MyBatis整合双数据源

有时候在项目中会遇到需要连接两个数据库的情况。本文就结合Spring和Mybatis来讲下怎么使用双数据源（或者是多数据源）。

背景知识介绍

本文中实现多数据源的关键是Spring提供的AbstractRoutingDataSource。这个类可以根据lookup key来实现底层数据源的动态转换。

public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {

	@Nullable
	private Map<Object, Object> targetDataSources;

	@Nullable
	private Object defaultTargetDataSource;

	private boolean lenientFallback = true;

	private DataSourceLookup dataSourceLookup = new JndiDataSourceLookup();

	@Nullable
	private Map<Object, DataSource> resolvedDataSources;

	@Nullable
	private DataSource resolvedDefaultDataSource;

	public void setTargetDataSources(Map<Object, Object> targetDataSources) {
		this.targetDataSources = targetDataSources;
	}

	public void setDefaultTargetDataSource(Object defaultTargetDataSource) {
		this.defaultTargetDataSource = defaultTargetDataSource;
	}

	public void setLenientFallback(boolean lenientFallback) {
		this.lenientFallback = lenientFallback;
	}

	public void setDataSourceLookup(@Nullable DataSourceLookup dataSourceLookup) {
		this.dataSourceLookup = (dataSourceLookup != null ? dataSourceLookup : new JndiDataSourceLookup());
	}


	@Override
	public void afterPropertiesSet() {
		if (this.targetDataSources == null) {
			throw new IllegalArgumentException("Property 'targetDataSources' is required");
		}
		this.resolvedDataSources = new HashMap<>(this.targetDataSources.size());
		this.targetDataSources.forEach((key, value) -> {
			Object lookupKey = resolveSpecifiedLookupKey(key);
			DataSource dataSource = resolveSpecifiedDataSource(value);
			this.resolvedDataSources.put(lookupKey, dataSource);
		});
		if (this.defaultTargetDataSource != null) {
			this.resolvedDefaultDataSource = resolveSpecifiedDataSource(this.defaultTargetDataSource);
		}
	}

	protected Object resolveSpecifiedLookupKey(Object lookupKey) {
		return lookupKey;
	}

	protected DataSource resolveSpecifiedDataSource(Object dataSource) throws IllegalArgumentException {
		if (dataSource instanceof DataSource) {
			return (DataSource) dataSource;
		}
		else if (dataSource instanceof String) {
			return this.dataSourceLookup.getDataSource((String) dataSource);
		}
		else {
			throw new IllegalArgumentException(
					"Illegal data source value - only [javax.sql.DataSource] and String supported: " + dataSource);
		}
	}


	@Override
	public Connection getConnection() throws SQLException {
		return determineTargetDataSource().getConnection();
	}

	@Override
	public Connection getConnection(String username, String password) throws SQLException {
		return determineTargetDataSource().getConnection(username, password);
	}

	@Override
	@SuppressWarnings("unchecked")
	public <T> T unwrap(Class<T> iface) throws SQLException {
		if (iface.isInstance(this)) {
			return (T) this;
		}
		return determineTargetDataSource().unwrap(iface);
	}

	@Override
	public boolean isWrapperFor(Class<?> iface) throws SQLException {
		return (iface.isInstance(this) || determineTargetDataSource().isWrapperFor(iface));
	}
    
	protected DataSource determineTargetDataSource() {
		Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");
		Object lookupKey = determineCurrentLookupKey();
		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 + "]");
		}
		return dataSource;
	}

	@Nullable
    //一般只需要用户实现这个方法。
	protected abstract Object determineCurrentLookupKey();

}

实现流程

step1：实现一个自定义的AbstractRoutingDataSource

public class DynamicDataSource extends AbstractRoutingDataSource {

    //这边定义了一个和线程绑定的ThreadLocal变量，用于存放需要使用的数据源的名称
    private static final ThreadLocal<String> dataSourceNameHolder = new ThreadLocal<>();

    public DynamicDataSource(DataSource defaultTargetDataSource, Map<Object, Object> targetDataSources) {
        super.setDefaultTargetDataSource(defaultTargetDataSource);
        super.setTargetDataSources(targetDataSources);
        super.afterPropertiesSet();
    }

