一、DataSource接口核心作用

DataSource是JDBC规范的核心接口，位于javax.sql包中，用于替代传统的DriverManager获取数据库连接。Spring框架通过org.springframework.jdbc.datasource包对该接口进行了增强，提供连接池管理、事务绑定等高级特性。

二、DataSource源码分析

核心接口javax.sql.DataSource

public interface DataSource  extends CommonDataSource, Wrapper {// 获取数据库连接Connection getConnection() throws SQLException;// 使用凭证获取连接Connection getConnection(String username, String password)throws SQLException;
}

可以看到，DataSource接口提供了获取连接的的方法，并且DataSource继承了两个父接口CommonDataSource和Wrapper，CommonDataSource定义如下：

public interface CommonDataSource {// 获取日志记录器PrintWriter getLogWriter() throws SQLException;// 设置日志记录器void setLogWriter(PrintWriter out) throws SQLException;// 设置登录超时时间（秒）void setLoginTimeout(int seconds) throws SQLException;// 获取登录超时时间int getLoginTimeout() throws SQLException;// 获取父Loggerdefault Logger getParentLogger() throws SQLFeatureNotSupportedException {throw new SQLFeatureNotSupportedException();}
}

这里CommonDataSource 提供了获取和设置日志的方法，连接超时管理以及获取父Logger的方法。

public interface Wrapper {// 检查是否实现指定接口boolean isWrapperFor(Class<?> iface) throws SQLException;// 获取接口实现<T> T unwrap(Class<T> iface) throws SQLException;
}

Wrapper主要用于获取特定扩展功能

AbstractDataSource抽象类，主要提供DataSource接口中的某些方法（如getLoginTimeout()、setLoginTimeout(int)等）的默认实现
主要的继承关系如下

AbstractDataSource
├── AbstractDriverBasedDataSource
│   ├── DriverManagerDataSource
│   └── SimpleDriverDataSource
├── AbstractRoutingDataSource└──IsolationLevelDataSourceRouter

DriverManagerDataSource核心方法

public class DriverManagerDataSource extends AbstractDriverBasedDataSource {@Overrideprotected Connection getConnectionFromDriver(String username, String password) throws SQLException {Properties mergedProps = new Properties();// 合并连接属性Properties connProps = getConnectionProperties();if (connProps != null) {mergedProps.putAll(connProps);}if (username != null) {mergedProps.setProperty("user", username);}if (password != null) {mergedProps.setProperty("password", password);}// 关键点：每次通过DriverManager新建连接return DriverManager.getConnection(getUrl(), mergedProps);}
}

说明：通过用户名密码从驱动获取连接，每次调用 getConnection() 都创建一条新连接，无连接池功能，适合测试环境。

2. SingleConnectionDataSource方法

public class SingleConnectionDataSource extends AbstractDriverBasedDataSource {private volatile Connection connection;@Overrideprotected Connection getConnectionFromDriver(String username, String password) throws SQLException {synchronized (this) {if (this.connection == null) {// 初始化唯一连接this.connection = doGetConnection(username, password);}return this.connection;}}protected Connection doGetConnection(String username, String password) throws SQLException {// 实际创建连接逻辑Properties mergedProps = new Properties();// ...属性合并逻辑与DriverManagerDataSource类似return DriverManager.getConnection(getUrl(), mergedProps);}
}

说明：单例模式来维护唯一连接，直接使用JDBC Driver实例，线程安全通过synchronized和volatile保证。

3.AbstractRoutingDataSource
AbstractRoutingDataSource实现动态数据源路由抽象类，主要属性如下

public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {// 目标数据源映射表private Map<Object, Object> targetDataSources;// 默认数据源private Object defaultTargetDataSource;// 解析后的数据源映射表private Map<Object, DataSource> resolvedDataSources;// 解析后的默认数据源private DataSource resolvedDefaultDataSource;// 数据源查找接口private DataSourceLookup dataSourceLookup = new JndiDataSourceLookup();// 是否宽松回退到默认数据源private boolean lenientFallback = true;
}

初始化方法(afterPropertiesSet)

@Overridepublic void afterPropertiesSet() {if (this.targetDataSources == null) {throw new IllegalArgumentException("Property 'targetDataSources' is required");}this.resolvedDataSources = CollectionUtils.newHashMap(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);}}

说明：将配置的targetDataSources转换为可用的resolvedDataSources
获取连接的逻辑：

@Override
public Connection getConnection() throws SQLException {return determineTargetDataSource().getConnection();
}
protected DataSource determineTargetDataSource() {Assert.notNull(this.resolvedDataSources, "DataSource router not initialized");// 获取当前查找键Object lookupKey = determineCurrentLookupKey();// 根据键查找数据源DataSource dataSource = this.resolvedDataSources.get(lookupKey);// 回退到默认数据源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;
}

AbstractRoutingDataSource定义了determineCurrentLookupKey()抽象方法，子类仅需实现此方法提供键值获取逻辑。

核心逻辑：

初始化阶段：

. 实现InitializingBean接口，在afterPropertiesSet()中解析targetDataSources，生成resolvedDataSources
将defaultTargetDataSource解析为resolvedDefaultDataSource

运行时路由：

通过determineCurrentLookupKey()抽象方法获取当前数据源标识
根据标识从resolvedDataSources中查找对应的数据源
未找到时根据lenientFallback决定是否使用默认数据源

4.IsolationLevelDataSourceRouter（基于事务隔离级别的路由）

public class IsolationLevelDataSourceRouter extends AbstractRoutingDataSource {private static final Constants constants = new Constants(TransactionDefinition.class);@Overrideprotected Object resolveSpecifiedLookupKey(Object lookupKey) {// 解析隔离级别配置if (lookupKey instanceof Integer) return lookupKey;if (lookupKey instanceof String) {String constantName = (String) lookupKey;if (!constantName.startsWith(DefaultTransactionDefinition.PREFIX_ISOLATION)) {throw new IllegalArgumentException("Only isolation constants allowed");}return constants.asNumber(constantName);}throw new IllegalArgumentException("Invalid lookup key");}@Overrideprotected Object determineCurrentLookupKey() {// 从当前事务同步管理器中获取隔离级别return TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();}
}