本文基於Spring 4.1.6 RELEASE原始碼 進行分析。
先看看之前咱們在spring-mybatis.xml檔案中配置的動態資料來源，如下：

<bean id="dataSource" class="com.ricky.codelab.spring.ds.DynamicRoutingDataSource">
    <property name="targetDataSources">
        <map key-type="com.ricky.codelab.spring.ds.RouteStrategy">
            <entry
 
 key="slave1" value-ref="slave1DataSource"/>
            <entry key="slave2" value-ref="slave2DataSource"/>
        </map>
    </property>
    <!-- 預設目標資料來源為主庫資料來源 -->
    <property name="defaultTargetDataSource" ref="masterDataSource"/>
</bean>

DynamicRoutingDataSource 繼承自AbstractRoutingDataSource 類，程式碼如下：

package com.ricky.codelab.spring.ds;

import org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource;

/**
 * 動態資料來源切換
 *
 * @author Ricky Fung
 * @create 2016-10-18 22:41
 */
public class DynamicRoutingDataSource extends AbstractRoutingDataSource {

    @Override
    protected Object determineCurrentLookupKey
 
() {

        return DynamicRoutingContextHolder.getRouteStrategy();
    }
}

AbstractRoutingDataSource 類的定義，原始碼如下：

package org.springframework.jdbc.datasource.lookup;

public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {
    private Map<Object, Object> targetDataSources;
    private Object defaultTargetDataSource;
    private boolean lenientFallback = true;
    private DataSourceLookup dataSourceLookup = new JndiDataSourceLookup();
    private Map<Object, DataSource> resolvedDataSources;
    private DataSource resolvedDefaultDataSource;
}

AbstractRoutingDataSource 實現了InitializingBean介面，Spring容器啟動時會回撥其afterPropertiesSet()方法，AbstractRoutingDataSource afterPropertiesSet()如下：

public void afterPropertiesSet() {
    if(this.targetDataSources == null) {
        throw new IllegalArgumentException("Property \'targetDataSources\' is required");
    } else {
        this.resolvedDataSources = new HashMap(this.targetDataSources.size());
        Iterator var1 = this.targetDataSources.entrySet().iterator();

        while(var1.hasNext()) {
            Entry entry = (Entry)var1.next();
            Object lookupKey = this.resolveSpecifiedLookupKey(entry.getKey());
            DataSource dataSource = this.resolveSpecifiedDataSource(entry.getValue());
            this.resolvedDataSources.put(lookupKey, dataSource);
        }

        if(this.defaultTargetDataSource != null) {
            this.resolvedDefaultDataSource = this.resolveSpecifiedDataSource(this.defaultTargetDataSource);
        }

    }
}

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);
    }
}

這裡主要把在xml中配置的targetDataSources解析到resolvedDataSources，defaultTargetDataSource 賦值到resolvedDefaultDataSource 。

另外，AbstractRoutingDataSource 繼承自AbstractDataSource，而AbstractDataSource 是DataSource的一個子類，如下：

package org.springframework.jdbc.datasource;

public abstract class AbstractDataSource implements DataSource {
    protected final Log logger = LogFactory.getLog(this.getClass());

    public AbstractDataSource() {
    }
}

因此，我們只需要看AbstractRoutingDataSource 的getConnection()方法實現即可，如下：

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

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

determineTargetDataSource()方法如下：

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;
    }
}

determineTargetDataSource方法決定返回哪個DataSource 物件，它根據determineCurrentLookupKey方法（這個方法正是我們在DynamicRoutingDataSource類中重寫的方法）獲取資料來源的key，然後從 resolvedDataSources中根據key取出DataSource ，如果返回的DataSource 為空，則使用預設的DataSource。