1、概述

应用场景：当要更新一条记录的时候，希望这条记录没有被别人更新

悲观锁和乐观锁区别：

• 悲观锁： 悲观的认为我要修改的数据一定会被他人修改，如果我要修改数据，我就会给数据上锁，别人都不能去修改，我修改完之后别人才可以修改
• 乐观锁：乐观的认为我修改的数据别人别人不会去改，但是在更新的时候会判断一下在此期间别人有没有去更新这个数据，可以使用版本号机制(version)和CAS算法实现

两种锁适用场景

乐观锁适用于写少读多的情况下（多读场景），即冲突真的很少发生的时候，这样可以省去了锁的开销，加大了系统的整个吞吐量

悲观锁适用于写多读少，多写，一般会经常产生冲突，这就会导致上层应用会不断的进行重试，这样反倒是降低了性能，所以一般多写的场景下用悲观锁就比较合适。

阿里巴巴建议以冲突概率20%这个数值作为分界线来决定使用乐观锁和悲观锁，
虽然说这个数值不是绝对的，但是作为阿里巴巴各个大佬总结出来的也是一个很好的参考。

2、实现（两步）

2.1 插件配置

@Bean
public OptimisticLockerInterceptor optimisticLockerInterceptor() {
    return new OptimisticLockerInterceptor();
}

2.2 注解实体字段 @Version 必须要!

实体类User添加 字段version
<font color='red'>同时数据库记得添加version字段</font>

public class User implements Serializable {

    private static final long serialVersionUID = 1L;

    /**
     * 主键ID
     */
    @TableId(value = "id", type = IdType.AUTO)
    private Integer id=null;

    /**
     * 姓名
     */
    private String name;

    /**
     * 年龄
     */
    private Integer age;

    /**
     * 邮箱
     */
    private String email;

    @Version
    private Integer version;


}

UserController
业务：id为n的一行数据的年龄age +1的操作

@RestController
@RequestMapping("/user")
public class UserController {
    @Autowired
    private IUserService service;
        @RequestMapping("/lock")
    public void lock(Integer id){
            User byId = service.getById(1);
            User user = new User();
            user.setId(1);
            user.setAge(byId.getAge()+1);
            user.setVersion(byId.getVersion());
            boolean b = service.updateById(user);
           if (b){
               System.out.println("-----------+++++++++++年龄增加成功++++++++++++++++++++++++++++--");
           }else{
               System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试");
           }
        }
}

测试类：
模拟开启50个线程模拟同时对id为n的数据操作

@Autowired
    private TestRestTemplate testRestTemplate;
    @Test
        //测试乐观锁
    void testLock() {

        String url = "http://localhost:8080/user/lock";
        for (int i = 0; i <50 ; i++) {
            new Thread(()->{

                MultiValueMap<String, String> params = new LinkedMultiValueMap<>();
                params.add("id","1");
                String result = testRestTemplate.postForObject(url, params, String.class);

            }).start();
        }
    }

测试结果：
数据库增加的年龄对应， 没有多+也没有少+

-----------+++++++++++年龄增加成功++++++++++++++++++++++++++++++++++++----------------------------------------
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
-----------+++++++++++年龄增加成功++++++++++++++++++++++++++++++++++++----------------------------------------
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
-----------+++++++++++年龄增加成功++++++++++++++++++++++++++++++++++++----------------------------------------
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
-----------+++++++++++年龄增加成功++++++++++++++++++++++++++++++++++++----------------------------------------
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$年龄增加失败，请重试

代码地址：https://github.com/jw-star/mybatis-plusDemo/tree/master/mybatisplusdemo
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