AbstractRoutingDataSource源码：

/*
 * Copyright 2002-2012 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.jdbc.datasource.lookup;
.....
/**
 * Abstract {@link javax.sql.DataSource} implementation that routes {@link #getConnection()}
 * calls to one of various target DataSources based on a lookup key. The latter is usually
 * (but not necessarily) determined through some thread-bound transaction context.
 *
 * @author Juergen Hoeller
 * @since 2.0.1
 * @see #setTargetDataSources
 * @see #setDefaultTargetDataSource
 * @see #determineCurrentLookupKey()
 */
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;
   /**
    * Specify the map of target DataSources, with the lookup key as key.
    * The mapped value can either be a corresponding {@link javax.sql.DataSource}
    * instance or a data source name String (to be resolved via a
    * {@link #setDataSourceLookup DataSourceLookup}).
    * <p>The key can be of arbitrary type; this class implements the
    * generic lookup process only. The concrete key representation will
    * be handled by {@link #resolveSpecifiedLookupKey(Object)} and
    * {@link #determineCurrentLookupKey()}.
    */
   public void setTargetDataSources(Map<Object, Object> targetDataSources) {
      this.targetDataSources = targetDataSources;
   }

   /**
    * Specify the default target DataSource, if any.
    * <p>The mapped value can either be a corresponding {@link javax.sql.DataSource}
    * instance or a data source name String (to be resolved via a
    * {@link #setDataSourceLookup DataSourceLookup}).
    * <p>This DataSource will be used as target if none of the keyed
    * {@link #setTargetDataSources targetDataSources} match the
    * {@link #determineCurrentLookupKey()} current lookup key.
    */
   public void setDefaultTargetDataSource(Object defaultTargetDataSource) {
      this.defaultTargetDataSource = defaultTargetDataSource;
   }

   /**
    * Specify whether to apply a lenient fallback to the default DataSource
    * if no specific DataSource could be found for the current lookup key.
    * <p>Default is "true", accepting lookup keys without a corresponding entry
    * in the target DataSource map - simply falling back to the default DataSource
    * in that case.
    * <p>Switch this flag to "false" if you would prefer the fallback to only apply
    * if the lookup key was {@code null}. Lookup keys without a DataSource
    * entry will then lead to an IllegalStateException.
    * @see #setTargetDataSources
    * @see #setDefaultTargetDataSource
    * @see #determineCurrentLookupKey()
    */
   public void setLenientFallback(boolean lenientFallback) {
      this.lenientFallback = lenientFallback;
   }

   /**
    * Set the DataSourceLookup implementation to use for resolving data source
    * name Strings in the {@link #setTargetDataSources targetDataSources} map.
    * <p>Default is a {@link JndiDataSourceLookup}, allowing the JNDI names
    * of application server DataSources to be specified directly.
    */
   public void setDataSourceLookup(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<Object, DataSource>(this.targetDataSources.size());
      for (Map.Entry<Object, Object> entry : this.targetDataSources.entrySet()) {
         Object lookupKey = resolveSpecifiedLookupKey(entry.getKey());
         DataSource dataSource = resolveSpecifiedDataSource(entry.getValue());
         this.resolvedDataSources.put(lookupKey, dataSource);
      }
      if (this.defaultTargetDataSource != null) {
         this.resolvedDefaultDataSource = resolveSpecifiedDataSource(this.defaultTargetDataSource);
      }
   }

   /**
    * Resolve the given lookup key object, as specified in the
    * {@link #setTargetDataSources targetDataSources} map, into
    * the actual lookup key to be used for matching with the
    * {@link #determineCurrentLookupKey() current lookup key}.
    * <p>The default implementation simply returns the given key as-is.
    * @param lookupKey the lookup key object as specified by the user
    * @return the lookup key as needed for matching
    */
   protected Object resolveSpecifiedLookupKey(Object lookupKey) {
      return lookupKey;
   }

   /**
    * Resolve the specified data source object into a DataSource instance.
    * <p>The default implementation handles DataSource instances and data source
    * names (to be resolved via a {@link #setDataSourceLookup DataSourceLookup}).
    * @param dataSource the data source value object as specified in the
    * {@link #setTargetDataSources targetDataSources} map
    * @return the resolved DataSource (never {@code null})
    * @throws IllegalArgumentException in case of an unsupported value type
    */
   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));
   }

   /**
    * Retrieve the current target DataSource. Determines the
    * {@link #determineCurrentLookupKey() current lookup key}, performs
    * a lookup in the {@link #setTargetDataSources targetDataSources} map,
    * falls back to the specified
    * {@link #setDefaultTargetDataSource default target DataSource} if necessary.
    * @see #determineCurrentLookupKey()
    */
   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;
   }

   /**
    * Determine the current lookup key. This will typically be
    * implemented to check a thread-bound transaction context.
    * <p>Allows for arbitrary keys. The returned key needs
    * to match the stored lookup key type, as resolved by the
    * {@link #resolveSpecifiedLookupKey} method.
    */
   protected abstract Object determineCurrentLookupKey();

}

其中的targetDataSource

 Map<Object, Object> targetDataSources

在配置文件中配置dataSource时，设置targetDataSources

<bean id="dataSource" class="com.taotao.manage.datasource.DynamicDataSource">
   <property name="targetDataSources">
      <map key-type="java.lang.String">
         <entry key="master" value-ref="masterDataSource"/>
         <entry key="slave1" value-ref="slave1DataSource"/>
      </map>
   </property>
   <property name="defaultTargetDataSource" ref="masterDataSource"/>
</bean>

根据targetDataSources可以在afterPropertiesSet方法中对targetDataSources进行解析，获取真正的datasources

@Override
public void afterPropertiesSet() {
   if (this.targetDataSources == null) {
      throw new IllegalArgumentException("Property 'targetDataSources' is required");
   }
   this.resolvedDataSources = new HashMap<Object, DataSource>(this.targetDataSources.size());
   for (Map.Entry<Object, Object> entry : this.targetDataSources.entrySet()) {
      Object lookupKey = resolveSpecifiedLookupKey(entry.getKey());
      DataSource dataSource = resolveSpecifiedDataSource(entry.getValue());
      this.resolvedDataSources.put(lookupKey, dataSource);
   }
   if (this.defaultTargetDataSource != null) {
      this.resolvedDefaultDataSource = resolveSpecifiedDataSource(this.defaultTargetDataSource);
   }
}