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1.? 引言

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讀寫分離要做的事情就是對于一條SQL該選擇哪個數據庫去執行，至于誰來做選擇數據庫這件事兒，無非兩個，要么中間件幫我們做，要么程序自己做。因此，一般來講，讀寫分離有兩種實現方式。

第一種是依靠中間件（比如：MyCat），也就是說應用程序連接到中間件，中間件幫我們做SQL分離；第二種是應用程序自己去做分離。這里我們選擇程序自己來做，主要是利用Spring提供的路由數據源，以及AOP。

然而，應用程序層面去做讀寫分離最大的弱點（不足之處）在于無法動態增加數據庫節點，因為數據源配置都是寫在配置中的，新增數據庫意味著新加一個數據源，必然改配置，并重啟應用。當然，好處就是相對簡單。

2.??AbstractRoutingDataSource

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基于特定的查找key路由到特定的數據源。它內部維護了一組目標數據源，并且做了路由key與目標數據源之間的映射，提供基于key查找數據源的方法。

3.? 實踐

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關于配置請參考《MySQL主從復制配置》

地址：www.cnblogs.com/cjsblog/p/9706370.html

3.1.? maven依賴

<?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><groupId>com.cjs.example</groupId><artifactId>cjs-datasource-demo</artifactId><version>0.0.1-SNAPSHOT</version><packaging>jar</packaging><name>cjs-datasource-demo</name><description></description><parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.0.5.RELEASE</version><relativePath/>?<!-- lookup parent from repository --></parent><properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding><java.version>1.8</java.version></properties><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-aop</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-jdbc</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>1.3.2</version></dependency><dependency><groupId>org.apache.commons</groupId><artifactId>commons-lang3</artifactId><version>3.8</version></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></dependencies><build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin><!--<plugin><groupId>org.mybatis.generator</groupId><artifactId>mybatis-generator-maven-plugin</artifactId><version>1.3.5</version><dependencies><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>5.1.46</version></dependency></dependencies><configuration><configurationFile>${basedir}/src/main/resources/myBatisGeneratorConfig.xml</configurationFile><overwrite>true</overwrite></configuration><executions><execution><id>Generate MyBatis Artifacts</id><goals><goal>generate</goal></goals></execution></executions></plugin>--></plugins></build> </project>

3.2.? 數據源配置

application.yml

spring:datasource:master:jdbc-url:?jdbc:mysql://192.168.102.31:3306/testusername: rootpassword:?123456driver-class-name: com.mysql.jdbc.Driverslave1:jdbc-url:?jdbc:mysql://192.168.102.56:3306/testusername: pig # 只讀賬戶password:?123456driver-class-name: com.mysql.jdbc.Driverslave2:jdbc-url:?jdbc:mysql://192.168.102.36:3306/testusername: pig # 只讀賬戶password:?123456driver-class-name: com.mysql.jdbc.Driver

多數據源配置

/*** 關于數據源配置，參考SpringBoot官方文檔第79章《Data Access》* 79. Data Access* 79.1 Configure a Custom DataSource* 79.2 Configure Two DataSources*/@Configuration public?class?DataSourceConfig {@Bean@ConfigurationProperties("spring.datasource.master")public?DataSource masterDataSource() {return?DataSourceBuilder.create().build();}@Bean@ConfigurationProperties("spring.datasource.slave1")public?DataSource slave1DataSource() {return?DataSourceBuilder.create().build();}@Bean@ConfigurationProperties("spring.datasource.slave2")public?DataSource slave2DataSource() {return?DataSourceBuilder.create().build();}@Beanpublic?DataSource myRoutingDataSource(@Qualifier("masterDataSource") DataSource masterDataSource,@Qualifier("slave1DataSource") DataSource slave1DataSource,@Qualifier("slave2DataSource") DataSource slave2DataSource) {Map<Object,?Object> targetDataSources =?new?HashMap<>();targetDataSources.put(DBTypeEnum.MASTER, masterDataSource);targetDataSources.put(DBTypeEnum.SLAVE1, slave1DataSource);targetDataSources.put(DBTypeEnum.SLAVE2, slave2DataSource);MyRoutingDataSource myRoutingDataSource =?new?MyRoutingDataSource();myRoutingDataSource.setDefaultTargetDataSource(masterDataSource);myRoutingDataSource.setTargetDataSources(targetDataSources);return?myRoutingDataSource;}}

這里，我們配置了4個數據源，1個master，2兩個slave，1個路由數據源。前3個數據源都是為了生成第4個數據源，而且后續我們只用這最后一個路由數據源。

MyBatis配置

@EnableTransactionManagement @Configuration public?class?MyBatisConfig?{@Resource(name =?"myRoutingDataSource")private?DataSource myRoutingDataSource;@Beanpublic?SqlSessionFactory?sqlSessionFactory()?throws?Exception?{SqlSessionFactoryBean sqlSessionFactoryBean =?new?SqlSessionFactoryBean();sqlSessionFactoryBean.setDataSource(myRoutingDataSource);sqlSessionFactoryBean.setMapperLocations(new?PathMatchingResourcePatternResolver().getResources("classpath:mapper/*.xml"));return?sqlSessionFactoryBean.getObject();}@Beanpublic?PlatformTransactionManager?platformTransactionManager()?{return?new?DataSourceTransactionManager(myRoutingDataSource);} }