    @Override
    //重写了AbstractRoutingDataSource的determineCurrentLookupKey方法
    protected Object determineCurrentLookupKey() {
        return getDataSource();
    }

    public static void setDataSource(String dataSource) {
        dataSourceNameHolder.set(dataSource);
    }

    public static String getDataSource() {
        return dataSourceNameHolder.get();
    }

    public static void clearDataSource() {
        dataSourceNameHolder.remove();
    }

}

step2：实现一个AOP对Service层方法进行AOP拦截，调用DynamicDataSource中的ThreadLocal变量，将当前请求需要使用的数据源名称设置进去。

//定义一个DataSource注解
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface DataSource {
    String name() default "";
}
//这边再定义一个常量
public interface DataSourceNames {
    String FIRST = "first";
    String SECOND = "second";

}

定义AOP处理DataSource注解

@Aspect
@Component
public class DataSourceAspect implements Ordered {
    protected Logger logger = LoggerFactory.getLogger(getClass());

    @Pointcut("@annotation(com.xx.yy.annotation.DataSource)")
    public void dataSourcePointCut() {

    }

    @Around("dataSourcePointCut()")
    public Object around(ProceedingJoinPoint point) throws Throwable {
        MethodSignature signature = (MethodSignature) point.getSignature();
        Method method = signature.getMethod();
        
        DataSource ds = method.getAnnotation(DataSource.class);
        //如果未指定数据源就使用第一个数据源
        if(ds == null){
            DynamicDataSource.setDataSource(DataSourceNames.FIRST);
            logger.debug("set datasource is " + DataSourceNames.FIRST);
        }else {
            DynamicDataSource.setDataSource(ds.name());
            logger.debug("set datasource is " + ds.name());
        }
        try {
            return point.proceed();
        } finally {
            DynamicDataSource.clearDataSource();
            logger.debug("clean datasource");
        }
    }
    @Override
    public int getOrder() {
        return 1;
    }
}

step3：对数据源进行配置

@Configuration
public class DynamicDataSourceConfig {

    @Bean
    @ConfigurationProperties("spring.datasource.druid.first")
    public DataSource firstDataSource(){
        return DruidDataSourceBuilder.create().build();
    }

    @Bean
    @ConfigurationProperties("spring.datasource.druid.second")
    public DataSource secondDataSource(){
        DataSource dataSource = DruidDataSourceBuilder.create().build();
        return dataSource;
    }

    @Bean
    @Primary
    @DependsOn(value = {"firstDataSource","secondDataSource"})
    public DynamicDataSource dataSource(DataSource firstDataSource,DataSource secondDataSource) {
        Map<Object, Object> targetDataSources = new HashMap<>();
        targetDataSources.put(DataSourceNames.FIRST, firstDataSource);
        targetDataSources.put(DataSourceNames.SECOND,  secondDataSource);
        return new DynamicDataSource(firstDataSource, targetDataSources);
    }
}

以上就是实现双数据源的全部配置。

使用

使用的时候非常简单，只需要在Service层的方法上加上@DataSource注解就可以了。

@DataSource(name = DataSourceNames.SECOND)
public String selectByInfoName(String name){
   //...
}

一些注意点

如果你使用了pageHelper等分页插件，请将方言设置成自动模式， autoRuntimeDialect: true

pagehelper:
  reasonable: false
  supportMethodsArguments: true
  params: count=countSql
  autoRuntimeDialect: true

如果你使用了Druid数据源，并通过下面的形式创建数据源，要保障数据源的用户名和密码字段不为null。不然DruidDataSourceWrapper这个Bean会检测这个字段的值，导致启动失败。

@Bean
    @ConfigurationProperties("spring.datasource.druid.first")
    public DataSource firstDataSource(){
        return DruidDataSourceBuilder.create().build();
    }

    @Bean
    @ConfigurationProperties("spring.datasource.druid.second")
    public DataSource secondDataSource(){
        DataSource dataSource = DruidDataSourceBuilder.create().build();
        return dataSource;
    }