古之学者必有师。师者，所以传道受业解惑也。
一个简单的需求，先判断表中是否存在对应的记录。不存在则插入，存在则忽略。
如果使用先select查询是否存在，然后使用insert语句插入数据这种方式。在事务并发执行下会出现插入多条的事务并发问题。这个问题就像之前遇到的更新丢失的问题一样。关于更新丢失可以看看这篇文章 https://www.jianshu.com/p/bfd7c684412d
原理如下： 事务并发执行。A事务和B事务开始时都看到的是数据是不存在的
image.png
接下来我们做个验证
准备数据环境
CREATE TABLE `jdbcstudy`.`test` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`a` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
`b` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
`c` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
`d` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 1 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;
写个service接口
package com.springboot.study.demo1.service;
/**
*@program: springboot_study
*@description:
*@author: yinkai
*@create: 2020-03-05 09:45
*/
public interface TestService {
void insertOrIgnore(String a,String b,String c,String d) ;
}
service实现类
- 使用 @Transactional注解表示本方法需在一个事务中执行，隔离级别是RR，传播属性是 REQUIRED
- insertOrIgnore方法先判断符合a,b,c,d字段的记录是不是存在，若不存在则插入，存在则忽略
- 为了测试结果更明显。在判断之后，插入之前添加了睡眠3秒
package com.springboot.study.demo1.service.impl;
import com.springboot.study.demo1.service.TestService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Isolation;
import org.springframework.transaction.annotation.Propagation;
import org.springframework.transaction.annotation.Transactional;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;
/**
*@program: springboot_study
*@description:
*@author: yinkai
*@create: 2020-03-05 09:46
*/
@Service
public class TestServiceImpl implements TestService {
@Autowired
JdbcTemplate jdbcTemplate;
@Transactional(rollbackFor = Exception.class,isolation = Isolation.REPEATABLE_READ,propagation = Propagation.REQUIRED)
@Override
public void insertOrIgnore(String a,String b,String c,String d) {
List<Map<String, Object>> maps = jdbcTemplate.queryForList("select count(1) isOne from test where a = ? and b = ? and c = ? and d = ? for update;",a,b,c,d);
Long isOne = (Long) maps.get(0).get("isOne");
//查询后睡三秒
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
//返回记录为0，插入
if(0 == isOne){
jdbcTemplate.update("INSERT INTO jdbcstudy.test(a, b, c, d) VALUES (?,?, ?, ?);",a,b,c,d);
}
}
}
写个测试类
- 测试类里开了三个线程去执行testService.insertOrIgnore()方法
package com.springboot.study.demo1;
import com.springboot.study.demo1.service.TestService;
import lombok.SneakyThrows;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.annotation.Rollback;
import org.springframework.test.context.junit4.SpringRunner;
/**
*@program: springboot_study
*@author: yinkai
*@create: 2020-02-29 14:23
*/
@RunWith(SpringRunner.class)
@SpringBootTest
public class Test {
@Autowired
TestService testService;
@Rollback(value = false)
@org.junit.Test
public void test() throws InterruptedException {
System.out.println("123");
Thread thread = null;
//开3个线程对应三个事务，并发插入。看看结果如何
for (int i = 1; i <= 3; i++) {
thread = new Thread(new Runnable() {
@Override
public void run() {
testService.insertOrIgnore("a1", "b1", "c1", "d1");
}
}, Integer.toString(i));
thread.start();
}
thread.join();
}
}
来看看执行结果，果然表中被插入了多条数据
image.png
既然问题出现了，那总得解决呀？
解决方法1，使用悲观锁、排他锁、X锁
将insert代码修改如下，后面加个 for update
List<Map<String, Object>> maps = jdbcTemplate.queryForList("select count(1) isOne from test where a = ? and b = ? and c = ? and d = ? for update;",a,b,c,d);
然后需要添加一个联合索引 index_a_b_c_d(a,b,c,d)。因为mysql的innodb引擎的行锁实现是依赖索引的，如果不加索引则会锁住全表。若插入a=a2,b=b2,c=c2,d=d2 也会被阻塞，这个问题等下再讨论~
image.png
完了再次执行程序，报出异常如下
Exception in thread "3" Exception in thread "1" org.springframework.dao.DeadlockLoserDataAccessException: PreparedStatementCallback; SQL [INSERT INTO jdbcstudy.test(a, b, c, d) VALUES (?,?, ?, ?);]; Deadlock found when trying to get lock; try restarting transaction; nested exception is
按照翻译理解就是事务发生死锁了,这个提示未免有些笼统。其实这里根本不是死锁问题呀，只是锁等到超时而已。因为只有一个事务持锁
不过程序的执行结果是ok的
image.png
解决方法二，使用mysql自带关键字解决
使用 IGNORE 关键字可以完美解决这个问题；注意也需要添加联合索引，且索引的类型必须是UNIQUE的，这是IGNORE 的特性
image.png
TestServiceImpl 代码可以改成这样，在insert语句上加上IGNORE 关键字
package com.springboot.study.demo1.service.impl;
import com.springboot.study.demo1.service.TestService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Isolation;
import org.springframework.transaction.annotation.Propagation;
import org.springframework.transaction.annotation.Transactional;
import java.util.concurrent.TimeUnit;
/**
* @program: springboot_study
* @description:
* @author: yinkai
* @create: 2020-03-05 09:46
*/
@Service
public class TestServiceImpl implements TestService {
@Autowired
JdbcTemplate jdbcTemplate;
@Transactional(rollbackFor = Exception.class, isolation = Isolation.REPEATABLE_READ, propagation = Propagation.REQUIRED)
@Override
public void insertOrIgnore(String a, String b, String c, String d) {
//查询后睡三秒
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
jdbcTemplate.update("INSERT IGNORE INTO jdbcstudy.test(a, b, c, d) VALUES (?,?, ?, ?);", a, b, c, d);
}
}
执行后查看表数据，完全ok
image.png
