一、日期类改进

1. SimpleDateFormat

• 线程不安全

package com.dreamer.multithread.day07;import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;public class Demo05 {public static void main(String[] args) {SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");for (int i = 0; i < 10; i++) {new Thread(() -> {try {Date parse = sdf.parse("2021-09-17");System.out.println(parse);} catch (ParseException e) {e.printStackTrace();}}).start();}}
}

• 加锁解决

package com.dreamer.multithread.day07;import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;public class Demo05 {public static void main(String[] args) {SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");for (int i = 0; i < 10; i++) {new Thread(() -> {synchronized (sdf) {try {Date parse = sdf.parse("2021-09-17");System.out.println(parse);} catch (ParseException e) {e.printStackTrace();}}}).start();}}
}

2. SimpleDateFormat

• JDK8之后提供了线程安全的类
• DateTimeFormatter

package com.dreamer.multithread.day07;import java.time.format.DateTimeFormatter;
import java.time.temporal.TemporalAccessor;public class Demo05 {public static void main(String[] args) {DateTimeFormatter dfm = DateTimeFormatter.ofPattern("yyyy-MM-dd");for (int i = 0; i < 10; i++) {new Thread(() -> {TemporalAccessor parse = dfm.parse("2021-09-17");System.out.println(parse);}).start();}}
}

二、不可变类

• 不可变类是线程安全的
• 类中所有成员变量都是final修饰，保证不可变，保证只能读不能写
• 类是final修饰，不能被重写