，1000个用户同时访问一个涉及到时间的接口，不应该因SimpleDateFormat造成并发转串行的严重性能问题。

2. 定义线程局部变量

变量是同一个，但是每个线程使用同一个初始值，存取数据只在本线程内有效（内部通过一个Map(实际是内部类ThreadLocalMap)存取数据，存取数据只在同一线程有效）

private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>(){
		@Override
		protected DateFormat initialValue(){
			return new SimpleDateFormat("yyyy-MM-dd");
		}
	};
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Java 8+以上可以用更简便的lambda表达式：

public static final ThreadLocal<DateFormat> df = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));
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使用方法：

new Thread(new Runnable() {

				@Override
				public void run() {
					try {
						Date date1 = df.get().parse(date1String);
						System.out.println(Thread.currentThread().getName() + ":" +date1);

					} catch (Exception e) {
						System.out.println("Thread error");
					}
				}
			}).start();
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3. jdk8推荐方法：

使用 Instant 代替 Date， LocalDateTime 代替 Calendar，
DateTimeFormatter 代替 SimpleDateFormat，官方给出的解释： simple beautiful strong
immutable thread-safe。

变量转换方式如下所示：参考[1] [2]

  /*转Instant*/
    Instant dateInstant = date.toInstant();
    //Timestamp转Instant
    Instant timestampInstant = timestamp.toInstant();
    //Calendar转Instant
    Instant calendarInstant = calendar.toInstant();
    //LocalDateTime转Instant
    Instant localDateTimeInstant = localDateTime.atZone(ZoneId.systemDefault()).toInstant();
    //ZonedDateTime转Instant
    Instant zonedDateTimeInstant = zonedDateTime.toInstant();
    //LocalDate转Instant
    Instant localDateInstant = localDate.atStartOfDay(ZoneId.systemDefault()).toInstant();
    
    /*转LocalDateTime*/
    //Date转LocalDateTime
    LocalDateTime dateLocalDateTime = LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault());
    //Timestamp转LocalDateTime
    LocalDateTime timestampLocalDateTime = timestamp.toLocalDateTime();
    //Calendar转LocalDateTime
    LocalDateTime calendarLocalDateTime = LocalDateTime.ofInstant(calendar.toInstant(), ZoneOffset.systemDefault());
    //Instant转LocalDateTime
    LocalDateTime instantLocalDateTime = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());
    //ZonedDateTime转LocalDateTime
    LocalDateTime zonedDateTimeLocalDateTime = zonedDateTime.toLocalDateTime();
    //LocalDate转LocalDateTime
    LocalDateTime localDateLocalDateTime = localDate.atStartOfDay();
    
    /*转LocalDate*/
    
    // date转LocalDate
    LocalDate dateLocalDate = LocalDate.ofInstant(date.toInstant(), ZoneId.systemDefault());  //jdk11
    LocalDate dateLocalDate = LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault()).toLocalDate();
    //Timestamp转LocalDate
    LocalDate timestampLocalDate = timestamp.toLocalDateTime().toLocalDate();
    //Calendar转LocalDate
    LocalDate calendarLocalDate = LocalDate.ofInstant(calendar.toInstant(), ZoneOffset.systemDefault());  //jdk11
    LocalDate calendarLocalDate = LocalDateTime.ofInstant(calendar.toInstant(), ZoneOffset.systemDefault()).toLocalDate();
    //Instant转LocalDate
    LocalDate instantLocalDate = LocalDate.ofInstant(instant, ZoneId.systemDefault());  //jdk11
    LocalDate instantLocalDate = LocalDateTime.ofInstant(instant, ZoneId.systemDefault()).toLocalDate();
    //LocalDateTime转LocalDate
    LocalDate localDateTimeLocalDate = localDateTime.toLocalDate();
    //ZonedDateTime转LocalDate
    LocalDate zonedDateTimeLocalDate = zonedDateTime.toLocalDate();
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