由于 Spring 容器中現在有4個數據源，所以我們需要為事務管理器和MyBatis手動指定一個明確的數據源。

3.3.? 設置路由key / 查找數據源

目標數據源就是那前3個這個我們是知道的，但是使用的時候是如果查找數據源的呢？

首先，我們定義一個枚舉來代表這三個數據源

package?com.cjs.example.enums;public?enum?DBTypeEnum {MASTER, SLAVE1, SLAVE2;}

接下來，通過ThreadLocal將數據源設置到每個線程上下文中

public?class?DBContextHolder?{private?static?final ThreadLocal<DBTypeEnum> contextHolder =?new?ThreadLocal<>();private?static?final AtomicInteger counter =?new?AtomicInteger(-1);public?static?void?set(DBTypeEnum dbType)?{contextHolder.set(dbType);}public?static?DBTypeEnum?get()?{return?contextHolder.get();}public?static?void?master()?{set(DBTypeEnum.MASTER);System.out.println("切換到master");}public?static?void?slave()?{// 輪詢int?index = counter.getAndIncrement() %?2;if?(counter.get() >?9999) {counter.set(-1);}if?(index ==?0) {set(DBTypeEnum.SLAVE1);System.out.println("切換到slave1");}else?{set(DBTypeEnum.SLAVE2);System.out.println("切換到slave2");}}}

獲取路由key

package?com.cjs.example.bean;import?org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource; import?org.springframework.lang.Nullable;public?class?MyRoutingDataSource?extends?AbstractRoutingDataSource?{@Nullable@Overrideprotected?Object?determineCurrentLookupKey()?{return?DBContextHolder.get();}}

設置路由key

默認情況下，所有的查詢都走從庫，插入/修改/刪除走主庫。我們通過方法名來區分操作類型（CRUD）

@Aspect @Component public?class?DataSourceAop?{@Pointcut("!@annotation(com.cjs.example.annotation.Master) "?+"&& (execution(* com.cjs.example.service..*.select*(..)) "?+"|| execution(* com.cjs.example.service..*.get*(..)))")public?void?readPointcut()?{}@Pointcut("@annotation(com.cjs.example.annotation.Master) "?+"|| execution(* com.cjs.example.service..*.insert*(..)) "?+"|| execution(* com.cjs.example.service..*.add*(..)) "?+"|| execution(* com.cjs.example.service..*.update*(..)) "?+"|| execution(* com.cjs.example.service..*.edit*(..)) "?+"|| execution(* com.cjs.example.service..*.delete*(..)) "?+"|| execution(* com.cjs.example.service..*.remove*(..))")public?void?writePointcut()?{}@Before("readPointcut()")public?void?read()?{DBContextHolder.slave();}@Before("writePointcut()")public?void?write()?{DBContextHolder.master();}/*** 另一種寫法：if...else... 判斷哪些需要讀從數據庫，其余的走主數據庫*/ // @Before("execution(* com.cjs.example.service.impl.*.*(..))") // public void before(JoinPoint jp) { // String methodName = jp.getSignature().getName(); // // if (StringUtils.startsWithAny(methodName, "get", "select", "find")) { // DBContextHolder.slave(); // }else { // DBContextHolder.master(); // } // } }

有一般情況就有特殊情況，特殊情況是某些情況下我們需要強制讀主庫，針對這種情況，我們定義一個主鍵，用該注解標注的就讀主庫

package?com.cjs.example.annotation;public?@interface?Master { }

例如，假設我們有一張表member

@Service public?class?MemberServiceImpl?implements?MemberService?{@Autowiredprivate?MemberMapper memberMapper;@Transactional@Overridepublic?int?insert(Member member)?{return?memberMapper.insert(member);}@Master@Overridepublic?int?save(Member member)?{return?memberMapper.insert(member);}@Overridepublic?List<Member>?selectAll()?{return?memberMapper.selectByExample(new?MemberExample());}@Master@Overridepublic?String?getToken(String appId)?{// 有些讀操作必須讀主數據庫// 比如，獲取微信access_token，因為高峰時期主從同步可能延遲// 這種情況下就必須強制從主數據讀return?null;} }

4.? 測試

@RunWith(SpringRunner.class) @SpringBootTest public?class?CjsDatasourceDemoApplicationTests?{@Autowiredprivate?MemberService memberService;@Testpublic?void?testWrite()?{Member member =?new?Member();member.setName("zhangsan");memberService.insert(member);}@Testpublic?void?testRead()?{for?(int?i =?0; i <?4; i++) {memberService.selectAll();}}@Testpublic?void?testSave()?{Member member =?new?Member();member.setName("wangwu");memberService.save(member);}@Testpublic?void?testReadFromMaster()?{memberService.getToken("1234");}}

查看控制臺

5.? 工程結構

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6.? 參考

https://www.jianshu.com/p/f2f4256a2310 http://www.cnblogs.com/gl-developer/p/6170423.html https://www.cnblogs.com/huangjuncong/p/8576935.html https://blog.csdn.net/liu976180578/article/details/77684583推薦文章1、一款高顏值的 SpringBoot+JPA 博客項目2、超優 Vue+Element+Spring 中后端解決方案3、推薦幾個支付項目！4、推薦一個 Java 企業信息化系統5、一款基于 Spring Boot 的現代化社區（論壇/問答/社交網絡/博客）

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