Spring的多数据源支持—-AbstractRoutingDataSource，AbstractRoutingDataSource定义了抽象的determineCurrentLookupKey方法，子类实现此方法，来确定要使用的数据源

public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {
     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;
    }
        // 确定当前要使用的数据源
        protected abstract Object determineCurrentLookupKey();
}

Druid 实现多数据源支持，核心是Overwrite AbstractRoutingDataSource 的 determineCurrentLookupKey 方法

  public class MultipleDataSource extends AbstractRoutingDataSource {
    public static final String DATA_SOURCE_A = "dataSourceTestA";  // A库
    public static final String DATA_SOURCE_B = "dataSourceTestB";  // B库
    private static final ThreadLocal<String> dataSourceKey = new ThreadLocal<String>();
    public static void setDataSourceKey(String dataSource) {
        dataSourceKey.set(dataSource);
    }
    @Override
    protected Object determineCurrentLookupKey() {
        return dataSourceKey.get();
    }
    public static void clear(){
        dataSourceKey.remove();
    }
}