SpringBoot 2.0 教程實戰 MySQL 讀寫分離

1. 引言

讀寫分離要做的事情就是對於一條SQL該選擇哪個數據庫去執行，至於誰來做選擇數據庫這件事兒，無非兩個，要麼中間件幫我們做，要麼程序自己做。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。這裡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）在於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，並重啟應用。當然，好處就是相對簡單。

2. AbstractRoutingDataSource

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

3. 實踐

MySQL主從複製配置(https://www.cnblogs.com/cjsblog/p/9706370.html)》

3.1. maven依賴

<code>
<project>
    <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>

    <parent>
        <groupid>org.springframework.boot/<groupid>
        <artifactid>spring-boot-starter-parent/<artifactid>
        <version>2.0.5.RELEASE/<version>
        <relativepath> 
    /<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>

            

        /<plugins>
    /<build>
/<project>/<code>

3.2. 數據源配置

application.yml

<code>spring:
  datasource:
    master:
      jdbc-url: jdbc:mysql://192.168.102.31:3306/test
      username: root
      password: 123456
      driver-class-name: com.mysql.jdbc.Driver
    slave1:
      jdbc-url: jdbc:mysql://192.168.102.56:3306/test
      username: pig   # 只讀賬戶
      password: 123456
      driver-class-name: com.mysql.jdbc.Driver
    slave2:
      jdbc-url: jdbc:mysql://192.168.102.36:3306/test
      username: pig   # 只讀賬戶
      password: 123456
      driver-class-name: com.mysql.jdbc.Driver/<code>

多數據源配置

<code>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; /** * 關於數據源配置，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> 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;
    }

}/<object>/<code>

這裡，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，而且後續我們只用這最後一個路由數據源。

MyBatis配置

<code>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);
    } 

}/<code>

由於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

<code>package com.cjs.example.enums; public enum DBTypeEnum {

    MASTER, SLAVE1, SLAVE2;

}/<code>

接下來，通過ThreadLocal將數據源設置到每個線程上下文中

<code>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");
        }
    }

}/<dbtypeenum>/<code> 

獲取路由key

<code>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();
    }

}/<code>

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）

<code>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，其餘的走主數據庫 */
// @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(); // } // }
}/<code>

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，用該註解標註的就讀主庫

<code> package com.cjs.example.annotation; 
public @interface Master {
}/<code>

例如，假設我們有一張表member

<code>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;
    }
}/<member>/<code>

4. 測試

<code>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");
    }

}/<code>

查看控制檯

5. 工程結構

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關鍵字: SQL 數據庫 XML