??點擊上方?好好學java?，選擇?星標?公眾號

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1. ?引言

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

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

MySQL主從復制配置(https://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

package com.cjs.example.config; import com.cjs.example.bean.MyRoutingDataSource; import com.cjs.example.enums.DBTypeEnum; import org.springframework.beans.factory.annotation.Qualifier; import org.springframework.boot.context.properties.ConfigurationProperties; import org.springframework.boot.jdbc.DataSourceBuilder; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import javax.sql.DataSource; import java.util.HashMap; import java.util.Map; /** * 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();}@Bean public 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

package com.cjs.example.config; import org.apache.ibatis.session.SqlSessionFactory; import org.mybatis.spring.SqlSessionFactoryBean; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.core.io.support.PathMatchingResourcePatternResolver; import org.springframework.jdbc.datasource.DataSourceTransactionManager; import org.springframework.transaction.PlatformTransactionManager; import org.springframework.transaction.annotation.EnableTransactionManagement; import javax.annotation.Resource; import javax.sql.DataSource;@EnableTransactionManagement @Configuration public class MyBatisConfig {@Resource(name = "myRoutingDataSource") private DataSource myRoutingDataSource;@Bean public SqlSessionFactory sqlSessionFactory() throws Exception {SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();sqlSessionFactoryBean.setDataSource(myRoutingDataSource);sqlSessionFactoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath:mapper/*.xml")); return sqlSessionFactoryBean.getObject();}@Bean public 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

package com.cjs.example.bean; import com.cjs.example.enums.DBTypeEnum; import java.util.concurrent.atomic.AtomicInteger; 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@Override protected 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）

package com.cjs.example.aop; import com.cjs.example.bean.DBContextHolder; import org.apache.commons.lang3.StringUtils; import org.aspectj.lang.JoinPoint; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Before; import org.aspectj.lang.annotation.Pointcut; import org.springframework.stereotype.Component;@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

package com.cjs.example.service.impl; import com.cjs.example.annotation.Master; import com.cjs.example.entity.Member; import com.cjs.example.entity.MemberExample; import com.cjs.example.mapper.MemberMapper; import com.cjs.example.service.MemberService; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Service; import org.springframework.transaction.annotation.Transactional; import java.util.List;@Service public class MemberServiceImpl implements MemberService {@Autowired private MemberMapper memberMapper;@Transactional@Override public int insert(Member member) { return memberMapper.insert(member);}@Master@Override public int save(Member member) { return memberMapper.insert(member);}@Override public List<Member> selectAll() { return memberMapper.selectByExample(new MemberExample());}@Master@Override public 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

package com.cjs.example; import com.cjs.example.entity.Member; import com.cjs.example.service.MemberService; 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.test.context.junit4.SpringRunner;@RunWith(SpringRunner.class) @SpringBootTest public class CjsDatasourceDemoApplicationTests {@Autowired private MemberService memberService;@Test public void testWrite() {Member member = new Member();member.setName("zhangsan");memberService.insert(member);}@Test public void testRead() { for (int i = 0; i < 4; i++) {memberService.selectAll();}}@Test public void testSave() {Member member = new Member();member.setName("wangwu");memberService.save(member);}@Test public void testReadFromMaster() {memberService.getToken("1234");}}

查看控制臺

5. ?工程結構

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

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