作者:黄青石 来源:cnblogs.com/huangqingshi/p/10165409.html
在写入数据库的时候需要有锁,比如同时写入数据库的时候会出现丢数据,那么就需要锁机制。
它们使用的场景如下:
他们实现的方式上有所不同。
乐观锁采用版本号的方式,即当前版本号如果对应上了就可以写入数据,如果判断当前版本号不一致,那么就不会更新成功,比如
update table set column = value
where version=${version} and otherKey = ${otherKey}
悲观锁实现的机制一般是在执行更新语句的时候采用for update方式,比如
update table set column='value' for update
这种情况where条件呢一定要涉及到数据库对应的索引字段,这样才会是行级锁,否则会是表锁,这样执行速度会变慢。
下面我就弄一个spring boot(springboot 2.1.1 + mysql + lombok + aop + jpa)工程,然后逐渐的实现乐观锁和悲观锁。并发控制--悲观锁和乐观锁详解。
假设有一个场景,有一个catalog商品目录表,然后还有一个browse浏览表,假如一个商品被浏览了,那么就需要记录下浏览的user是谁,并且记录访问的总数。
表的结构非常简单:
create table catalog (id int(11) unsigned NOT NULL AUTO_INCREMENT COMMENT '主键',name varchar(50) NOT NULL DEFAULT '' COMMENT '商品名称',browse_count int(11) NOT NULL DEFAULT 0 COMMENT '浏览数',version int(11) NOT NULL DEFAULT 0 COMMENT '乐观锁,版本号',PRIMARY KEY(id)) ENGINE=INNODB DEFAULT CHARSET=utf8;
CREATE table browse (id int(11) unsigned NOT NULL AUTO_INCREMENT COMMENT '主键',cata_id int(11) NOT NULL COMMENT '商品ID',user varchar(50) NOT NULL DEFAULT '' COMMENT '',create_time timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '创建时间',PRIMARY KEY(id)) ENGINE=INNODB DEFAULT CHARSET=utf8;
POM.XML的依赖如下:
<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>2.1.1.RELEASE</version> <relativePath/> <!-- lookup parent from repository --> </parent> <groupId>com.hqs</groupId> <artifactId>dblock</artifactId> <version>1.0-SNAPSHOT</version> <name>dblock</name> <description>Demo project for Spring Boot</description>
<properties> <java.version>1.8</java.version> </properties>
<dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency>
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-devtools</artifactId> <scope>runtime</scope> </dependency> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <scope>runtime</scope> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-data-jpa</artifactId> </dependency> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> </dependency> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <optional>true</optional> </dependency>
<!-- aop --> <dependency> <groupId>org.aspectj</groupId> <artifactId>aspectjweaver</artifactId> <version>1.8.4</version> </dependency>
</dependencies>
<build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> </plugin> </plugins> </build>
</project>
介绍一下项目的结构的内容:
咱们看一下核心代码的实现,参考如下,使用dataJpa非常方便,集成了CrudRepository就可以实现简单的CRUD,非常方便,有兴趣的同学可以自行研究。
public interface CatalogRepository extends CrudRepository<Catalog, Long> {
@Query(value = "select * from Catalog a where a.id = :id for update", nativeQuery = true) Optional<Catalog> findCatalogsForUpdate(@Param("id") Long id);
@Lock(value = LockModeType.PESSIMISTIC_WRITE) //代表行级锁 @Query("select a from Catalog a where a.id = :id") Optional<Catalog> findCatalogWithPessimisticLock(@Param("id") Long id);
@Modifying(clearAutomatically = true) //修改时需要带上 @Query(value = "update Catalog set browse_count = :browseCount, version = version + 1 where id = :id " + "and version = :version", nativeQuery = true) int updateCatalogWithVersion(@Param("id") Long id, @Param("browseCount") Long browseCount, @Param("version") Long version);
}
还有我写的测试类,方便大家进行测试:
package com.hqs.dblock;
import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.boot.test.context.SpringBootTest;import org.springframework.boot.test.web.client.TestRestTemplate;import org.springframework.test.context.junit4.SpringRunner;import org.springframework.util.LinkedMultiValueMap;import org.springframework.util.MultiValueMap;
@RunWith(SpringRunner.class)@SpringBootTest(classes = DblockApplication.class, webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)public class DblockApplicationTests {
@Autowired private TestRestTemplate testRestTemplate;
@Test public void browseCatalogTest() { String url = "http://localhost:8888/catalog"; for(int i = 0; i < 100; i++) { final int num = i; new Thread(() -> { MultiValueMap<String, String> params = new LinkedMultiValueMap<>(); params.add("catalogId", "1"); params.add("user", "user" + num); String result = testRestTemplate.postForObject(url, params, String.class); System.out.println("-------------" + result); } ).start(); } }
@Test public void browseCatalogTestRetry() { String url = "http://localhost:8888/catalogRetry"; for(int i = 0; i < 100; i++) { final int num = i; new Thread(() -> { MultiValueMap<String, String> params = new LinkedMultiValueMap<>(); params.add("catalogId", "1"); params.add("user", "user" + num); String result = testRestTemplate.postForObject(url, params, String.class); System.out.println("-------------" + result); } ).start(); } }}
调用100次,即一个商品可以浏览一百次,采用悲观锁,catalog表的数据都是100,并且browse表也是100条记录。采用乐观锁的时候,因为版本号的匹配关系,那么会有一些记录丢失,但是这两个表的数据是可以对应上的。
乐观锁失败后会抛出ObjectOptimisticLockingFailureException,那么我们就针对这块考虑一下重试,下面我就自定义了一个注解,用于做切面。
package com.hqs.dblock.annotation;
import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;
@Target(ElementType.METHOD)@Retention(RetentionPolicy.RUNTIME)public @interface RetryOnFailure {}
针对注解进行切面,见如下代码。我设置了最大重试次数5,然后超过5次后就不再重试。
package com.hqs.dblock.aspect;
import lombok.extern.slf4j.Slf4j;import org.aspectj.lang.ProceedingJoinPoint;import org.aspectj.lang.annotation.Around;import org.aspectj.lang.annotation.Aspect;import org.aspectj.lang.annotation.Pointcut;import org.hibernate.StaleObjectStateException;import org.springframework.orm.ObjectOptimisticLockingFailureException;import org.springframework.stereotype.Component;
@Slf4j@Aspect@Componentpublic class RetryAspect { public static final int MAX_RETRY_TIMES = 5;//max retry times
@Pointcut("@annotation(com.hqs.dblock.annotation.RetryOnFailure)") //self-defined pointcount for RetryOnFailure public void retryOnFailure(){}
@Around("retryOnFailure()") //around can be execute before and after the point public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { int attempts = 0;
do { attempts++; try { pjp.proceed(); } catch (Exception e) { if(e instanceof ObjectOptimisticLockingFailureException || e instanceof StaleObjectStateException) { log.info("retrying....times:{}", attempts); if(attempts > MAX_RETRY_TIMES) { log.info("retry excceed the max times.."); throw e; } }
} } while (attempts < MAX_RETRY_TIMES); return null; }}
大致思路是这样了。
关注Java技术栈微信公众号,在后台回复关键字:分布式,可以获取更多栈长整理的分布式技术干货。