十、不可变类

• 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();
        }

    }
}

• 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();
        }

    }
}

• 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