1. DynamicDataSource 继承 org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource，实现抽象方法 protected Object determineCurrentLookupKey()

   protected Object determineCurrentLookupKey() {
       return DatabaseContextHolder.getDatabaseType();
   }
   

   DatabaseContextHolder 中定义一个静态的 ThreadLocal<DatabaseType> 的contextHolder

   public class DatabaseContextHolder {
   private static final ThreadLocal<DatabaseType> contextHolder = new ThreadLocal<DatabaseType>();
   
   public static void setDatabaseType(DatabaseType databaseType) {
   
       contextHolder.set(databaseType);
   
   }
   
   public static DatabaseType getDatabaseType() {
   
       return (DatabaseType) contextHolder.get();
   
   }
   
   public static void clearDatabaseType() {
   
       contextHolder.remove();
   
   }
   

}

```
`DatabaseType` 是一个枚举，枚举了项目要连接的所有的数据库,

创建各个数据库的`DataSource`
```java
    com.alibaba.druid.pool.DruidDataSource dataSource = new com.alibaba.druid.pool.DruidDataSource()
    dataSource.setDriverClassName(env.getProperty("xxx.driverClassName"));
    dataSource.setUrl(env.getProperty("xxx.url"));
    dataSource.setUsername(env.getProperty("xxx.username"));
    dataSource.setPassword(env.getProperty("xxx.password"));

    dataSource.setInitialSize(Integer.parseInt(env.getProperty("datasource.initialSize")));
    dataSource.setMinIdle(Integer.parseInt(env.getProperty("datasource.minIdle")));
    dataSource.setMaxActive(Integer.parseInt(env.getProperty("datasource.maxActive")));      
    dataSource.setMaxWait(Long.parseLong(env.getProperty("datasource.maxWait")));
    dataSource.setRemoveAbandoned(Boolean.parseBoolean(env.getProperty("datasource.removeAbandoned")));
    dataSource.setRemoveAbandonedTimeout(Integer.parseInt(env.getProperty("datasource.removeAbandonedTimeout")));
    dataSource.setTimeBetweenEvictionRunsMillis(Long.parseLong(env.getProperty("datasource.timeBetweenEvictionRunsMillis")));
    dataSource.setMinEvictableIdleTimeMillis(Long.parseLong(env.getProperty("datasource.minEvictableIdleTimeMillis")));
    dataSource.setPoolPreparedStatements(Boolean.parseBoolean(env.getProperty("datasource.poolPreparedStatements")));
    dataSource.setMaxPoolPreparedStatementPerConnectionSize(Integer.parseInt(env.getProperty("datasource.maxPoolPreparedStatementPerConnectionSize")));
    dataSource.setValidationQuery(env.getProperty("datasource.validationQuery"));
```

`datasource` 设置`targetDataSources`
```java
 Map<Object, Object> targetDataSources = new HashMap<>();
 targetDataSources.put(DatabaseType.xxx, mailServerDataSource());
 datasource.setTargetDataSources(Map<Object,Object>)
 ```