读写分离有必要吗？
实现读写分离势必要与你所做的项目相关，如果项目读多写少，那就可以设置读写分离，让“读”可以更快，因为你可以把你的“读”数据库的innodb设置为MyISAM引擎，让MySQL处理速度更快。
实现读写分离的步骤
监听MybatisPlus接口，判断是写入还是读取
在这里我使用的是AOP的方式，动态监听MybatisPlus中Mapper的方法。
import com.supostacks.wrdbrouter.DBContextHolder;
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 MyBatisPlusAop {@Pointcut("execution(* com.baomidou.mybatisplus.core.mapper.BaseMapper.select*(..))")public void readPointCut(){}@Pointcut("execution(* com.baomidou.mybatisplus.core.mapper.BaseMapper.insert*(..))" +"||execution(* com.baomidou.mybatisplus.core.mapper.BaseMapper.update*(..))" +"||execution(* com.baomidou.mybatisplus.core.mapper.BaseMapper.delete*(..))")public void writePointCut(){}@Before("readPointCut()")public void readBefore(){DBContextHolder.setDBKey("dataread");}@Before("writePointCut()")public void writeBefore(){DBContextHolder.setDBKey("datawrite");}
}
定义介绍：
DBContextHolder中使用了ThreadLocal存储数据库名
readPointCut定义读的切点，如果调用的是BaseMapper.select*(…)则判断是读数据，则调用读库。
writePointCut定义写的切点，如果调用的是BaseMapper.insert|update|delete*(…)则判断是写数据，则调用写库
自定义MyBatis的DataSourceAutoConfiguration
DataSourceAutoConfiguration是Mybatis官方使用的SpringBootStarter，因为我这边自定义了Mybatis连接的相关属性名用来切换数据源，所以我需要自构一个DataSourceAutoConfig，代码如下：
@Configuration
public class DataSourceAutoConfig implements EnvironmentAware {private final String TAG_GLOBAL = "global";/*** 数据源配置组*/private final Map<String, Map<String, Object>> dataSourceMap = new HashMap<>();/*** 默认数据源配置*/private Map<String, Object> defaultDataSourceConfig;public DataSource createDataSource(Map<String,Object> attributes){try {DataSourceProperties dataSourceProperties = new DataSourceProperties();dataSourceProperties.setUrl(attributes.get("url").toString());dataSourceProperties.setUsername(attributes.get("username").toString());dataSourceProperties.setPassword(attributes.get("password").toString());String driverClassName = attributes.get("driver-class-name") == null ? "com.zaxxer.hikari.HikariDataSource" : attributes.get("driver-class-name").toString();dataSourceProperties.setDriverClassName(driverClassName);String typeClassName = attributes.get("type-class-name") == null ? "com.zaxxer.hikari.HikariDataSource" : attributes.get("type-class-name").toString();return dataSourceProperties.initializeDataSourceBuilder().type((Class<DataSource>) Class.forName(typeClassName)).build();} catch (ClassNotFoundException e) {throw new RuntimeException(e);}}@Beanpublic DataSource createDataSource() {// 创建数据源Map<Object, Object> targetDataSources = new HashMap<>();for (String dbInfo : dataSourceMap.keySet()) {Map<String, Object> objMap = dataSourceMap.get(dbInfo);// 根据objMap创建DataSourceProperties,遍历objMap根据属性反射创建DataSourcePropertiesDataSource ds = createDataSource(objMap);targetDataSources.put(dbInfo, ds);}// 设置数据源DynamicDataSource dynamicDataSource = new DynamicDataSource();dynamicDataSource.setTargetDataSources(targetDataSources);// db0为默认数据源dynamicDataSource.setDefaultTargetDataSource(createDataSource(defaultDataSourceConfig));return dynamicDataSource;}@Overridepublic void setEnvironment(Environment environment) {String prefix = "wr-db-router.spring.datasource.";String datasource = environment.getProperty(prefix + "db");Map<String, Object> globalInfo = getGlobalProps(environment, prefix + TAG_GLOBAL);assert datasource != null;for(String db : datasource.split(",")){final String dbKey = prefix + db; //数据库列表Map<String,Object> datasourceProps = PropertyUtil.handle(environment,dbKey, Map.class);injectGlobals(datasourceProps, globalInfo);dataSourceMap.put(db,datasourceProps);}String defaultData = environment.getProperty(prefix + "default");defaultDataSourceConfig = PropertyUtil.handle(environment,prefix + defaultData, Map.class);injectGlobals(defaultDataSourceConfig, globalInfo);}public Map getGlobalProps(Environment env, String key){try {return PropertyUtil.handle(env,key, Map.class);} catch (Exception e) {return Collections.EMPTY_MAP;}}private void injectGlobals(Map<String,Object> origin,Map<String,Object> global){global.forEach((k,v)->{if(!origin.containsKey(k)){origin.put(k,v);}else{injectGlobals((Map<String, Object>) origin.get(k), (Map<String, Object>) global.get(k));}});}DynamicDataSource 这个类继承了AbstractRoutingDataSource，通过获取ThreadLocal中的数据库名，动态切换数据源。
public class DynamicDataSource extends AbstractRoutingDataSource {@Value("wr-db-router.spring.datasource.default")private String defaultDatasource;@Overrideprotected Object determineCurrentLookupKey() {if(null == DBContextHolder.getDBKey()){return defaultDatasource;}else{return DBContextHolder.getDBKey();}}
}
我们通过重写determineCurrentLookupKey方法并设置对应的数据库名称，我们就可以实现切换数据源的功能了。
AbstractRoutingDataSource 主要源码如下：
public Connection getConnection() throws SQLException {return this.determineTargetDataSource().getConnection();}...protected DataSource determineTargetDataSource() {Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");Object lookupKey = this.determineCurrentLookupKey();DataSource dataSource = (DataSource)this.resolvedDataSources.get(lookupKey);if (dataSource == null && (this.lenientFallback || lookupKey == null)) {dataSource = this.resolvedDefaultDataSource;}if (dataSource == null) {throw new IllegalStateException("Cannot determine target DataSource for lookup key [" + lookupKey + "]");} else {return dataSource;}}
自定义MyBatisPlus的SpringBoot自动配置
MybatisPlus是默认使用的Mybatis的自带的DataSourceAutoConfiguration，但是我们已经将这个自定义了，所以我们也要去自定义一个MyBatisPlusAutoConfig，如果不自定义的话，系统启动将报错。代码如下：
@Configuration(proxyBeanMethods = false
)
@ConditionalOnClass({SqlSessionFactory.class, SqlSessionFactoryBean.class})
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties({MybatisPlusProperties.class})
@AutoConfigureAfter({DataSourceAutoConfig.class, MybatisPlusLanguageDriverAutoConfiguration.class})
public class MyBatisPlusAutoConfig implements InitializingBean {
xxx
}
这个代码是直接拷贝了MyBatisPlusAutoConfiguration，只是将@AutoConfigureAfter({DataSourceAutoConfiguration.class, MybatisPlusLanguageDriverAutoConfiguration.class})改为了
@AutoConfigureAfter({DataSourceAutoConfig.class, MybatisPlusLanguageDriverAutoConfiguration.class})
这样启动就不会报错了。
其他步骤
上面这些开发完，就差不多可以实现数据库的动态切换从而实现读写分离了，不过其中有一个方法PropertyUtil，这是自定义的一个可以读取properties某个前缀下的所有属性的一个工具类。代码如下：
import org.springframework.core.env.Environment;
import org.springframework.core.env.PropertyResolver;import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;public class PropertyUtil {private static int springBootVersion = 2;public static <T> T handle(final Environment environment,final String prefix,final Class<T> clazz){switch (springBootVersion){case 1:return (T) v1(environment,prefix);case 2:return (T) v2(environment,prefix,clazz);default:throw new RuntimeException("Unsupported Spring Boot version");}}public static Object v1(final Environment environment,final String prefix){try {Class<?> resolverClass = Class.forName("org.springframework.boot.bind.RelaxedPropertyResolver");Constructor<?> resolverConstructor = resolverClass.getDeclaredConstructor(PropertyResolver.class);Method getSubPropertiesMethod = resolverClass.getDeclaredMethod("getSubProperties", String.class);Object resolverObject = resolverConstructor.newInstance(environment);String prefixParam = prefix.endsWith(".") ? prefix : prefix + ".";return getSubPropertiesMethod.invoke(resolverObject, prefixParam);} catch (final ClassNotFoundException | NoSuchMethodException | SecurityException | InstantiationException| IllegalAccessException | IllegalArgumentException | InvocationTargetException ex) {throw new RuntimeException(ex.getMessage(), ex);}}private static Object v2(final Environment environment, final String prefix, final Class<?> targetClass) {try {Class<?> binderClass = Class.forName("org.springframework.boot.context.properties.bind.Binder");Method getMethod = binderClass.getDeclaredMethod("get", Environment.class);Method bindMethod = binderClass.getDeclaredMethod("bind", String.class, Class.class);Object binderObject = getMethod.invoke(null, environment);String prefixParam = prefix.endsWith(".") ? prefix.substring(0, prefix.length() - 1) : prefix;Object bindResultObject = bindMethod.invoke(binderObject, prefixParam, targetClass);Method resultGetMethod = bindResultObject.getClass().getDeclaredMethod("get");return resultGetMethod.invoke(bindResultObject);}catch (final ClassNotFoundException | NoSuchMethodException | SecurityException | IllegalAccessException| IllegalArgumentException | InvocationTargetException ex) {throw new RuntimeException(ex.getMessage(), ex);}}
}我将路由切换的功能逻辑单独拉成了一个SpringBootStarter，目录如下：
顺便介绍一下如何将以个项目在SpringBootStarter中自动装配
1.在resources中创建文件夹META-INF
2.创建spring.factories文件
3.在该文件中设置你需要自动装配的类
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\com.xxx.wrdbrouter.config.DataSourceAutoConfig,\com.xxx.wrdbrouter.config.MyBatisPlusAutoConfig就先记录这些，目前正在进行主从库同步的相关内容。
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